4. Electricity and magnetism Flashcards
Describe the forces between magnets
Magnets attract or repel other magnets
Describe the forces between magnets and magnetic materials
Magnets will always attract other magnetic materials
What causes magnetic forces?
Interactions between magnetic fields
What are 3 magnetic materials (ferrous)?
Iron, cobalt, nickel
What are 3 non-magnetic materials (non-ferrous)?
Plastic, aluminium, copper
What are 3 methods of magnetisation?
Stroking with a magnet
Use of direct current (d.c.) in a coil
Hammering in a magnetic field
What are 3 methods of demagnetisation?
Hammering
Heating
Use of alternating current (a.c.) in a coil and slowly remove the material
What are magnetic field lines?
- A way of picturing the space around a magnet
- They show its strength and direction
Describe an experiment to identify the pattern of magnetic field lines, including the direction
Place the magnetic on plain paper
Draw around the magnet using a pencil
Make a dot one end
Place a compass needle in line with the dot
Make a dot where the compass needle points to
Point the compass needle to the next point
Make 4 more dots and connect them
Repeat with the compass needle on other points along the magnet
What are 2 rules to follow when drawing magnetic field lines?
- They always go from north to south
- They never touch or cross other field lines
What are 2 magnetic properties of soft iron?
- Easy to magnetise
- Easy to demagnetise
What are 2 magnetic properties of steel?
- Hard to magnetise
- Hard to demagnetise
What are permanent magnets made out from?
Hard magnetic materials
What are electromagnets made out from?
Soft magnetic materials
What is meant by the term hard magnetic material?
Magnetic materials that can be permanently magnetised
What is meant by the term soft magnetic material?
Magnetic materials that can only be temporarily magnetised
What is the design and use of a permanent magnet?
- Hard magnetic material
- Fridge doors
What is the design and use of an electromagnet?
- Solenoid
- Scrap metal moving
What are the 2 types of charges?
Positive and negative
What happens between unlike charges?
Attraction
What happens between like charges?
Repulsion
What is the SI unit for charge?
Coulombs (C)
Describe simple experiments to show the production and detection of electrostatic charges
Suspend one of the materials using a cradle and a length of spring
Rub on end of the material using a cloth
Rub the second piece of insulating material using a cloth
Hold the charged end of the second piece close to the charged end of the suspended piece
If the first piece is repelled, the materials have the same charge
What does charging a body involve?
Addition or removal of electrons
What is the direction of an electric field at a point?
The direction of the force on a positive charge at that point
Describe an electric field?
A region in which an electric charge experiences a force
Describe charging by induction
- Charged object is held close to the conductor
- Electrons in the conductor are able to move towards or away from the object
What is an electrical conductor?
A material that allows charge to flow through it
What is an electrical insulator?
A material that does not allow charge to flow through it
What are 2 examples of electrical conductors?
Metals and graphite
What are 2 examples of electrical insulators?
Non-metals and plastics
Non-metals and plastics
A negatively charged particle
What is current?
Rate of flow of charge
What is used to measure current?
Ammeter
What is the equation for current?
I = Q ÷ t
What causes current in metals?
Flow of electrons
What is the SI unit of current?
Amperes (A)
How is an ammeter connected to a circuit?
Always in series
What is conventional current?
Positive to negative
What is the electromotive force (e.m.f.) of an electrical source of energy measured in?
Volts (V)
What is the definition of e.m.f.
Energy supplied by a source in driving 1C of charge round a complete circuit
What is potential difference (p.d.) measured in?
Volts (V)
What is 1 V equivalent to?
1 J/C
What is used to measure potential difference?
Voltmeter
What is potential difference?
Amount of energy transferred by each unit of charge passing between those two points
How is a voltmeter connected to a circuit?
In parallel with the part of the circuit you want to measure the potential difference of
What is resistance?
Opposition to current
What is the formula for resistance?
R = V ÷ I
Describe an experiment to determine resistance using a voltmeter and an ammeter
Set the power supply to a low voltage (1 or 2 V)
Use a voltmeter and ammeter to obtain voltage and current readings
Substitute readings into the equation R = V ÷ I
What is the relationship between length and resistance?
Length is proportional to resistance
What is the relationship between cross-sectional area of a wire and resistance?
Cross-sectional area of a wire is inversely proportional to resistance
Cross-sectional area of a wire is inversely proportional to resistance
Battery/power source → circuit components → surroundings
What is the formula for power?
P = VI
What is the formula for energy transferred?
E = Pt / E = VIt
Describe the current in a series circuit?
The same at every point
What is the sum of combined resistances in a series circuit equal to?
The sum on individual resistances
What is the sum of the p.d.s across the components in a series circuit equal to?
Total p.d. across the supply
Describe current in a parallel circuit?
Current from the source is larger than the current in each branch
Describe current in a parallel circuit?
Current from the source is larger than the current in each branch
Describe combined resistance of two resistors in parallel?
Less than that of either resistor by itself
What is current from the source in a parallel circuit equal to?
Currents in separate branches
What are the advantages of connecting lamps in parallel in a lighting circuit?
Lamps can be individually controlled, using their own switches
If one lamp stops to function the other will continue to work
Each lamp gets maximum p.d.
What is a variable potential divider also known as?
Potentiometer
What is a variable potential divider also known as?
Potentiometer
What is the relationship between p.d. and resistance across 2 resistors?
The resistor with a largest resistance will have greater p.d. than the other
What does a potentiometer consist of?
A coil of wire with a sliding contact along it
What does moving the sliding contact do in a potentiometer?
- Increases or decreases the length of wire
- Increases or decreases resistance (and hence p.d.)
What is a thermistor?
A resistor that changes its resistance when it is heated
What is the relationship between temperature and resistance in thermistors?
Temperature is inversely proportional to resistance
What is a light-dependent resistor?
A resistor that changes its resistance when light shines upon it
What is the relationship between light and resistance in an LDR?
Light is inversely proportional to resistance
What is the formula to find V.out from a potentiometer?
V.out = V.in × (R2 ÷ R.Total)
What is the function of a transducer?
Takes in physical energy and converts it into an electrical signal that can be read
What does a relay consist of?
A coil and magnetic switch
Describe the action of a relay
When there is a current in the coil it creates a magnetic effect
This attracts the switch, closing it
This can be used to control a switch in a separate circuit
What is a diode?
A component which only allows charge to flow in one direction through it
In what direction must the diode point to to have a current?
Positive to negative
What does a diode do to an a.c. power supply?
Rectification
What is rectification?
Conversion of a.c. to d.c.
How is an LDR used, along with a relay, to control a bulb?
*Light sensitive switch
When it gets darker, resistance increases
This causes an increase in p.d.
Increase in p.d. causes an increase in current in the coil
A large enough current in the coil will attract the switch, closing it
This allows a current to pass through
Exam tip ; )
Temperature-controlled (temperature-operated alarm) circuits work the same way as LDR-controlled circuits
What are the 2 types of signal?
Analogue and digital
What is an analogue signal?
One that is continuously varying and can take any value
What is a digital signal?
One that can only take one of two (discrete) states
What is a logic gate?
An electronic component that gives a particular output, depending on the input(s) to it
What is the hazard of damaged insulation?
Electric shock
What is the hazard of overheating of cables?
Fire
What is the hazard of damp conditions?
Electrocution
What does a fuse do?
Protects a circuit
Explain the use of fuses
If the current in the wire becomes too large
The wire heats up and melts
The wire breaks, stopping the current and breaking the circuit
Explain choosing appropriate fuse ratings
The fuse should always have a higher current rating than that needed by the appliance - without being too high
Explain the use of circuit breakers
An automatic switch which
Current rises over a specified value
Electromagnet pulls the contacts apart
Breaking the circuit
Explain the action of earth wire
Earth wire provides a low resistance path to the earth
This causes a surge of current in the earth wire and also in the live wire
High current in the fuse causes it to melt and break
This cuts off the supply of electricity to the appliance
How is an e.m.f. induced in a conductor?
When the conductor is moved across the magnetic field, it cuts through the field lines
This induces an e.m.f. in the conductor
What does the direction of the induced e.m.f. always do?
Opposes the change that produces it
Describe an experiment to demonstrate electromagnetic induction
If a wire is passes across a magnetic field/changing magnetic field
A small e.m.f. is induced in the wire
This can be detected by a galvanometer
What are 3 ways to increase the magnitude of an induced e.m.f.?
Moving the wire faster
Using a stronger magnet
Adding more turns to the coil
What is alternating current (a.c.)?
One that continuously changes its direction
What is direct current (d.c.)?
One that is steady and only flows in one direction
What is direct current (d.c.)?
One that is steady and only flows in one direction
What is direct current (d.c.)?
One that is steady and only flows in one direction
Which electrical components produce alternating current?
Mains electricity
Which electrical components produce direct current?
Electric cells (batteries)
Describe and explain a rotating-coil generator and the use of slip rings
As the coil rotates, it cuts through field lines
This induces an e.m.f. and hence a current
The size of the e.m.f. could be increased by: (List)
Slip rings transfer current to the metal brushes whilst allowing the coil to rotate freely
What are slip rings in a rotating-coil generator attached to?
The ends of the coil
When is induced e.m.f. greatest in a rotating-coil generator?
When the coil is horizontal
Why is induced e.m.f. greatest when the coil is horizontal in a rotating-coil generator?
The coil cuts through magnetic field lines at the fastest rate
Exam tip ; )
Remember to say ‘add more turns to the coil’ and not ‘add more coils’ – if you give the second one it will be marked wrong.
What is a transformer?
An electrical device that can be used to increase or decrease the voltage of an a.c.
What can a transformer not work with?
Direct current
What does a step-up transformer do?
Increases the voltage of a power source
What does a step-down transformer do?
Decreases the voltage of a power source
How does a transformer work?
A.c. is supplied to the primary coil
A changing magnetic field is produced by the coil
The field passes through the soft iron core and secondary coil
Changing field in the secondary coil induces an e.m.f.
When is a transformer used?
High-voltage transmission of electricity
What is are 2 advantages of high-voltage transmission?
- Reduces power loss
- Thinner, cheaper cables can be used
Explain why power losses in cables are lower when the voltage is high
Current causes heating in wires
Resulting in power loss
Increasing voltage reduces current
Smaller current causes less heating in wires
Less power is lost
What is created when there is a current in a wire or solenoid?
A magnetic field around the wire
What do magnetic field lines created by a current in a wire or solenoid look like?
Concentric circles around the wire
What do strength and direction of a magnetic field created by a current depend on?
Direction and magnitude of current
Exam tip ; )
The strength and direction of the field depend on the size and direction of the current:
- If the current is increased, the field will get stronger
- If the direction of the current is changed, the direction of the field will change
The strength of the magnetic field is related to the distance between the field lines:
As the field lines spread out, the field gets weaker
The field lines around a wire get further apart the further they are from the wire
What is the direction of a magnetic field line at a point?
The direction of the force on the N pole of a magnet at that point
What is the effect of reversing current on a current carrying conductor in a magnetic field?
Direction of force reverses
What is the effect of reversing direction of field on a current carrying conductor in a magnetic field?
Direction of force reverses
How does a magnetic field deflect a charged particle?
- Moving charges forms a current
- The field exerts a force on the particle, deflecting it
How can the deflection of charged particles be demonstrated?
- Using a cathode ray tube and a pair of magnets
- Passing a collimated beam of beta particles between the poles of a horseshoe magnet
How can the turning effect of a current carrying coil in a magnetic field be increased?
Increasing the number of turns on the coil
Increasing the current
Increasing the strength of the magnetic field
Describe the action of a d.c. motor
Current passes through the coil
Current creates a magnetic field around the coil
This interacts with the field of the magnets
Force is exerted on the coil
This pushes one side of the coil up and the other down
Creating a turning effect
The split-ring commutator reverses the direction of current in the current every half turn
Why does the split-ring commutator reverse the direction of current in a d.c. motor every half turn?
To reverse the direction of force and keep the coil spinning
What materials are magnets attracted to?
iron
steel
nickel
cobalt
What are the forces between magnets?
Same poles repel
Opposite poles attract
Magnetic field lines important points:
- when the magnetic field lines are close together what does this indicate?
- what does the direction of the magnetic field show?
- what should field lines never do?
- Magnetic field is strongest when field lines are closer together
- The direction of the magnetic field shows which direction a compass would point
- Magnetic field lines point away from north and towards south
- Field lines never cross
What is a uniform field?
constant magnetic field - constant direction + strength
arrows point in same direction, field lines are equally spaced apart, magnetic field strength = constant
can be made in the space between opposite poles of a magnet
Describe the properties of magnetically hard and soft materials
permanent magnets are made of magnetically hard materials eg steel, which retain their magnetism once magnetised
temporary magnets eg electromagnets are magnetically soft eg iron, which lose their magnetism once they are no longer exposed to a magnetic field and are easy to magnetise and de-magnetise
What does a magnetic field line do?
Indicate the size and direction of the magnetic force
Describe the method for investigating the magnetic field pattern for a single magnet
Place a bar magnet on the centre of a piece of paper
Bring a plotting compass close to end of North Pole of magnet but stop it touching magnet
Mark where the compass needle is pointing next to compass
Lift up plotting compass and place the centre of it on the mark made on the paper
Repeat until the South Pole is reached, join up dots
Repeat for at least another 5 lines, starting each line near the North Pole but at a different place
Describe the method for finding the magnetic field pattern for two interacting bar magnets (north to south)
Place a sheet of paper on a horizontal surface and place two bar magnets North Pole to South Pole next to each other
Leave a gap between them at least four times width of plotting compass
Bring plotting compass close to end of North Pole and draw a small dot next to where arrowhead is pointing
Move and repeat
Repeat for at least another four lines from the North Pole
What happens when some materials are placed in a magnetic field?
Magnetism is induced
Why does magnetism occur?
movement of electrons around atom
therefore a wire carrying a current (conductor) produces a magnetic field around it
Why does magnetism occur?
movement of electrons around atom
therefore a wire carrying a current (conductor) produces a magnetic field around it
Describe the construction of solenoids
soft iron core wrapped in a coil of wire
when current flows through coil of wire it becomes magnetic
How can a coil be strengthened?
more turns in coil
iron core
increase current
What happens when you reverse the current in a solenoid?
reversing the current reverses the poles of the electromagnet
Why is iron used for the core in a solenoid, not steel?
iron is magnetically soft/ gains/loses magnetism easily
Describe how a relay works
Relay = switch controlled by electromagnet
Electromagnet with small current passing through it closes switch by pushing an armature into the contacts
Closing the switch using an electromagnet reduces chance of electric shock
Describe how a circuit breaker works
Current passes through electromagnet
Magnetic field around electromagnet not strong enough to attract contact
If there is a surge in current strength of magnetic field increases and electromagnet attracts contact
Breaks circuit
Describe how a loudspeaker works
- how can the pitch and loudness of the sound be changed?
Alternating current is passed through electromagnet
Changes polarity of electromagnet
Electromagnet attracts and then repels permanent magnet
Causes magnet and cone to vibrate creating a soundwave
pitch = change frequency of alternate current which change frequency of vibration
loudness = increase current in electromagnet, stronger magnetic field around it, larger force of attraction and repulsion between it and fixed magnet -> increased amplitude of vibration
How does an electric bell work?
1.button pressed
2. Current flows
3. Electromagnet operates
4. Attracts armature
5. Strikes gong
6. Circuit broken at screw
7. Electromagnet turns off
What is a conventional current?
Current that flows from + to - battery terminals
What is the motor effect?
force on charged particle (eg in a wire carrying an electric current) when it moves in a magnetic field as long as its motion is not parallel to the field
this is because magnetic field produced by moving charges interacts w/ magnetic field of permanent magnet
How can the force on a current carrying conductor in a magnetic field be increased?
Increasing size of magnetic field
Increasing size of current
Making angle between conventional current and magnetic field closer to 90’’ (if current (conducting wire) was moving parallel to field then there would be no force)
Use longer wire
How are alpha beta and gamma deflected by force which is perpendicular to a magnetic field?
alpha deflected in opposite direction (have opposite charge)
gamma not deflected
beta is deflected the other way to alpha
What happens to a coil in a magnetic field?
When a current flows in the coil a force is applied to it
Current in each side of the loop is in opposite directions
Force on one side is upwards and on other side it is downwards
One side of the loop will be forced to the top, the other to the bottom (rotate clockwise)
The coil will then oscillate and eventually line up vertically with one side at the top and one at the bottom
How can an electric motor’s strength be increased?
increase current
increase number of turns in coil
increase strength of magnetic field
increase length of wire
How does a simple DC motor work?
Current flows in a coil of wire
Creates a magnetic field around coil of wire
Interacts with field from permanent magnet
Causes force on loop of wire which causes it to start rotating
Split ring commutator changes direction of current every half turn as it spins, reversing direction of current and reversing direction of force so it can keep spinning
What is the function of the coil of wire in dc motor
Directs current through magnetic field
What do the brushes do
Prevents wires getting tangled allowing coil of wire to rotate
What does the split ring commutator do
changes direction of current every half turn
changes direction of force, allowing it to rotate for more than half a turn
What happens when a conductor is moved across a magnetic field? (electromagnetic induction)
conductor/wire cuts through magnetic field lines
potential difference is induced
if wire is part of complete circuit a current will flow
wire and magnetic field must move perpendicular to each other (no field lines are cut if move parallel to each other, no potential difference is therefore induced)
How can a larger potential difference be induced?
More turns in coil
Stronger magnetic field
Faster movement
How can the direction of the current be changed?
Changing direction of magnetic field
Changing direction of movement
What is a generator?
Device that electrically transfers kinetic store of generator into other stores (opposite of electric motor)
Produce electricity
Describe how an AC generator works
coil of wire is spun in magnetic field
coil cuts through magnetic field lines so potential difference is induced
as coil spins each part of wire sometimes movs up and sometimes down
because it changes direction current that is induced is an alternating current (changes direction)
How does a DC generator (dynamo) generate a direct current?
split ring commutator converts alternating current into direct current
How does a bike dynamo work?
spins magnet in a coil as wheels move instead of coil of wire
magnetic field lines cut through sides of coil inducing potential difference in coil
magnetic field constantly changing direction as it rotates, output current is also alternating - so unlike normals dynamos it produces a ac not dc current
What do the slip rings do in an AC generator?
connect coil to brushes, maintains a/c inside coil to a/c in outer circuit
what kind of materials are electrical conductors?
silver, copper, alumininum, steel (delocalised electrons)
what kind of materials are insulators?
rubber, plastic, glass, wood
but can conduct a bit of static electricity, just not a flow of charge
what happens when a cloth and rod are rubbed together?
initially neutral
force of friction causes electrons to move
if the cloth transfers some electrons to the rod, what charge will the objects now have?
cloth: more protons -> positive
rod: more electrons -> negative
what charge does polythene get when charged with cloth?
negative
why are charges evenly distributed in a conductor?
electrons are repelled by other electrons, and these are attracted to protons
is it possible to move positive charges?
no
is it possible to move negative charges?
yes! electrons are not within nucleus
charging via induction
What happens if you bring a negatively charged rod near a piece of paper (neutral)?
rod approaches paper, electrons in paper are repelled away from rod
makes one side of paper negative and other positive
positive side is closer to rod
resultant force = attractive as opposite charges are closer
paper sticks to rod
works the same if rod is positive -> electrons attracted to one side
charging by contact
How can a neutral object become charged by touching a charged object?
electrons flow onto (if object positively charged) or off of object (if negatively charged, as the elctrons repel each other and move away from each other)
what is earthing?
charge on object is neutralised by transfer of electrons
why would electrons try to flow off of a negatively charged object?
they are repelled
why would electrons try to flow onto a positively charged object?
they are attracted
what is the purpose of a photocopier?
to print copies of an image/document
how does a photocopier work?
- what is done to the drum before toner is added?
image of document is projected on to positively charged drum
drum loses charge in light areas and keeps positive charge in dark areas (light areas conduct electricity)
negatively charged toner sticks to where there is a positive charge on drum
transferred onto paper
how do inkjet printers work?
ink droplets positively charged via charging electrodes
pass between oppositely charged plates
computer controls plate voltage
droplets attracted towards negatively charged plate + repelled from positive one (the deflection can be changed by altering strength of plate charges)
direction changed by changing the charge of the plates
how does electrostatic paint sprayer work?
paint droplets given charge by paint sprayer so repel each other-> fine mist and covers large area
oppositely charge car
less waste, more even coverage
how does a dust precipitator work?
reduces pollution
dust becomes charged by passing thru negatively charged plates
dust removed from air as attracted to positively charged plates
dust removed from collecting plates as it is neutralised upon touching the plates, falls into dust collection
how is lightning made?
droplets + ice crystals rub against each other in cloud during storm, creates static electrical charge
clouds have positive and negative side
negative side is at the bottom -> when there is enough charge in the cloud it is released via lightning bolt, which is goes to anything oppositely charged (eg lightning conductor, positively charged, earths it)
why is refuelling airplane potentially dangerous?
build up of static charge : friction between fuel and pipe, fuel gains charge -> could spark and fuel could ignite and explode (also friction with air whilst flying can cause a charge)
explosion prevented by fuel tank being connected to earth w conductor
carriers charge through to earth to reduce sparking risk
what is the electric current?
Rate of flow of charge
what is the equation to calculate current from time and charge?
give units
current = charge(c)/time(s)
I=q/T
what is the definition of potential difference?
(voltage)
energy gained or lost by each electron (energy transferred per unit charge)
how do you calculate potential difference from energy transferred and charge?
potential difference = energy transferred (J) /charge (C)
v= e/q
what does an ammeter do?
measure current
what does a voltmeter do and where should it be in a circuit?
measures voltage
should be paralell to component
what is resistance?
what determines/reduces flow of current
what happens to the current when the potential difference increases?
increases (electrons gain more energy, move faster)
what happens to the current when the resistance of a circuit increases?
decreases
what is the equation v=ir?
potential difference = current x resistance
how do you convert from mA to A?
divide by 1000
there are 1000 milliamps in one amp
what is the unit for resistance?
ohm
what affects the resistance of a wire?
just list 4 things
material
length
thickness
temperature
why does length affect resistance?
length of wire is increased, electrons have to travel further
chance of collisions increases
causes resistance to increase
why does thickness affect resistance?
increasing thickness of wire increases area that electrons can flow through
decreases chance of collisions w metal ions
causes resistance to decrease
why does temperature affect resistance?
ions vibrate faster
harder for electrons to flow
temp increases -> resistance increases
what is ohms law?
current is directly proportional to poterntial difference if temp is constant
thus component that obeys ohms law has constant resistance (the gradient)
why does the resistance of a wire increase when the current increases?
current increases -> more FREQUENT collisions of electrons w ions
rate of energy transer to ions increases
ions vibrate faster
resistance of wire increases
what can a diode or LED be used to determine?
whether a current is flowing
what are the similarities between resistors and bulbs?
reduce the current in a circuit
have some resistance
energy is transferred
what are the similarities between resistors and bulbs?
reduce the current in a circuit
have some resistance
energy is transferred
what are the differences between resistors and bulbs?
resistor has fixed resistance
bulb has a changing resistance
bulb emits light
what are the similarities between bulbs and light emitting diodes?
emit light
resistance isn’t constant
energy transferred
what are the differences between bulbs and light emitting diodes?
current can only flow in one direction
bulb -> current can flow in either direction
what is the use of a thermistor?
sense diff temps and switch off an oven when it gets too hot
why does the resistance of a thermistor drop with temperature?
as it gets hotter -> more electrons get excited + become available for conduction
resistance drops
what are LDRS used for
sense light levels and switch on streetlamps when it gets dark
why does the resistance decrease with the increase of light intensity?
as it gets brighter, more electrons get excited and become available for conduction -> R drops
why would a larger potential difference increase total amount of energy that is transferred?
each electron transfers more energy to the bulbs
why would a larger current increase the total amount of energy that is transferred?
more electrons pass through bulb each second
what causes heating?
electric current
what is the equation for total electrical energy transferred?
energy transferred= current x voltage x time
E=IVT (energy in joules)
what is the equation for electrical power?
P=IV
power in watts
what is the equation for electrical power?
P=IV
power in watts
what equation links power energy and time?
power = energy/time
in a series circuit there is ____ possible _ for an electron to ___
there is only one path
in a paralell circuit
the electrons can…
take different paths around the circuit
why is a paralell circuit used for normal room lights?
so they can be switched on and off independently
if one bulb blows in a paralell circuit…
the other bulbs will still work
whereas in a series they would all go out
it is possible to turn on individual bulbs in a _ circuit
paralell
What are the rules in a series circuit?
current is the same throughout circuit
potential difference is shared between components (bulbs will get dimmer the more you add)
total resistance = adding resistances of individ components
what are the rules for a paralell circuit?
potential difference is same throughout circuit
current splits at each junction (more current flows through the branch with smaller resistor)
use v=ir to calculate current
what happens when more components are added to a paralell circuit?
increases current
resistance of circuit decreases
what is alternating current?
electrons continually change direction
mains electricity is ac
what type of current is battery?
dc (direct, electrons flow in same direction)
what are the differences between batteries and mains?
cells have chemical stores which are transferred electrically, mains depends
constant low voltage, alternating high voltage
DC, AC
50Hz is typical freq for UK mains
why is the wire in the cable made from copper?
conductor, flexible
why is the wire casing made from flexible plastic?
insulator, flexible
why are the plug pins made from brass?
conductor, rigid
why is the plug casing made from stiff plastic?
insulator, rigid
what colour is the live wire and what does it do (plug)
brown
high voltage
carries current that alternates between neg and pos volatge
what colour is the earth wire and what does it do?
green
safety wire used to earth appliances w a metal case
what colour is the neutral wire?
blue
completes circuit
kept at 0 voltage
how do fuses work?
why are they needed and explain fuse ratings
surge in current could damage an electrical appliance
fuse contains thin wire -> melts if current too hgh
this breaks cirucit and elecrticity unable to flow
fuse rating gives current for which the fuse will melt eg 3A fuse
should be slightly larger than normal operating current
what is earthing?
reduces chance of shock
earth wire connected to casing of metal device to provide alternative path for current if fault occues
if live wire becomes loose and touches metal case, v large current flows (low resistance) to eath and melts the fuse wire breaking circuit
what are double insulated appliances?
plastic casess
case cannot become live, doesn’t conduct
if one layer of insulation damaged the appliance is still safe
what are circuit breakers?
circuit broken by using an electromagnet to pull two wires apart
How does an induction cooker work?
use alternating magnetic fields to induce currents in saucepan, which heats saucepan
little energy transferred to thermal store of surroundings, making it v efficient
Describe the structure of a transformer
Alternating current in primary coil produces continually changing magnetic field within iron core
Iron core guides magnetic field into secondary coil
Field lines continually moving because magnetic field is changing strength
Potential difference is induced across secondary coil because secondary coil cuts through field lines
If secondary coil is attached to a circuit then current will flow
What is a transformer used for?
transforming alternating voltage from a larger/smaller value to a smaller/larger value
What does a step up transformer do to voltage current and power output, and why
more turns in secondary coil
increases voltage
decreases current
no effect on output power (because P=IV)
What does a step down transformer do to voltage, current and power?
fewer turns in secondary coil
decreases voltage
increases current
power stays same
What happens when voltage is increased using step up transformer?
Because power = current x voltage, current decreases
What is the relationship between input (primary) and output (secondary) voltages and the turns ratio for a transformer?
Vp/Vs = Np/Ns
P= primary
S= secondary
N= number of turns
What is the relationship between input and output power when a transformer is 100% efficient?
input power (IpVp) = output power (IsVs)
How do you calculate efficiency of transformer?
Power in secondary/power in primary x 100
How do you calculate efficiency of transformer?
Power in secondary/power in primary x 100
Why do transformers not work with direct current?
In direct current, magnetic field would be constant so no field lines would be cut and there would be no potential difference generated
How is energy wasted in transformers (3 ways)
Heat is produced in coils, since current is flowing through them
Magnetic field in core produces current which also causes heating
Magnetic field may leak out of core if there is an air gap between primary and secondary
How do you make transformers efficient?
heating in wires -> use low resistance wires
eddy currents in iron core -> laminate iron core
leaking magnetic field -> use an iron core to direct current through coils of wire
How are transformers used on an industrial scale?
used to increase voltage of electricity supply after it has left power station -> lower current, less heating of wires, less energy wasted
voltage reduced at sub stations before reachers consumer, as this is safer
Ion
A charged atom
Rubbing a polythene rod with a dry cloth…
Transfers electrons to the surface atoms of rod from the cloth, so the rod becomes negatively charged.
Electrons gained in the polythene rod.
Atoms in dry cloth becomes positive as a result of losing electrons.
Rubbing a perspex rod with a dry cloth…
Transfers electrons from the surface atoms of the rod to the cloth, so the rod becomes positively charged.
Atoms become positive in the perspex rod as a result of losing electrons.
Electrons gained by dry cloth.
Battery symbol
Battery.
A cell is necessary to push electrons around a complete circuit. A battery consists of two or more cells.
Switch symbol
Switch.
A switch enables the current in a circuit to be switched on or off.
Indicator symbol
Indicator.
An indicator is designed to emit light as a signal when a current passes through it or as a light source such as a bulb.
Ammeter symbol
Ammeter.
Used to measure current.
Resistor symbol
Resistor.
Limits the current in a circuit.
Variable Resistor symbol
Variable Resistor.
allows the current to be varied.
Voltmeter symbol
Voltmeter.
Used to measure voltage.
Diode symbol
Diode
allows current through in one direction only.
LED symbol
LED
Emits light when a current passes through it.
Fuse symbol
Fuse
designed to melt and break the circuit if the current through it is greater than a certain amount.
Heater symbol
__ITTTTTTI__
LLLLLLLI
Heater.
Designed to transfer electrical energy to heat the surroundings.
What way in a circuit does current flow?
+ to -
The size of an electric current…
… is the rate of flow of an electric charge.
What is the unit of electric charge?
Coulombs (C)
Current =
Charge flow (C) / Time taken (s)
Potential difference across the component (V) =
Energy transferred (J) / Charge (C)
What causes resistance?
Electrons having to push past lots of vibrating ions in a metal filament.
resistance (Ω) =
Voltage / Current
Define Ohm’s Law
The current through a resistor at constant temperature is directly proportional to the voltage across the resistor.
2 Non-Ohmic Conductors
Filament Bulb
Diode
1 Ohmic Conductor
Resistor
Thermistor symbol
Thermistor
A resistor that depends on temperature.
Light Dependent Resistor symbol
Light Dependent Resistor
A resistor that depends on the intensity of light.
In a series circuit, how much current passes through each component?
The same amount.
In a series circit, what happens to the voltage?
It is shared between all of the components.
In a parallel circuit, what happens to the current?
It is split up, and runs through separate components.
In parallel, what is the voltage like for each component?
the same.
Direct Current
Electric current in a circuit that is in one direction only.
Alternating Current
Electric current in a circuit that repeatedly reverses its direction.
What is the mains frequency in the UK?
50Hz (AC)
Live wire
The mains wire that has a voltage that alternates between +325 and -325v (in Europe)
Neutral Wire
The wire of a mains circuit that is earthed at the local substation so its potential is close to zero.
Half wave rectification
The use of a diode in a circuit with an alternating supply pd to allow current in only one direction every other half cycle of the supply pd.
Socket
A mains socket is used to connect the mains plug of a mains appliance to the mains circuit.
What are plug pins made of?
Brass (Its a good conductor, and it doesn’t rust or oxidise)
What is the longest pin of a three-pin plug and why?
The earth plug, because it is the first pin that connects with the wall socket, so the plug is earthed even before the other pins connect.
Fuse
A fuse contains a thin wire that melts and cuts the current off if too much current passes through it.
What does the earth wire protect?
The user
What does the fuse protect?
The appliance and the wires of a circuit.
Circuit Breaker
An electromagnetic switch that opens and cuts the current off off if too much current passes through it.
Why are circuit breakers better than fuses?
They work faster
They can be reset
Residual Current Circuit Breaker
(RCCB)
An RCCB cuts off the current in the live wire when it is different from the current in the neutral wire.
More sensitive than fuses/ normal circuit breakers.
A fuse is always connected in…
…series.
Power Supplied (W) =
Current (A) x Voltage (V)
When charge flows through a resistor…
energy is transferred to the resistor, so the resistor becomes hotter.
Kilowatt-hour
(kwh)
Electrical energy supplied to a 1kW device in 1 hour.
What is a kWh?
A unit of energy
Are low power lamps more or less efficient than high power ones?
More, because they give the same amount of light energy for the same input of electrical energy.
Filament Bulb
+ and -‘s
very inefficient
heat from the bulb makes the plastic casing go brittle and crack.
CFL’s
+ and -‘s
More efficient that filament bulbs
LED’s
+ and -‘s
Use a low voltage and low power
V. Efficient
Last much longer.
Step Up transformers
raise voltage
lower current
Step Down transformers
Lower voltage
Raise current
Describe the magnetic field patterns with like poles facing each other.
Bend away from the midpoint.
Describe the magnetic field patterns with unlike poles facing each other.
The field between the poles is uniform.
What is special about a uniform field?
The magnetic field lines are parallel to each other.
What metal is used for the core of electromagnets?
Iron (Easily magnetised and demagnetised)
Name for metals that can be magnetised/ demagnetised easily
ferrous metals
4 examples of electromagnets
> Scrapyard cranes > Circuit breakers > Electric bells > Relays
Increasing _____ makes the magnetic field of an electromagnet stronger.
Current
What surrounds an electromagnets core?
A coil of insulated wire
How can the size of the electromagnetic force be increased
Increasing the current Using a stronger magnet
In the motor effect, when is the force greatest?
When the wire is perpendicular to the magnetic field.
What are the magnetic field lines around a wire?
Circles centred on the wire in a plane perpendicular to the wire.
What are the magnetic field lines around a wire?
Circles centred on the wire in a plane perpendicular to the wire.
Reversing the direction of the current…
…reverses the magnetic field lines
The Motor Effect
When a CURRENT is passed along a wire in a magnetic field, A FORCE IS EXERTED on the wire by the magnetic field.
In the motor effect, when is the force zero?
When the wire is parallel to the magnetic field.
Fleming’s Left Hand Rule
Movement
Field (N to S)
Current
Commutator for DC
Split Ring- Reverse the current on every half turn to keep the motor spinning constantly.
Commutator for AC
Slip-ring - Doesn’t alternate the current because it alternates already.
Electromagnetic Induction
The process of inducing a potential difference/voltage in a wire by moving the wire so it cuts across the lines of force of a magnetic field.
The Generator Effect
The production of voltage using a changing magnetic field.
What never changes in a DC generator?
The Voltage’s Polarity
What is a simple AC generator made up of?
A coil that spins in a uniform magnetic field.
When is the size of an induced p.d/voltage on an AC generator zero?
When the coil is perpendicular to the magnetic field lines.
When is the size of the induced p.d/voltage on an AC generator greatest?
When the coild is parallel to the direction of the magnetic field.
What types of current does a generator work with?
AC
The Solenoid Rule
Direction of coil tells us which pole it corresponds to.
North- iN
sOUTh- OUT
4 facts about electromagnetic induction.
No current when the wire is stationary
A current is generated when the magnet moves but not the wire.
Faster movement = More current
Current reverses when the direction of motion reverses.
What must there be for electromagnetic induction to occur?
A changing magnetic field.
3 Acronyms for Transformers
Vicky needs 5 A’s
Voltage Can Make Vicky Cry
PC Ian Can’t Solve Crime
A A A A A
V C M V C
PC IC SC
3 steps of a transformer based on the acronyms
An alternating voltage in the primary coil creates an alternating current there.
This creates an alternating magnetic field in the iron core
This induces an alternating voltage in the secondary coil, which induces an alternating current.
3 steps of a transformer based on the acronyms
An alternating voltage in the primary coil creates an alternating current there.
This creates an alternating magnetic field in the iron core
This induces an alternating voltage in the secondary coil, which induces an alternating current.
A step up transformer…
Has more turns on the secondary coil, so increases voltage.
A step down transformer…
Has less turns on the secondary coil, so decreases voltage.
The higher the grid voltage…
… the better the efficiency of the transformer.
Equasion for transformer efficiency:
Vp / Ip = Vs / Is
V = Voltage
I = Current
The transformer equation:
Vp / Vs = (No. of turns) Np / Ns
The faster the coil rotates…
…The greater the frequency of the AC current
+
The larger the peak value of the AC current.
Switch mode transformer.
A transformer that works between 50 000 Hz and 200 000 Hz
What is the core of a switch mode transformer made of?
Ferrite
Advantages of a switch mode transformer
> Lighter and Smaller
> Higher frequency
Voltage of the National Grid
132 000 V
Why does the voltage of the national grid being higher make it more efficient?
If the voltage was lower more curren would be needed for the same amount of power to be transferred. The cables would overheat and result in more wasted energy.
What direction are the magnetic field lines in?
North to South
Which equation do we tend to use to compare the power of resistors that are connected in series?
P= I^2 R
Which equation do we tend to use to compare the resistors that are connected in parallel?
P= V^2/ R
What is direct current?
Current flowing in one direction
What is alternating current?
Changes direction
What is a variable resistor?
It is a controlled resistor and it is used as volume control and as potentiometer- controls the current.
What does a diode do?
Allows current to flow in one way only
How should the diode be placed for the circuit to work?
The straight line should be on the side of the negative - terminal of the battery
What is diode made to do?
Change ac to dc by rectification
What are short circuits?
Is a low resistance path for the current to follow
What is the earthing wire used for in our homes?
Safety precautions to prevent electric shock
What are dangers of electricity?
Damaged insulation
Overheating of cables
Damp conditions since water is a conductor
What is a transducer?
A device for converting a non-electrical input into an electrical signal
What does the input sensor do?
Detects changes in the environment and converts them to electrical signal
Give examples for input devices
Thermistor
LDR
Thermocouple
Give examples for output devices
LED’s
Heater
Bell
Loudspeaker
What are analogue signals?
Handled information which is represented by continuous change and flow, such as voltage and current
What are digital signals?
Represent data in discrete units
Give the truth table for an OR gate
0 1 1
1 0 1
1 1 1
0 0 0
Give the truth table for an AND gate
0 0 0
0 1 0
1 0 0
1 1 1
Give the truth table for a NOT gate
0 1
1 0
Give the truth table for a NAND gate
0 0 1
1 0 1
0 1 1
1 1 0
Give the truth table for a NOR gate
0 0 1
1 0 0
0 1 0
1 1 0
Draw on a piece of paper ALL of the logic gates
.
List magnetic materials
Iron
Steel
Nickel
Cobalt
What are ferromagnetic materials?
Iron
Steel
Give an example of a hard magnet material
Steel
What does a hard magnetic material mean?
Hard to magnetize- long time to magnetize
Permanent magnet
What does a soft magnetic material mean?
Easy to magnetise- short time to magnetize
Temporary magnet
Why are some materials attracted to magnets?
Because of the existence of tiny magnets inside. When a magnetic is brought near it, its tiny magnets are arranged uniformly
How can magnetism be made?
By induction
By stroking
By electrically
How can you magnetise electrically?
Place magnetic material in a solenoid connected to dc current
How can magnetism be destroyed?
Hammering a magnet
Heating
Dropping a magnet
Demagnetising by using reduced AC through a coil of wire wrapped round a magnet
What is the magnetic field?
The area around a permanent magnet or a wire carrying a current.
Describe the straight wire’s magnetic field
Concentric circles
What can a solenoid be used for?
Circuit breaker
Magnetic relay
What is the purpose of a relay?
Control high voltage system with low voltage or control high current system with low current
Describe the relay process
The low voltage circuit is on so there is a current in the solenoid. The magnetic field in the solenoid attracts the armature and the current flows through the contacts so the high voltage is on.
What is a circuit breaker?
A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit.
What increases the magnetic force?
Increasing current
Stronger magnet
Increasing length of wire in the field
What are parts of an electrical motor
Coil of wire
Two magnets
Split ring commutator
Brushes
How does the electrical motor work?
The current flows in opposite directions on each side of the armature which will create two opposite forces act as different line making the armature only spin half a cycle
The split-ring reverses the current direction through an armature every half turn so the armature will continue to spin in the same direction
What increases the turning effect?
Increasing current
Increasing number of turns
Increasing area of the coil
Using a stronger magnet
What happens when the coil is horizontal?
The turning effect will be maximum
What happens as the turning effect as the coil rotates from horizontal to vertical?
It decreases
What is faraday’s law?
The EMF induced in a conductor is proportional to the rate at which magnetic field lines are cut by the conductor
What increases the induced EMF?
Moving wire faster
Stronger magnet
Increasing the length of wire inside the magnetic field
What is a galvanometer?
A very sensitive ammeter
What is Lenz’s Law?
An induced current always flows in the direction such opposes the change which produced it
How can the size of the current be made bigger?
Magnet at a faster speed
Stronger magnet
More turns of wire on the coil
What does an AC generator convert?
Mechanical work into electrical work
How does an AC generator work?
The wire cuts the m.f lines inducing an emf that causes induced current in the coils.
If the wire is moved in the opposite direction the induced current also moves in the opposite direction
This means that as a coil is rotated in magnetic field, the induced current reverses direction every half turn. This is called alternating current.
How do I increase EMF in the AC generator?
Increase number of turns
Use a stronger magnet
Increase the area of the coil
Rotating the coil faster
What are the types of transformers?
Step up transformer
Step down transformer
What is a transformer made from?
Two coil and one on each side of a soft iron core
Explain how the transformer can work
AC current passes through the primary core which creates a changing magnetic field in the iron core. The changing magnetic field then induces alternating current in the secondary coil. The induced emf depends on the number of coils in the secondary coils.
Transformer equation
Voltage A/ Voltage B= Number of turns in A/ Number of turns in B
Why do transformers only work with alternating current?
Using direct current will create a mf in the core but it will not be a changing mf and so no voltage will be induced in the second coil
Power in the transformer
V1 * I1= V2 * I2
Why does a transformer produce a very high voltage?
Minimise energy loss as current decreases therefore it is less expensive
What does a current carrying wire create?
A magnetic field
The larger the electric current…
The stronger the magnetic field
What does the direction of the magnetic field depend on?
The direction of the current
What magnetic field is around a straight wire?
A field made of of concentric circles with the wire in the centre
What is the magnetic field around a flat circular coil like?
Around the centre of the flat circular coil of wire it is similar to that of a bar magnet
Around the coil there are concentric ellipses (stretched circles)
Describe the magnetic field around a solenoid like?
Strong and uniform inside current carrying solenoid
- Outside the coil it is like a bar magnet (ends of solenoid act as a north and south pole making an electromagnet)
What can magnetic materials either be?
Hard or soft
Why are magnetic materials like iron considered soft?
It looses its induced magnetism quickly (temporary magnet)
Why are magnetic materials like iron considered soft?
It looses its induced magnetism quickly (temporary magnet)
Why are magnetic materials like steel considered hard?
The magnetism is kept permanently (permeant magnet)
Why is iron used in transformers?
Because of its properties - it needs to magnetise and demagnetise 50 times per second
How can you increase the strength of a magnetic field around a solenoid?
Add soft iron core
Why are electromagnets far more useful than permanent magnets?
Turn on and off
Change the magnetic field strength
Change the shape of the magnet
What is an example of a use of electromagnets?
Lifting metal objects and the electric bell
Describe the construction of a electromagnet
Current carrying wire wrapped into coil, wrapped around an iron core
What is electromagnetic induction?
The creation of a voltage in a wire which is experiencing a change in magnetic field
What is the dynamo effect?
Transforming kinetic energy into electrical energy (in a power station this kinetic energy is provided by turbines)
How can you get EM induction?
When an electrical conductor moves through a magnetic field (cutting the magnetic field lines) causing the electrical conductor changes
How can you test EM induction?
By connecting an ammeter to a conductor and moving the conductor through a magnetic field - it will show the magnitude and direction of the induced current
What happens if the direction of movement is reversed?
The induced voltage/current will be reversed too
What three things can you increase to get a bigger voltage?
The strength of the magnet
The number of turns on a coil
The speed of the movement
What is the difference between a simple electric motor and a generator?
Motor - you have a current in the wire and magnetic field which causes movement
Generator - You have a magnetic field and movement which induces a current
What does AC generators do?
They rotate a coil in a magnetic field which produces a current
What is there construction similar to?
A motor
What happens as the coil spins?
A current is induced in the coil. The current changes direction every half turn
What do generators have instead of a split-ring commutator?
An ac voltage - the faster the revolutions produce more peaks and higher overall voltage
How do power stations use ac generators?
They produce electricity - they get the energy needed to turn the coils or magnetic field in different ways
A magnet is held above a coil. The coil is connected to a voltmeter. The magnet is released and falls into the coil
Explain why the voltmeter records a reading
As the magnet falls through the coil, there is a changing magnetic field acting on the coil. This inducers a voltage across the coil and a current through it
The magnet is released from a greater height. How does this affect the voltmeter. Explain your answer
The magnet will be moving faster so the induced voltage will be greater
State how the voltmeter reading changes when the same magnet?
(i) moves more slowly into the coil
(ii) moves into a coil with more turns
(iii) is reversed so that the S pole enters the coil first
(i) Reading decreases - less voltage
(ii) Reading increases - more voltage
(iii) Reading will be negative
A student investigates how to produce a voltage. He hangs a magnet from the spring, above a coil that is connected to a data logger. The student pulls the magnet through the coil then releases it
Explain why the data is varied?
How do you know it is an alternating current?
1) The magnetic field is being cut forcing the amplitude to decrease as the spring with the magnet attached, slows down gradually
2) It goes from positive to negative
Why does the voltage change on a graph?
Direction of magnet changes
Amount of field lines cut changes
Speed of magnet changes
(Any one of these answers)
How does a motor effect happen?
When you put a current carrying wire in a magnetic field (causes the wire to move)
Why does the wire move?
It is because charged particles moving through a magnetic field will experience a force as long as they’re not moving parallel to the field lines
To experience a full force what does the wire have to be at?
90 degrees to the magnetic field (wont work if the wire runs along the magnetic field)
What direction does the force always act towards?
The same direction relative to the magnetic field of the magnets and the direction of the current in the wire (north-south poles)
The direction of the force depends on what?
The direction of the magnetic field
- Direction of the current
What is a good way showing the direction of a force?
Apply a current to a set of rails inside a horseshoe magnet. A bar is placed on the rails which completes the circuit - this generates a force that rolls the bar along the rails
What does the magnitude of the force increase?
The strength of the magnetic field
How else does the force increase?
The amount of current flowing through the conductor
What happens if you reverse the current or magnetic field?
It also reverse the direction of the magnetic field
What does flemings left hand rule tell you?
Which way the force acts
What do the thumb, first finger and second finger represent?
Thumb - Motion (force)
First finger - Field
Second finger - Current
How do you use flemings left hand rule?
Using your left hand point your first finger in the direction of the field and your second finger in the direction of the current. Your thumb will then point in the direction of the force (motion)
A thin strip of aluminium foil held between the poles of a strong magnet
When the switch is closed the aluminium foil moves. Explain way
The magnetic field due to the current in the foil interacts with the permanent magnetic field of magnet, causing a force on the foil so, the foil moves.
What do magnetics produce?
Magnetic fields
What do all magnets have?
Two poles (north and south)
- The poles are the strongest part of a magnet
What is a magnetic field?
A region where magnetic materials (iron) experience a force
What do magnetic field lines show?
The size and the direction of the magnetic fields - always point from north to south
What happens if you place the north and south poles of two permanent magnets near each other?
It creates a uniform field between the two magnets (straight lines going from north to south)
What can you use to look at magnetic field lines?
Compasses and ion fillings
What do compasses and iron fillings do to make us see the magnetic fields?
Align themselves with the magnetic field
How can see the magnetic field with iron fillings?
Put a magnet(s) under a piece of paper and scatter the iron fillings on top
How can you see the magnetic field lines with compasses?
Use more than one compass/ a compass and move it around a magnet (trace the pathway on a piece of paper)
What do magnets affect?
Any magnetic materials or other magnets
Likes poles…
Repel
Opposite poles…
Attract
What happens when a magnet is brought near a magnetic material?
That material acts as a magnet (the magnetism has been induced by the original magnet)
What happens the closer the magnet and magnetic material get?
The stronger the induced magnetism will be
Far from the magnet…
Little induced magnetism
Close to the magnet…
Magnetism induced
What four factors will speed up an DC electric motor?
More current
More turns on the coil
Stronger magnetic field
A soft iron core in the coil
How can a simple electric motor be built?
Using a coil of wire that is free to rotate between two opposite magnetic poles. When an electric current flows through the coil, the coil experiences a force and moves (interaction)
Describe how an electric motor works
A force is acting on the two side arms of the coil. Because the coil is on a spindle (one side acts up and the other down) it rotates
(it is the forces between the two magnetic fields that cause the coil to turn, transferring electrical energy to kinetic energy)
What is used to make sure when the coil rotates, it doesn’t get stuck?
Split- ring commutator - clever why of swapping the contacts every half turn to keep the motor rotating in the same direction
How can the direction of the motor be reverses?
Swap the polarity of the dc supply
- Swap the magnetic poles over
What can be used to work out which way the current will turn?
Flemings left hand rule
What works because of the motor effect?
Loudspeakers
How does the loudspeaker work?
AC electrical signals from an amplifier are fed to a coil of wire in the speaker which is wrapped around the base of the cone
What is the coil in the loudspeaker surrounded by?
A permanent magnet - the ac signals cause a force on the coil and make it move back and forth
What do the movements of the coil produces?
The cones vibrate which creates sounds
What do transformers do?
Change the size of the voltage of an alternating current
What do all transformers have?
Two coils, the primary and the secondary, joined with an iron core
What happens when you apply an alternating voltage across a primary coil?
The magnetically soft (iron) core magnetises and demagnetises quickly. This induces an alternating voltage in the secondary coil
The ratio between the primary and secondary voltages is the same as what ratio?
The ratio between the number of turns on the primary and secondary coils
What are step up transformers?
They step the voltage up - they have more turns on the secondary coil than the primary coil
What are step down transformers?
They step the voltage down - they have more turns on the primary coil than the secondary
What do you need to know to calculate the output voltage from a transformer?
The input voltage and the number of turns on each coil
What is the word equation for transformers?
Input (primary) voltage /Output (secondary) voltage = Number of turns on primary/ number of turns on secondary
What are the other two equations you can use for transformers?
Vp/Vs = Np/Ns or both equations flipped around
If transformers are nearly 100% what does that mean?
Power in = power out
What is the general formula for power supplied?
Power = voltage x current or P= V x I
How can you rewrite input power = output power for transformers?
VpIp = VsIs
Vp = Primary voltage
Ip = Primary current
Vs = Secondary voltage
Is = Secondary current
What do transformers make more efficient?
Transmitting mains electricity
What are the main purposes of using step up and step down transformers?
Used when transmitting electricity across the country
Why is the voltage not transmitted efficiently in power stations?
It is too low - the lower the voltage the higher the current for a given amount of power, causing the wire to heat up
What are step up transformers used for in the power station?
To boost the voltage before it is transmitted
What are step down transformers used for in power stations?
Used at the end of the journey to reduce the voltage so its more useful and safer to use
Charge = ?
units
Charge = current * time
Q = I * t
Q = C (coulombs)
I = A (current)
t = S (seconds)
Ferrous materials?
4
Attracted by a magnet and can be magnetised
Iron
Nickel
Steel
Cobalt
Steel = Permanent
Iron = Temporary
Non - ferrous metals
5
Not attracted by magnet and can’t be magnetised
Copper
Silver
Aluminium
Wood
Glass
Electrical field intensity
+ Equation
Amount of force exerted by the charge on a unit charge (Q) placed at a point in the field
E = F / Q
Ohms law
+ Equation
Voltage across resistor is directly proportional to current
R = V / I
R (measured in Ohms)
P.d (potential difference) - Voltage
V = Energy / Charge
V = E / Q
E.M.F (electromotive force)
EMF = lost volts inside the power source + terminal p.d
EMF = Ir + IR
Resistance and resentivity
R = resentivity * (Length of a resistor / Area of cross section of the resistor)
R = resentivity * (L / A)
Current:
Series
Parallel
Series = Stays the same
- Parallel = Current divides
Voltage:
Series
Voltage
Series = Divides
- Parallel = Stays the same
Total Resistance in series
R = R1 + R2 + R3
The R’s are all resistances of resistors in Ohms
Total Resistance in parallel
1 / R = 1 / R1 + 1 / R2 + 1 / R3
Potential divider or potentiometer
V1 / V2 = R1 / R2
Potential divider
V2 = (R2 / (R1 + R2)) * V
Power
4
P = V * I
P = I^2 * R
P = V^2 / R
P = Energy / time
Diode function
Semiconductor device
Current pass only in one direction, rectifier
Transistor function
Semiconductor device works as a switch, collector, base, emitter
Light dependent resistor function
LED
LED is dependent on light
Brightness increase = resistance decrease
Thermister function
Dependent on temperature
Temp increase = resistance decrease
Capacitor function
Parallel conductor with insulator in between to store changes
Relay function
Electromagnetic switching device
Fleming’s left hand rule
Thumb = Motion
First finger (index) = Field
Second finger = Current
Transformer equation
voltage
Np / Ns = Vp / Vs
Transformer equation
(Current)
(2)
Vp / Vs = Ip / Is
Vp * Ip = Vs * Is
E.M induction
EMF or current is induced in a conductor when it cuts the magnetic field lines
A.C generator
Produces current
Fleming’s RIGHT hand rule
D.C generator
Consumes current
Fleming’s LEFT hand rule
Name the 5 common logic gates and how they work
AND (both input needs to be 1 for output to be 1)
OR (output will be 1 if one or both inputs is 1)
NOT (opposite of the input)
NAND (output is 1 when input = either 1/0 0/1 0/0)
NOR (output will be 1 when input = 0/0)
Cathode rays
Stream of electrons emitted from heated metal (cathode)
Process is called thermionic emission
CRO
Horizontal or y-plates for vertical movement of electron beam
Timebase for x-plates for horizontal movement
Describe an experiment to measure magnetic fields.
Use iron filings and a sheet of plastic.
Place the magnet under the plastic.
Sprinkle iron filings over.
Tap gently to evenly distribute.
The filings will show the field lines.
Which direction you do draw the arrows on magnetic field diagrams?
from North to South
Describe another experiment to measure magnetic fields.
Place compass at end of magnet.Mark position of the end of compass. Move end of compass needle to new mark and repeat
What does it mean if the field lines are closer together?
The magnetic field is stronger where lines are closer together
How do you create a uniform magnetic field?
Use two permanent bar magnets. Place the south of one magnet opposite the north of the other magnet Place them at the correct distance apart to get evenly spaced lines.
A straight wire carrying an electric current has a circular ___________________ around it.
magnetic field
How does the magnetic field around a straight wire look?
The field around a straight wire forms concentric circles
What happens to the magnetic field aroud the wire if we reverse the current on a wire
It changes direction/ reverses
How does the field around a single flat coil or loop of wire look?
The field produced round a single flat coil or loop of wire looks like:
How does the magnetic field around a solenoid look?
.
If we place a conductor (a wire or metal) inside a magnetic field then something amazing happens – what?
The conductor (wire) moves
Explain what the motor effect is
If a current carrying wire is placed in a magnetic field it experiences a force and moves.
For a current carrying wire in a magnetic field to experience the maximum force, it has to be————to the magnetic field.
At right angles/ perpendicular
Where does your thumb point in Flemings left hand rule?
Thrust, movement, force.The direction the wire will move if placed in a magneti field.
What does your first finger point to in Flemings left hand rule.
Field - magnetic field. From North to South.
Where does your second finger point in Flemings left hand rule?
Current
What happens to the force on the motor effect if the wire is made into a coil?
A turning force is produced if the wire is made into a coil. This effect is used in simple DC (direct current) electric motors.
Name 3 things which use an electric motor?
fans
electric drills
food processors
washing machines
CD and DVD players
How does a simple dc motor work?
When a current carrying coil is placed in a magnetic field it will rotate the current flowing through the coil creates a magnetic field the field around the current interacts with the field produced by the magnet the forces combine to make the coil rotate
How can you increase the speed of a motor?
increase the size of the magnetic field increase the size of the current increase the number of turns on the coil
What is a split-ring commutator?
Is used in an electric motor to change the direction of current every half turn to keep the coil rotating.
What is electromagnetic induction?
When a magnet is moved into a conductor (such as a coil of wire) a current will be induced.
How can you increase the size of the voltage induced by electromagnetic induction (such as in a generator)?
increase the strength of the magnetic field increase the speed of rotation of the magnet increase the number of coils add iron core inside coil
What is meant by the term “magnetic material”?
A material that is attracted by a magnet. Can become magnetised when placed in a magnetic field.
What is a magnetically hard material?
A material that stays magnetic after it has been magnetised.
What is a name for a magnetically hard material that has been magnetised?
A permanent magnet
What is a magnetically soft material?
A material that does not stay magnetic once it has been magnetized.
What is a use for a soft magnet?
Used in electromagnets - as they can be switched on and off
Describe a method of inducing a magnet in a bar of iron
take a bar of iron (of the hard magnetic type) and stroke it repeatedly in the same direction with a magnet.This will induce a magnetic field in the magnet.
Describe another method of inducing a magnet in a bar of iron
place the iron bar inside a coil of wire carrying a current.Again this will induce a field in the bar.
The head of an iron nail is placed close to the north pole of a bar magnet. The head of the nail is attracted toward the bar magnet until they tuch an it sticks to the magnet. Explain what causes this to happen.
The magnet induces magnetism in the nail and it becomes magnetised.
Is iron a magnetic material
yes
Is cobalt a magnetic material
yes
Is nickel a magentic material
yes
Is copper a magnetic material
no
The formula for calculating electrical power is:
Power = Current x VoltageorP = IV Where I – current in amps (A) V – voltage in volts (V)
A bulb operates at 230V. The current is 0.08A. Calculate the power of the bulb.
P = IVP = 230 x 0.08= 18.4 W (this would probably be an energy-saving bulb)
A hairdryer has a rating of 2kW. What current does it use?
P = IVI = P/VI = 2000/230I = 8.7A
A power supply produces a current of 0.5A and has a power output of 2.5W. Calculate its voltage output.
P = IVV = P/IV = 2.5/0.5V = 5V
A pair of curling tongs is rated at 1kW. Calculate the current it uses.
P = IVI = P/VI = 1000/230I = 4.35A
A current of 0.5A flows through a power supply producing 12V. Calculate the output power.
P = IVP = 12 x 0.5P = 6 W
Calculate the power of an electric can opener that uses 0.2 A.
P = IVP = 230 x 0.2P = 46 W
Two equations for calcultaing Energy
E = IVtWhereI – current in amps (A)V – voltage in volts (V)t – time of flow in seconds (s)E – energy used in joules (J)Notice that: P = IVSo we can replace the It in the equation above with: E = Pt
Names 5 Electrical Hazards
Frayed Cables
Long cables
Damaged Plugs
Water
Metal Objects
What wire is connected to the metal casings of appliances?
Explain Why
The Earth wire
What size of fuse would you use for an appliance rated at 2.5 kW?
13 A
What size of fuse would you use for an appliance rated at 500W?
3 A
What does this symbol mean?
Double Insulated
When electricity flows in one direction only then the circuit is using
DC
Direct Current
When a circuit has a current flow that is constantly changing direction then the circuit is using
AC
alternating Current
What kind of circuit is this?
Series
What kind of circuit it this?
Parallel
LDR
What is this?
What is it for?
Light-dependent Resistor
LDR
LDRs, change their resistance according to the amount of light that falls on them. As a result they can be used to monitor light levels. Say you want a light to come on automatically when it gets dark.
Thermistor
What is this?
What is it for?
Thermistor
Thermistors change their resistance according to the temperature around them.
These devices can be used to respond to temperatures, for instance, if you want a cooling fan to come on when a device gets too hot.
An LED
An LED
Light-emitting Diode
These are devices that produce light when a current is passed through them. Think of your computer. You might have an LED for the power being on.
What is the relationship between resistance and current?
As we add more components into a circuit the resistance of the circuit increases. This has the effect of reducing the current that flows.
Essentially:
more resistance →→→→→→→→→→→ less current
What is the name of the device that can measure current in a circuit?
An Ameter.
What is the relationship between current and voltage?
As voltage increases the current increases.
What is the formula for current?
Current = charge transferred/time taken or I = Q/t Where Q – charge measure in coulombs (C) I – current measured in amps (A) t – time measured in seconds (s)
A charge of 60 coulombs is transferred along a conductor in two and a half minutes. Calculate the current.
I = Q/t
I = 60/150 (we have changed the minutes into seconds)
I = 0.4A
If a current of 0.2A flows for an hour, how much charge will be transferred?
I = Q/t
Q = It
Q = 0.2 x 60 x 60 (Note that one hour is 60 minutes of 60 seconds each: 60 x 60 = 3600 s)
Q = 0.2 x 3600
= 720 C
A charge of 200 coulombs is transferred along a conductor in half an hour. Calculate the current.
I = Q/t
I = 200/30 x 60
I = 200/1800
I = 0.111A
If a current of 4A flows for six hours, how much charge will be transferred?
I = Q/t
Q = It
Q = 4 x 6 x 60 x 60
Q = 86400 C
What is Ohms Law?
We can use a formula known as Ohm’s law to relate current voltage and resistance in circuits.
R = V/I
This is usually written as:
V = IR
V – voltage in volts (V)
I – current in amps (A)
Calculate the resistance of a circuit where the voltage is 12V and the current is 0.2A.
R = V/I
R = 12/0.2
R = 60 Ω
An appliance has a resistance of 3000 Ω. Calculate the current that flows through at normal domestic voltage.
R = V/I
I = V/R
I = 230/3000
I = 0.077A
An appliance has a resistance of 200 Ω and a current of 0.5 A. Calculate the voltage of the appliance.
V = IR
V = 0.5 x 200
= 100 V
A bulb has a resistance of 650 Ω and is operated at a voltage of 40 V. Calculate the current in this bulb.
R = V/I
I = V/R
I = 40/650
I = 0.062A
A circuit of resisatnce 26.1 ohms has a voltage across it of 36 volts.
Calculate the current through the circuit.
Give your answer to 3 decimal places.
Include the correct unit in your answer.
1.4 A
One volt is defined as being one ________ per _______
joule/coulomb
Current flow in metals is a flow of _____ which have a _______ charge.
Current flow in metals is a flow of electrons which have a negative charge.
current is a flow of ______________
current is a flow of charge
When a LDR is illuminated its resistance _______.
A circuit component designed to change its resistance as its temperature changes is know as a _______.
One component designed to produce light when current passes through it is a _____ emitting _____. This is usually abbreviated to the letters ___________.
When a LDR is illuminated its resistance decreases.
A circuit component designed to change its resistance as its temperature changes is know as a thermistor.
One component designed to produce light when current passes through it is a light emitting diode. This is usually abbreviated to the letters LED.
In a circuit 6262 coulombs of charge are transferred in 7 minutes.
Calculate the size of the current.
Include the correct unit in your answer.
Give the answer to one decimal place.
14.9 A
A circuit has a resistance of 487 ohms and a current flowing through it of 7.4 amps.
Calculate the voltage across the circuit.
Include the unit in your answer - to one decimal place.
3603.8V
A circuit has a current of 5.0 amps flowing through it when the applied voltage is 21 volts.
Calculate the resistance of the circuit - to one decimal place.
4.2
A current of 5.4 amps flows for 102 seconds.
Calculate the charge transferred in this time.
Include the correct unit in your answer.
Give the answer to one decimal place.
550.8C
How long will it take for a 7.7 amp current to transfer 2477 coulombs of charge?
321.7s
What is voltage?
V = JC-1 volt = joule per coulomb
How can you create a static charge?
Rub two insulators together
A rod is rubbed by a cloth and gains electrons. Is it positively or negatively charged?
Negatively charged
A different rod is rubbed by a cloth and becomes positively charged. Has it lost or gained electrons?
It has lost electrons
Give one hazard caused by electrostatic charges and state how the risk from this hazard can be reduced.
Risk of a spark causing an explosion (e.g. lorries containing fuel) - reduce risk by ensuring that lorry is earthed
Risk of a getting a shock - use insultation, such as wear insulating shoes or use an insulating mat.
State one example where static is a nuisance and suggest a solution.
Dust attracted to computer or tv screen - use anti-static spray or cloth.
Materials clinging to skin - use anti-static cleaner
Can you charge a metal rod by rubbing it?
No, because the electrons in metal can flow easily so if electrons were transferred onto the metal rod they would flow away rather than staying on surface of object and making the metal charged.
Suggest two useful uses for static electricity in everyday life
2 from:
Spray Painting
Smoke Precipitators
Defibrillators
There are two types of charge.
Describe how you could demonstrate this using different insulating rods and a cloth.
Method: Suspend the two rods. Rub both rods with a cloth.If they carry the same charge they will be repelled .If they carry different charge they will be attracted.
How could you show that a plastic rod gains charge?
Try to pick up small peices of paper with it
Use it to deflect a stream of water.
Try to deflect gold leaf
Describe what happens when a plastic rod is rubbed with a cloth.
before rubbing both are neutral
electrons move from the cloth to the rod
plastic rod gains electrons becomes negatively charged
cloth looses electrons becomes positively charged
Can rods made of different insulating materials gain a different charge?
Yes, some materials will become positively charged, some will become negatively charged.
What happens to something that is negatively charged?
It has gained electrons
Silver
Conductor (best)
Carbon
Conductor (moderate)
quartz
Insulator
silver
conductor
water
conductor (poor)
concrete
conductor
brick
insulator
aluminium
conductor
silicon
semi-conductor
conductor (moderate)
plastic
insulator
copper
conductor
iron
conductor
teflon
insulator (best)
graphite
conductor
oil
insulator
glass
insulator
humans
conductor (poor)
rubber
insulator
china
insulator
air
insulator
polythene
insulator
PVC
insulator
perspex
insulator
gold
conductors
steel
conductor
ceramic
insulator
brass
conductor
mercury
conductor
bronze
conductor
dry cotton
insulator
asphalt
insulator
dry paper
insulator
dry wood
insulator
(but living tree conductor as contains water)
diamond
insulator
2 advantages of a series circuit
- Fewer wires 2. Lower resistance
2 disadvantages of a series circuit
- One element fails - the circuit fails 2. No independent control
2 advantages of a parallel circuit
- a failure of one component does not lead to the failure of the other components
- more components may be added in parallel without the need for more voltage
2 disadvantages of a parallel circuit
- power remains at the same voltage as the voltage of a single power source
- Lower resistance
- split of an energy source across the entire circuit
What is this symbol?
open switch
What is this symbol?
LDR
Light-dependent resistor
What is this symbol?
A cell
What is this symbol?
A battery
What is this symbol?
A lamp or bulb
What is this symbol?
A fixed resistor
What is this symbol?
An ammeter
What is this symbol?
Voltmeter
What is this symbol?
A variable resistor
What is this symbol?
A fuse
(melts or blows when the current is too big)
What is this symbol?
A diode
What is this symbol?
LED
Light Emitting Diode
What is this symbol?
thermistor
What is this symbol?
variable resistor
What is this symbol?
voltmeter
What does a magnet have around it?
a magnetic field
How can a magnetic field be created around a wire?
Pass a current through it
What is a magnetically hard material?
Steel - retains its magnetism once magnetised
What is a magnetically soft material?
Iron - loses its magnetism easily and is therefore a useful temporary magnet
How can magnetism be induced in a magnetic material?
leave it in a magnetic field
What is a magnetic field?
The volume of space around a magnet where magnetism can be detected
How can we show the shape of a magnetic field?
Using iron fillings or compasses which will align with the direction of the magnetic field
What happens when two magnets are placed near each other?
Their magnetic fields affect each other
What is electromagnetism?
A phenomenon where a magnetic field is created around a wire that has current passing through it
What is the shape of the field around a current carrying wire?
Circular
How can you increase the strength of a magnetic field around a current carrying wire?
increase the current in the wire
wrap the wire into a coil or a solenoid (a long coil)
What happens when the direction of current flowing through a solenoid is reversed?
Its poles are also reversed
How can you increase the strength of a field around a solenoid?
increase the current flowing through
increase the number of turns of the solenoid
wrap the solenoid around a magnetically soft core such as iron
What is an electromagnet?
the combination of soft iron core and solenoid
How does an electric bell work?
Bell push pressed; circuit complete and current flows
Soft core of electromagnet is magnetised and attracts the iron armature
Armature moves so hammer strikes bell
Gap created at contact screw
Circuit incomplete and current stops flowing
Electromagnet turned off so spring’s armature returns to original position
How does a circuit breaker work?
If current is too high the electromagnet is strong enough to pull the iron catch out of position so that the contacts open
Circuit breaks
But can be reset with reset button
What is the purpose of a relay switch?
In circuits with a large current there is a danger of receiving a large electric shock.
A relay switch uses a small current in one circuit to turn on a second that may have a much large current
How does a relay switch work?
Switch of small circuit closed so current flows
As current passes through coil C the iron core is magnetised and attracts the iron armature
The armature is pivoted so its lower end pushes the contacts of the second circuit together
Now the second circuit is complete and current flows.
How does a relay switch turn off the second circuit?
If switch S is opened the electromagnet is turned off
Iron armature moves back to its original position and contacts spring apart
Circuit off b/c current can no longer flow
What happens to a charged particle moving in a magnetic field?
It experiences a force
When will a charged particle moving in a magnetic field not experience a force?
If its motion is not parallel to the field
How to make a wire move?
Pass a current through a wire held at right angles to the mag.field of a magnet.
Why does a current carrying wire in a field experience a force?
The two magnetic field lines interact around the wire and the magnet.
In certain places, the fields are in the same direction and so reinforce each other giving a strong magnetic field.
Where the fields are in opposite directions weaker fields are produced.
The wire experiences a force pushing it from the strong to the weak field. MOTOR EFFECT
How can a stronger force be produced to move the wire?
stronger mag.field
increased current
What happens if the current or direction of the mag.field is reversed?
The wire experiences a force in the opposite direction
How does a moving-coil loudspeaker use the MOTOR EFFECT?
Signals from amplifier fed into coil as a.c
The coil now has a changing magnetic field that interact with those of the magnet creating varying forces on the wires of the coil.
This makes the speaker cone vibrate
The vibrations create the sounds waves we hear
How does a split ring help a motor work?
Lets the connections to the supply through the brushed swap over so current continues to flow in the same direction and the motor turns consistently
What is the effect of the arrangement of the brushes and the split ring?
Changes the direction of the current every half turn which means that the rotation can be continuous
How can you increase the rate at which the motor turns?
increase the number of turn of wire COIL
increase the strength of mag.field
increase the current flowing
How are practical motors different from lab ones?
1 - CURVING
Curved electromagnets which are capable of producing strong mag.fields
How are practical motors different from lab ones?
2 - COILS
Single loop is replaced with several coils of wire wrapped on the same axis; makes the motor powerful
allows it to run more smoothly
How are practical motors different from lab ones?
3 - LAMINATED
Coils are wrapped on a laminated soft iron core to make motor more efficient and powerful
What do motors do vs generators?
Motors use electricity to produce movement….generators movement to produce electricity
How is the mains supply produced?
By large generators in power statioms
What is the generator effect?
When a wire is moved across a magnetic field at right angles, a voltage is induced in the wire.
How can current flow from a generator?
If the wire is part of a complete circuit
How can you increase the size of the induce voltage?
move the wire more quickly
use a stronger magnet
wrap the wire into a coil (move wires move in the mag.field)
What is the other version of the generator effect?
a voltage can be generated by pushing a magnet into a coil
How can you increase the the size of the induced voltage using the other version of the generator effect?
move the magnet more quickly
use a stronger magnet
using a coil with more turns
using a coil with a large cross sectional area
What does the size of induced voltage depend on?
the rate at which the magnetic lines of flux are being cut
What is directly prop. relationship in electromagnetic induction?
The size induced voltage is dir.prop to the rate at which the flux lines are cut
What happens to the direction of induced current if motion direction is reversed?
it too, reverses
How does a dynamo work?
As the cyclist pedals the wheel rotates and a magnet in the dynamo spins around.
As it turns its field lines cut through the surrounding coil inducing a current in it
This current is used to work the cyclist’s lights
Why is the induced voltage from a generator greater than that of a dynamo?
Uses:
STRONGER MAGNETS
MORE COILS OF WIRE
SPINS COIL FASTER
With generators, the wire alternates consistently due to slip rings. What does this mean?
ALTERNATING CURRENT IS PRODUCED
What is generator than produces alternating current called?
ALTERNATOR
What is the frequency of mains supply?
50 Hz
How does a transformer work?
AC current passed through coil
Its magnetic field is changing; as current increases field grows and visa versa
If a second coil placed near first, changing mag.field of first coil interacts with the second’s coil inducing and a.c. current
Size and direction of current induced changes depending on current provided to first coil
What is the name for the combination of two magnetically linked coils?
TRANSFORMER
Equation for V and turns on coil?
input V/output V = turns on primary coil/turns on secondary coil
What is true if a transformer is 100% efficient?
p1 = p2
v1 x I1 = v2 x I2
What happens in the national grid as current is transported?
energy is lost in the form of heat
Why is energy lost?
The wires on pylons are long and have a high resistance
How are heat loses reduced?
By transporting at low I and high V
What happens to current after it is generated in alternators?
Step-up transformer to increase V and decrease I
Why is current transported high off ground?
High voltages in the cables is v.dangerous
What happens to current when it is going into homes?
STEP DOWN TRANSFORMER - increase current
How does current get to our homes?
via underground cables
Why is iron used in transformers?
it can magnetise and demagnetise 50 times/second because it is magnetically soft
At what angle does a current-carrying wire need to cut a magnetic field of a magnet to experience max.F?
90 degrees
What direction do we use for the motor effect for motion of electrons?
conventional current (+ to -)
How to we say the current is flowing in a motor?
from positive to negative
What is electromagnetic induction?
creation of a voltage (maybe current) in a wire experiencing a change in mag.field
What is the dynamo effect?
Using electromagnetic induction to transfer kinetic energy to electrical energy
How can you test if a current has been induced in the conductor?
Attach an ammeter to it; when moving it through the magnetic field the ammeter will shows the magnitude and direction of the induced current
In a generator what do faster revolutions generate?
Higher overall voltage and more peaks (higher frequency)
When will current be generated from a generator?
If the conductor is connected to a circuit
Why do step-down transformers reduce the voltage?
So that it is more useful and safer to use
At what V is electricity transported?
400 000V
At what voltage is electricity received at factories?
33000 V
At what voltage does electricity run at before entering a step up transformer?
25000 V
At what voltage is electricity supplied to our homes?
230V
What are the advantages of circuit breakers?
respond more rapidly to current surges than fuses do
more reliable
more sensitive
can be reset (no need to replace)
How can a circuit breaker be made more sensitive and be made to respond quicker to current surges?
wrap more coils around the iron core to create a stronger electromagnet that can attract the iron armature quicker and with more force
How does a loudspeaker work in terms of a moving coil?
Wire from amplifier carries ac which makes the coil move backwards and forwards at the same frequency as the changing current.
The coils mag.field and that of the magnet interact producing a force which is alternating due to the a.c current and thus changing mag.field of the amplifier wire.
The paper cone is then forced to move which creates sound.
Why is current reduced when transported electricity along pylons?
P = VI
The heating effect of a high current would cause too much heat loss so current is reduced and if P is constant, then V must increase
How is an electromagnet created?
Wrap wire around an iron core. When current is passed through the wire, a mag.field is created.
This mag.field means that the wire magnetizes the iron core strongly.
How will a compass be positioned around a current carrying wire?
Compass always points perpendicular to the wire (flow of current)
Advantages of an electromagnet?
Can be made stronger
Field strength can be varied
Can be turned on/off
What would happen to the motor if the split ring didn’t have a split?
The motor wouldn’t spin as the current would short circuit across the ring and back to the power supply.
What is the purpose of brush contacts in a motor?
Made of carbon; soft and good electricity conductor
Reduced friction between it and split ring commutator.
In an experiment checking the strength of an electromagnet how can you increase its strength?
use a larger battery with a greater voltage
add another battery in series
thicker iron nail
more coils of wire
How can you measure the strength of an electromagnet?
how much compass deviates
mass of iron filings picked up
no of paperclips it can pick up
What might have caused a systematic error that made the strength of the electromagnet to start above 0?
Experiment already carried out before and the iron still slightly magnetized
She did not zero the balance when measuring the mass of iron filings picked up
What would happen if a motor didn’t have a split ring?
It would rotate to vertical position and vibrate until it came to rest
What does the split ring do?
Changes the direction of current flowing through the coil after every half turn by swapping the contacts at the DC supply
What happens when a magnet moving in a conductor stops moving?
There is no induced voltage and no current flows b/c the flux lines are no longer being cut
Why is a N pole created at the end of a coil when the N pole of a magnet is brought near?
It generated a mag.field to try and oppose that that is induced by the current in order to oppose the motion of the bar magnet’s mag.field
What is the purpose of a transformer?
Transformers are devices that convert electricity from one voltage to another through electromagnetic induction.
What does a transformer consist of?
two solenoids (coils) and a soft magnetic material (iron core) and an ac (alternating current supply)
Explain the use of step up and step down transformers in the transmission of electricity.
Electricity from a power station passes through a step up transformer. This increases the voltage but decreases the current. This saves energy losses in the cables due to heating of the metal cables. The transmitted electrcity passes through a step down transformer which decreases the voltage but increases the current before it reaches the industrial or domestic users.
What is the transformer equation?
Vp / Vs = (No. of turns) Np / Ns
A transformer has 4000 turns on the primary coil and 1000 on the secondary. The input voltage is 5V. Calculate the output voltage.
1.25V
A transformer needs to output a voltage of 24v for an input voltage of 240v. The primary coil has 600 turns. How many does the secondary coil have?
60
A transformer has 15000 turns on the primary coil and 5000 on the secondary. The input voltage is 20v. Calculate the output voltage.
6.67V
A transformer needs to output a voltage of 40v for an input voltage of 800v. The primary coil has 4000 turns. How many does the secondary coil have?
200
A transformer has 1000 turns on the primary coil and 200 turns on the secondary coil. If a voltage of 250V is applied to the primary coil, what is the voltage across the secondary coil?
50V
What is the transformer efficiency equation?
input power = output power or VP IP = Vs Is
A transformer has an input current of 0.4A at a voltage of 240v. The output voltage is 12v. Calculate the output current.
8A
A transformer has an output current of 0.1A at a voltage of 6000v. The input current is 6A. Calculate the input voltage.
100V
A transformer has an input current of 0.02A at a voltage of 230v. The output voltage is 10v. Calculate the output current.
0.46A
A transformer has an output current of 0.003A at a voltage of 25000v. The input current is 15A. Calculate the input voltage.
5V
What is produced when a magnet moves in a coil of wire?
A voltage
What do transformers change?
Alternating voltage
What is the name given to a transformer that increases voltage
a step up transformer
What is the name given to a transformer that decreases voltage
step down transformer
What is a conductor?
Has delocalised electrons that flow freely.
What is an insulator?
Has localised electrons that are held tightly in place.
Name two examples of conductors:
gold, copper
Name two examples of insulators:
rubber, wood, plastic
What is charging by friction?
Applying a frictional force between two insulators, transferring electrons, giving a ‘permanent’ charge.
What is charging by induction?
Bringing another charged object close to an insulator to create a ‘temporary charge’.
Why do the electrons transfer when charging by friction?
Because they are lightest and have the shortest distance to travel.
Explain how the insulators have become charged (3).
The insulators have become charged by friction
The electrons have been transferred
From rod to cloth
What does charging by induction NOT involve?
Transfer of electrons
Describe what happens if a negatively charged balloon is held up to a wall:
As the balloon moves closer to the wall, the electrons inside the wall are repelled creative a positive surface closest to the balloon and a force of attraction.
Describe what happens when a positively charged balloon is moved towards the wall:
the electron in the wall would move towards the the balloon as they are attracted and move to the closest surface. This means that the surface of the wall becomes negatively charged and attracts the balloon.
How does a Van de Graaff generator build up charge?
The generator rubs plastic and rubber together to produce a charge on the belt which is then transferred via a cone onto the conductor dome to produce a charged dome.
How does spray painting of cars using static electricity work?
The vehicle is grounded and given a positive charge whilst the paint is given a negative charge. They attract each other, creating an even coat.
What are the benefits of using static electricity to spray paint cars?
Paint droplets repel each other and are attracted to car creating even coat.
East to pain odd objects.
Less waste of paint.
What is the function of a dust precipitator?
To remove pollutants from the air.
How do dust precipitators work?
smoke travels up precipitator and travels past strongly charged grid that causes particles to have a negative charge
Further up the precipitator, positively charged plates attract the charged pollutants and collect them in collecting plates.
What are the dangers of electrostatics and planes?
It can cause sparks which can then cause fires and explosions.
How could charge build up on a plane?
Friction from air resistance causes them to become statically charged
Why is an earthing wire important when re-fuelling?
A copper wire can provide a low resistance path for the charge to travel to the ground and leave the plane.
What is current?
The rate of flow of charge
What is the equation linking charge, current and time?
current (I) = charge (Q) / time (t)
What is voltage/potential difference?
The energy transferred per unit of charge.
What is the equation linking energy, voltage and charge??
voltage (V) = energy transferred (E) / charge (Q)
How do we measure current?
Ammeter (in series)
How do we measure voltage?
Voltmeter (in parallel)
What does Ohm’s law state?
That the voltage is proportional to the current for a component with a fixed resistance.
What is the equation linking current, voltage and resistance?
V=IR
How does length affect resistance?
The longer the wire, the greater the resistance, because there is a greater number of collisions involving the delocalised electrons
How does the thickness of a wire affect the resistance?
The thicker the wire the lower the resistance, therefore increasing the number of pathways for the delocalised electrons.
How would temperature affect the resistance?
The higher the temp, the higher the resistance as ions can vibrate more, increasing NO. collision.
what is charging by friction?
applying a frictional force between two insulators giving a ‘permanent’ charge
What is charging by induction?
bringing another charged object close to an insulator to create a ‘temporary’ charge
What is the current?
the rate of flow of charge
What is the voltage?
the energy transferred per unit of charge
How does temperature affect resistance?
The higher the temperature the higher the resistance as ions can vibrate more, increasing collisions involving ions
How does length of wire affect resistance?
The longer the wire the higher the resistance as there are a greater amount of collisions
How does cross sectional area of wire affect the resistance?
The thicker the wire the lower the resistance as there are more pathways
What does a diode do?
It only allows current to flow in one direction.
What does an LED do?
Emits light when current flows in right direction
What does a thermistor do as temperature increases?
the resistance decreases
What does an LDR do as the light intensity increases?
The resistance decreases
What happens to the resistance as the voltage increases?
The resistance also increases, as the ions in the filament also gain more energy, meaning they vibrate more increasing the number of collisions.
In series: current…
is the same everywhere in the circuit
In series: potential difference…
is shared between components
In series: total resistance…
is the sum of resistance of components
In parallel: current…
is the sum of the current in each loop
In parallel: potential difference…
is the same in each loop
What is the conventional current?
from positive to negative
what is the actual electron flow?
from negative to positive
What colour is the earth wire?
green and yellow
What colour is the live wire?
brown
What colour is the neutral wire?
blue
What is the function of the earth wire?
directs current into ground in case of fault and current flows through casing
What is current measured in?
Amperes (Amps)
What is charge measured in?
Coulombs
What is energy measured in?
Joules
What is resistance measured in?
Ohms
What is potential difference measured in?
Volts
What is power measured in?
Watts
What does current in a resistor cause?
Transfer of electrical energy to heat energy and an increase in temperature (e.g. in a kettle’s element)
Power =
P=IV
Energy =
E=IVt
Is mains electricity AC or DC?
Alternating Current
What does current depend on in a series circuit?
Voltage of cell
Does current vary in a series circuit after passing through components?
NO
What is the relationship between current, resistance and voltage?
V=IR
What is the relationship between current, resistance and power?
P=I^2R
What is the proportionality between current and resistance?
They are inversely proportional to one another
Voltage =
V=E/Q=P/I=IR
What is current?
The rate of flow of charge
Charge =
Q=It
What do magnetic field lines represent?
Direction and intensity of a magnetic field
What does electric current in a conductor induce?
A magnetic field in the anticlockwise direction around the wire
What happens when a wire carries a current in a magnetic field?
A force is exerted
What increases the force on a wire carrying a current in a magnetic field?
strength of magnetic field; current
what is induced in a solenoid when it moves through a magnetic field?
a voltage
How can you reduce heat loss by convection?
Use a vacuum; convection needs a fluid to work in.
Use trapped gas or liquids which restrict motion of convection currents.
How can you reduce heat loss by radiation?
Use shiny surfaces which will reflect the IR rays away. Shiny surfaces are also poor radiators of heat; black-bodies absorb and radiate the most heat.
Describe how a simple electric motor works
A single coil of wire is in a magnetic field. A current passes through the wire which makes it move to and fro between 180°. A split ring commutator ensures that the current continues to change direction (otherwise the coil wouldn’t spin)
How can you make a DC motor spin faster?
Increase number of turns on the coil; Increase number of cells in battery; increase the strength of the magnetic field.
Describe Fleming’s left hand rule
If you make a left-hand gesture where the thumb, index finger and middle finger are all orthogonal to one another (perpendicular), if the index finger is the direction of the magnetic field (North to South), and the middle finger is the direction of the current, the thumb will be pointing in the direction of the force exerted on the wire.
Describe the right hand grip rule
If you make a fist with the thumb pointing upwards with your right hand, if the thumb is the direction of the current, the fingers will be pointing in the direction of the magnetic field produced surrounding the wire.
How does a generator work?
A conducting coil moves in a changing magnetic field induces a voltage; since the coil in which the voltage is being induced will be spinning, the electrical connections are made by the carbon brushes which slide over the commutator rings.
State the transformer equation
Vp/Vs = Np/Ns = Is/Ip
What does a step down transformer do?
It decreases the voltage and increases the current.
What does a step up transformer do?
It increases the voltage and decreases the current.
Why might step up transformers be used when transporting electricity across the country in the national grid?
Because using a step up transformer, the current will decrease, and power lost = I^2R, so the current needs to be as low as possible to minimise power lost.
Describe how a photocopier works.
A statically charged drum is exposed to light, reflected from the document to be copied, which discharges the drum everywhere except where the the dark print does not reflect the light. The charged parts of the drum attract the toner which is then bonded (by heat) to the paper.
Describe how static electricity can be used in an inkjet printer.
The ink droplets are charged and are allowed to be directed to particular places on the paper by deflecting them between charged plates.
Describe how static electricity can be used in paint spraying.
The droplets of paint are given a static charge and the object to be painted is given the opposite charge. Therefore, the paint droplets will be attracted to the surface of the object and they will be evenly spread out from each other - this minimises paint used.
What is voltage?
Energy transferred per unit charge passed
Volts = energy transferred ÷ charge
What is current?
Rate of flow of charge
Amps = charge ÷ time
What is resistance?
Ohms = voltage ÷ current
More resistance = less current.
What is power?
Watts = energy ÷ time
Watts = voltage x current
What does the current voltage graph of a resistor look like?
straight line through origin ∴ V directly proportional to I
constant gradient ∴ constant resistance
constant resistance because constant temperature
What is voltage?
Energy transferred per unit charge passed
Volts = energy transferred ÷ charge
What is current?
Rate of flow of charge
Amps = charge ÷ time
What is resistance?
Ohms = voltage ÷ current
More resistance = less current.
What is power?
Watts = energy ÷ time
Watts = voltage x current
What does the current voltage graph of a resistor look like?
straight line through origin ∴ V directly proportional to I
constant gradient ∴ constant resistance
constant resistance because constant temperature
What does the current voltage graph of a bulb look like?
s shape, with rotational symmetry
at bottom gradient steep ∴ low resistance. This is due to low temperature, so the bulb will also be dimmer
at top gradient shallow ∴ high resistance. This is due to high temperature, so bulb will be bright
What does the current voltage graph of a diode look like?
high resistance in one direction so current can only go in other direction
only turns on at 0.6V
What is a thermistor?
A resistors which varies with temperature. Cold = high res. Warm = low res.
What is an LDR?
A resistor whose value depends on light intensity. Dark = high res. Light = low res.
What order should you do circuit calculations in?
Resistance
Current
Voltage
Power
What is the electron flow?
Negative to positive
What is the current direction?
Positive to negative
What is the difference between dc and ac?
Dc, e.g. Cell, only goes one way can’t change direction
Ac, e.g. Mains, goes both ways ∴ can change direction, above and below axis
How can resistors be used in daily life?
they slow down current ∴ the KE of the electrons turned into thermal energy
can be used in appliance, e.g. Hair dryers
How can you investigate how current varies w voltage in wires, resistors, lamps and diodes?
use an ammeter and measure current w and wo these elements.
measure different voltage levels using a voltmeter w these elements
What is current like in series and parallel circuits?
Series: current is the same
Parallel: current divided by num of components
How do you read an oscilloscope?
x axis is time base- time per cm. Can find freq of wave: 1/time
y axis is y-gain. This is volts per cm. We can work out the amplitude of the wave, e.g. If y-gain is 2 V/cm, and it is 3 cm high, then V = 6
What does the earth wire do?
Longest pin ∴ goes in first and open other holes
If live wire becomes loose and touches the case, case becomes live. If you touch case, instead of current going through you, it takes path of least resistance through case, earth wire and to the ground ∴ creates a large current, which blows the fuse
What is double insulation?
No external bits of metal in appliance ∴ case can’t become live ∴ don’t need earth wire
What do the flex and cable grip do?
Flex is plastic coating on wire going in, as its an insulator
Cable grip: holds in flex and keeps it down
What does the fuse do?
- if current going through it is too large it melts causing gap in circuit, thus preventing fire
Comes in 3A, 5A, and 13A. Use smallest possible, but only acc to 10%
What does a circuit breaker do? What are its advantages over fuses?
spring loaded switch held closed by soft iron bolt. An electromagnet is turned on if I too large and pulls bolt away ∴ switch opens and circuit breaks.
more reliable (fuse only acc to 20%)
more sensitive
switch can be reset unlike fuse which breaks and so must be replaced
respond mor rapidly to current surges than fuses
What are the pros and cons of parallel circuits?
Pros: * can disconnect or connect new elements without disrupting other elements, e.g. Can turn off a light without all the lights in your house going off
* if one part breaks, it wont all. Current can flow through other parts
Cons: * can’t increase or decrease the voltage
* requires lots of wires
What are the pros and cons of series circuits?
Pros: * more devices, the higher the voltage
* don’t overheat easily ∴ better around flammable things like clothes
Cons: * if one part breaks, it all breaks
* greater the num components, higher the resistance
What is the unit for current?
ampere / amps or A
Define current
The rate of flow of charge OR the number of coulombs passing a point in a circuit per unit time (It is not the speed of the electrons!)
Describe what it means when 1 A is passing through a circuit
1 A = 1 Coulomb per second
What is the unit for energy?
joules
How is energy related to current, voltage and time?
E = IVt
What is voltage measured in?
Volts or V
Define voltage
It is the energy per unit charge either supplied by a cell, or transferred to a component.
How does voltage related to current and resistance?
V=IR
What is a volt?
1 V = 1 J/C 1 volt = 1 joule per coulomb
What is resistance?
a measure of how much voltage is needed per amp of current. It is a measure of the is a measure of the tendency of a material to resist the flow of an electrical current.
How is power related to current and voltage?
P=IV
What is power measure in?
Watts (W)
What is a watt?
1 W = 1 J/s I Watt = 1 joule per second
How is voltage related to energy and charge?
V=E/Q
Voltage = Energy / Coulomb
Voltage lost across a component is a measure of the energy lost across the component per coulomb
What is the voltage across a cell or power supply a measure of?
The energy transferred from chemical store to electrical in the wires per coulomb
What type of circuit is this? Series or parallel?
Series circuit
Only one path or branch in the circuit
What component symbols are shown below?
battery/two cells
&
solar cell
What component symbol is shown below?
variable resistor
What component symbol is shown below?
thermistor
What component symbol is shown below?
ammeter
What component symbol is shown below?
diode
What component symbol is shown below?
buzzer
What component symbol is shown below?
motor
What component symbol is shown below?
voltmeter
What component symbol is shown below?
bulb
What component symbol is shown below?
Fuse
What component symbol is shown below?
Light emitting diode
What component symbol is shown below?
Light dependent resistor
How does voltage across bulb in a series and parallel circuit differ?
Current is conserved in all circuits
Series - current is the same everywhere in the circuit
Parallel - current splits between the branches- current into a junction equals current out of a junction
How does current differ between series and parallel circuits?
Current is conserved in all circuits
Series - current is the same everywhere in the circuit
Parallel - current splits between the branches- current into a junction equals current out of a junction
Each bulb provides 3 V. In which circuit (series or parallel are the bulbs brighter?
Series- voltage is shared - VCell = V1 + V2
Parallel - each bulb gets the full voltage of the cell - Vcell = V1 = V2
Bulbs in parallel are brighter = both have 3V across them (brighter)
Bulbs in series have 1.5 v across each (dimmer)
Bulbs in series provide a larger resistance than bulbs in parallel
With this in mind, which circuit draws a larger current from the cell?
V = IR
I = V / R
as resistance decreases, the current increases for the same voltage
The second circuit draws a larger current from the cell, bulbs are in parallel and the total resistance is smaller than the first circuit (series circuit)
Why are the bulbs dimmer in the second circuit?
Voltage is shared between bulbs in series
Less voltage across cell therefore bulb is dimmer.
Explain why the second circuit draws a lower current from the cell?
circuit with two bulbs in series - has larger resistance
V=IR
I = V/R
as resistance increases, current decreases for same voltage
What is true about the current in a series circuit?
Current is the same everywhere in a series circuit
What happens to the total resistance and current in a circuit as more bulbs are added in parallel
Adding bulbs in parallel decreases the total resistance
V=IR
I = V / R
as resistance decreases, current increases for the same voltage
How does the resistance of an LDR vary with light intensity?
As light intensity increases, resistance decreases
Describe the shape of the magnetic field around a wire
Circular field shape
field lines getting futher apart- field getting weaker as you move away from the wire
How do you determine the direction of a magnetic field around a wire?
Use the right hand grip rule
Thumb- direction of current
Fingers- wrap in direction of magnetic field (clock wise or anti-clockwise)
How would you construct a simple electromagnet?
1) wrap a wire around an iron nail
2) connect a cell across the wire
What can be done to increase the magnetic field around a solenoid?
1- more turns
2- increase the current
3- add an iron core
Draw the shape of a magnetic field around a coil.
.
Draw the magnetic field around a solenoid.
.
What happens to the magnetic field around a wire if more wires are added side by side- How can you tell from the image that the magnetic field is stronger?
More wire- stronger field
The closer the magnetic field lines the stronger the field.
iron filing will line up more easily and more closely together in a stronger magnetic field.
If a vertical wire has the current upward in the wire, work out the direction of the magnetic field using your Right Hand Grip Rule
Thumb- up and in direction of current
Finger wrap in direction of magnetic field.
How would you demonstrate the direction of the magnetic field around a single wire?
Place compasses around the wire
The compass will point in the direction of the magnetic field
What happens to the direction of the magnetic field around a magnet if the direction of the current it reversed?
The direction of the magnetic field is reversed.
How do we use dot and crosses to represent the direction of a mangetic field or current?
dot means out of the page
cross means into the page
How do you use the right hand grip rule to work out which end of a solenoid is the north end?
Fingers- wrap your fingers in the direction of the current
Thumb- will point to the North end of the solenoid
What happens to the magnetic field around a solenoid if you add an iron core (magnetically soft)
Stronger magnetic field
Why is iron a better core to use in an electromagnet than steel?
Iron is magnetically soft- the domains in iron line up easily - it can magnetise and demagentise easily
Steel is magnetically hard- the domains do not line up easily- it cannot magnetise easily and it can be used to make permananet magnets because it also does not easily demagnetise
Electromagets are used in electric bells.
When the button is pushed, a current flows in the electromagnet -This creates a magnetic field around the electromagnet
The electromagnet attracts the soft iron armature which moves to the right
This causes the hammer to hit the bell.
What happens next?
The moment the iron armature moves to the right this creates a _break in the circuit- t_he circuit is no longer complete.
The current drops to zero and the magnetic field around the electromagnet disappears.
The iron armature is no longer attracted and springs back to the left
The armature completes the circuit again and the whole cycle repeats alowing the bell to repeatedly sound.
A negatively charged particle is moving horizontally to the right in a uniform magnetic field that is pointing in the same direction as the velocity. What is the direction of the magnetic force on the charge?
1. Left
2. Right
3. Up
4. Down
5. Into the page
б. Out of the page
7. No force at all.
- No force at all
Note: Particle is moving parallel to the field- no force.
Describe the left hand rule (LHR)
Thumb = Motion
First finger (index) = Field
Second finger = Current
Complete the explanation of why the comutator allows the motor to rotate
There is a current carrying coil
90º to a magnetic field
Using LHR- left side of coil experiences an upward force.
Using LHR- current is travelling the opposite direction on the right side of coil- it experiences a downward force
Coil rotates clockwise
When coil is at 90º, the current in the coil reverses due to the commutator and brushes.
Current continues to travel IN on the left and OUT on the right
8 Using LHR- left side of coil still experiences an upward force and the right hand side still experiences a downward force
Coil continues to rotate clockwise
When the magnet is moved into the coil, a current is induced and the needle on the ammeter moves to the left.
Describe what happens when the magnet is pulled out of the coil.
The needle with move to the right
Reversing the direction of the magnet, reverses the induced current
When a magnet is moved in and our of a coil, a current is induced in the coil. What is this phenomenon called?
Electromagnetic induction
Explain why a current is induced in the coil when the magnet is pushed into the coil
The magnet is moving (thuMb- MOVEMENT)
The magnetic field lines of the magnet are cutting the wires of the coil at 90º - (First finger- FIELD)
This induces a current in the coil - (seCond finger- CURRENT)
Explain why a current is induced in a wire when it it is moved downward through a magnetic field
Wire is moved downward (thuMb - MOVEMENT)
The wire cuts the magnetic field lines at 90º - First finger- FIELD)
This induces a current in the wire- (seCond finger- CURRENT)
When a wire is moved down through a magnetic field, the wires cut the magnetic field lines at 90º and this induces a current in the wire which moves the ammeter needle to the left.
What happens when the wire is pulled up and out of the magnetic field?
The needle will move to the right
Reversing the direction of the movement of the wire will reverse the direction of the induced current.
When a wire is moved downward through a magnetic field, the wire cuts the magnetic field lines at 90º and induces a current in the wire.
What would happen if the wire is moved more quickly?
A large current would be induced
When a wire is moved downward through a magnetic field, the wire cuts the magnetic field lines at 90º and induces a current in the wire.
What would happen if the strength of the magnets were increased?
A large current would be induced
When a wire is moved downward through a magnetic field, the wire cuts the magnetic field lines at 90º and induces a current in the wire.
What would happen if the number of turns in the wire was increased?
A large current would be induced
When a magnet is moved in and out of a coil, the magnetic field lines of the magnet cut the coil at 90º , which induces an a.c. current.
State three way in which you could increase the current induced?
More turns in the coil
Stronger magnet
Move the magnet faster
Use the LHR to explain why a current is induced in the coil
Magnet is moved (thuMb- MOVEMENT)
Magnetic field lines around magnet cut the coil at 90º (First finger- FIELD)
Due to LHR a current is induced in the coil (seCond finger- CURRENT)
Use the LHR to explain why a current is induced in the wire.
Wire is moved (thuMb- MOVEMENT)
90º to a magnetic field (First finger- FIELD)
Due to LHR a current is induced in the wire (seCond finger- CURRENT)
Use the LHR to explain why the wire moves to the left.
Current carrying wire (seCond finger- CURRENT)
90º to magnetic field (First finger field)
Using LHR this creates a force to the left- wire moves left (thuMb movement)
What are the ends of a bar magnet called?
north pole and south pole
In which region around a bar magnet is the magnetic field the strongest? How can the magnetic field lines indicate this?
The magnetic field is strongest at the poles, the field lines are close together at the poles indicating a stronger field.
How can you create a uniform field using two bar magnets?
Place the bar magnets so that the N pole of one magnet is facing the S pole of the other.
Move them so that they are close but not touching
There is a uniform field between the poles
Describe the magnetic field created when the N-pole of one bar magnet is near the S-pole of another.
Uniform field
Magnetic field lines are parallel AND evenly spaced
What is a magnetic field?
A region of space where a magnetic material experiences a force.
Name a metal which is magnetically hard.
Steel
Name a metal which is magnetically soft
iron
What is the point called between two adjacent N-poles of a magnet where their magnetic field cancels?
neutral point
What is the law of magnetism?
Like poles repel
Unlike poles attract
What is the difference between the domains in a magnetically hard material and a magnetically soft material?
Magnetically hard- difficult for domains to align in a field (also difficult for domains which are lined up to de-align)
Magnetically soft- easy for domains to align in a magnetic field and then de-align when field is removed.
What is a permanent magnet made of? Magnetically hard material or magnetically soft?
Magnetically hard- so that it does not lose its magnetism easily
How can you observe THE SHAPE of a magnetic field around a magnet?
Pace paper over top the magnet
Sprinkle iron filing on the paper
Tap the paper to help the iron filings line up in the field
What happens when an iron nail is brought near the N Pole of a magnet.
The nail is attracted to the N-pole
The domains in the iron nail line up (point away) in the field creating a S-pole at the top of the nail closest to the magnet. (Magnetism is induced in the iron nail)
Opposite poles attract and the nail sticks to the magnet.
What is the difference in the orientation of the domains in a magnetised and demagnetised material?
Magnetised- domains are lined up
De-magnetised- domains are randomly arranged
How can you show the shape AND direction of a magnetic field around a magnet?
place a magnet on a piece of paper
place a plotting compass at one corner of the N-pole of the magnet
draw a dot where the compass is pointing.
move the compass forward so that the tail is adjacent to the point you just plotted
draw another dot where the compass is pointing
Continue this until you reach the S-pole and connect the dots with a smooth curve.
Repeat steps 3-6 drawing multiple field line around the magnet
Draw an attracting field between a North and South pole.
.
Draw a repelling field between two N-poles
.
How can an iron nail be magnetised?
Stroke the iron nail with a magnet for several minutes.
This will align the domains in the iron nail and magnetise it.
How can you demagnetise a permanent magnet?
Heat it, hammer it or drop it repeatedly.
This will de-align the domains (demagnetise it)
In which direction do magnetic field lines run?
North to South
OUT of the North
INTO the South
What do we call it when ‘all’ the domains in a magnetic material are aligned and magnet cannot become any stronger?
saturated
