UNIT 4 Electricity and magnetism Flashcards
magnetic forces are due to
due to interactions between magnetic fields
Two bar magnets can repel or attract
left-hand = find force
right hand = find current
exam tip
- adding arrows pointing away from the north pole and towards the south pole
- making sure the magnetic field lines are further apart as the distance from the magnet increases
the relative strength of a magnetic field is represented by
represented by the spacing of the magnetic field lines
Properties of magnets
types of magnets
- ends of a magnet are called poles
- have two poles: a north and a south
- Permanent magnets
Induced (temporary) magnets
Permanent Magnets
- made out of permanent magnetic materials, for example steel
- will produce its own magnetic field
It will not lose its magnetism
Temporary (Induced) Magnets
- When magnetic material is placed in a magnetic field, the material can TEMPORARILY be turned into a MAGNET: INDUCED magnetism
steel obj -> can be magnetised and will remain magnetic for a while
- other obj, such as electromagnets or transformers (;;soft iron) will be demagnetised as soon as the cause of the induced magnetism is removed
When magnetism is induced on a material:
- One end of the material will become a north pole, the other end south pole
- Magnetic materials will always be attracted to a permanent magnet; so end of the material closest to the magnet will have the OPPOSITE pole to magnetβs pole closest to the material
- When magnetic material removed from magnetic field it will lose most/all of its magnetism quickly
there are positive and negative charges
what repels what
FOR MAGNETIC materials (that arenβt magnets) - attracted to either pole, doesnβt matter if N or S
positive charges repel other positive charges,
negative charges repel other negative charges,
but positive charges attract negative charges
LIKE REPEL. OPP ATTRACT
define magnetic material
Only a magnet can ____ another magnet
[Non-magnetic materials do not experience a force when placed in a magnetic field]
- experience a force when placed in a magnetic field
- attracted to a magnet when unmagnetised
- can be magnetised to form a magnet
only magnet can repel
electrical charge, Q, measured in
charge is measured in
Coulomb (C)
charging of solids by friction involves
ONLY
only a transfer of negative charge (electrons)
electric field as β¦
- shown by electric field lines
TOWARDS NEG. CHARGES
a region in which an
electric charge experiences a force
- charged object creates an electric field around itself; experiences electrostatic force
- Fields lines always point AWAY from positive charges and towards negative charges
direction of an electric field at a
point is β¦
the direction of the force on a positive
charge at that point
[i.e. βField lines show the direction that a positive charge would experience if it was at that pointβ]
- in demonstrations it is always electrons (negative charges) which are free to move according to that force
- strength of an electric field depends on the distance from the object creating the field:
[!!] field is strongest close to the charged object - shown by the field lines being closer together
[!!!] the field becomes weaker further away from the charged object - shown by the field lines becoming further apart
Electric Field Patterns
- objs in electric field experience an electrostatic force [force=vector β΄ the direction of this force depends on whether the charges are the same or opposite]
The force is either ATTRACTIVE or REPULSIVE,
meaningβ¦
- if charges are same (both pos./both neg.), this force = repulsive & the second charged object will move away from the charge creating the field
- if the charges are the opposite (negative and positive), this force will be ATTRACTIVE and the second charged object will move TOWARD the CHARGE CREATING THE FIELD
size of the force depends on the strength of the field at that point
so FORCE BECOMES
- STRONGER as the distance between the two charged objects DECREASES
& weaker - as the distance between the two charged objects increases
[relatshp between strength of the force and the distance applies to both the force of attraction and force of repulsion, i.e.
two negative charges brought close together will have a stronger repulsive force than if they were far apart]
complete recap:
- field lines around a POINT CHARGE
- field lines - Between Two Oppositely Charged Parallel Conducting Plates [uniform electric field]
- field lines - Around a Charged Conducting Sphere [demoβed using a Van der Graaff Generator, using streamers, small pieces of paper, polystyrene beads, aluminium foil containers]
- field lines going towards neg. away pos., evenly spaced, with arrow showing direction (towards neg)
- field lines are:
Directed from the positive to the negative plate
Parallel
Straight lines
- field lines are:
- field lines are - symmetrical, as with a point charge
- bc the charges on the surface of the sphere are evenly distributed.
charges are the same, so they repel.
the surface is conducting, allowing them to move
so just arrows pointing away from pos. evenly spaced around circle
electric current is related to the
happens bcβ¦
flow
of charge
two oppositely charged conductors are connected together (by a length of wire), charge will flow between the two conductors
direct current (d.c.) - straight horizontal line
and alternating current (a.c.) - parabola up and down dipping below x-axis too
difference?
voltage y-axis, time x-axis
direct: charge flow in 1 direction while in alternating the direction of charge flow changes regularly
dc
ELECTRONS. NEG TO POS
[state in circuit whether current/electron flow is clockwise or anticlockwise]
produced when using dry cells and batteries (and sometimes generators, although these are usually ac)
The electrons flow in one direction only, from the negative terminal to the positive terminal
ac
typically comes from mains electricity and generators
- needed for use in transformers in the National Grid
The direction of electron flow changes direction regularly
A typical frequency for the reversal of ac current in mains electricity is 50 Hz
Define electric current
I = Q/t
Q = It
the [amount of] charge passing a
point [in a circuit] per unit time [per second]
charge/second CHARGE PER SECOND
current eqn
I = Q/t
Current (A) = charge (C)/time (s)
current = power [of appliance] / [mains] voltage
conventional current is fromβ¦
CONVENTIONAL current is from positive to negative
and that the flow of free electrons is
from negative to positive
clockwise/anticlockwise in circuit?
π¨ Ammeters should always be connected in SERIES with the part of the circuit you wish to measure the current through
- bc ammeters measure the amount of charge passing through them per unit time, so the ammeter has to be in series so that all the charge flows through it
AMMETER - analogue: Always double check exactly where the marker is before an experiment, if not at zero, you will need to subtract this from all your measurements. They should be checked for zero errors before using
- are also subject to parallax error
Always read the meter from a position directly perpendicular to the scale
digital:
can measure very small currents, in mA or Β΅A
show measurements as digits and are more accurate than analogue displays
- easy to use because they give a specific value and are capable of displaying more precise values
- may βflickerβ back and forth between values and a judgement must be made as to which to write down
Digital ammeters should be checked for zero error. Make sure the reading is zero before starting an experiment, or subtract the βzeroβ value from the end results
Define electromotive force (e.m.f.) - voltage supplied by a power supply
the electrical work done by a source in moving a unit charge around a complete circuit
-measured in volts (V)
eqn for emf
E = W/Q
same for voltage!
emf = converted energy [work] / positive charge
Define potential difference (p.d.)
p.d. between two points is
measured in volts (V)
the work done by a unit charge passing through a component
eqn for p.d.
V = W/Q
voltage (v) = work (J)/charge (C)
π¨ Voltmeters are connected in PARALLEL with the component being tested
The potential difference is the difference in electrical potential between two points, therefore the voltmeter has to be connected to two points in the circuit
VOLTMETER - analogue:
Analogue voltmeters subject to PARALLAX ERROR
- ALWAYS read the meter from a position directly perpendicular to the scale
Typical ranges are 0.1-1.0 V and 0-5.0 V for analogue voltmeters although they can vary - ALWAYS double check exactly where the marker is before use in an experiment, if not at zero, you will need to subtract this from all your measurements
digital: [SAME AS OTHER]
can measure very small potential differences, in mV or Β΅V
- more accurate, easy to use,
- judgement must be made as to which to write down, check for zero error, make sure the reading is zero before starting an experiment, or subtract the βzeroβ value from the end results
define resistance
eqn for Resistance
Resistance is the opposition to current
For a given potential difference, the higher the resistance, the lower the current
Therefore resistors are used in circuits to control the current
-> R = V/I
ohmβs law
Current is directly proportional to potential difference as long as the temperature remains constant
consequences of ohmβs law
- resistors are used in circuits to control either 1) the current in branches of the circuit (through certain components), or 2) the potential difference across certain components
due to consequences of ohmβs law -
The current in an electrical conductor decreases as its resistance increases (for a constant p.d.)
The p.d. across an electrical conductor increases as its resistance increases (for a constant current)
following relationship for a
metallic electrical conductor:
(a) resistance is directly proportional to length - R inc., L inc.
(b) resistance is inversely proportional to
cross-sectional area - R inc., CS-A dec.
I-V Graphs for Ohmic Resistors, Filament Lamps & Diodes [current on y-axis]
resistor: straight line with positive correlation, diagonal through origin
filament lamp: curved like β/β through origin
- because the resistor has a constant resistance
- for filament lamps,
The current INCREASES at a proportionally SLOWER rate than the potential difference
The current INCREASES at a proportionally SLOWER rate than the potential difference
BECAUSE
The current causes the filament in the lamp to heat up
As the filament gets hot, its resistance increases
This opposes the current, causing it to increase at a slower rate
I-V Graph for a Diode
I
FORWARD bias: looks like β―, shows a sharp increase in voltage and current
reverse BIAS: where its horizontal+ straight line along x-axis like ____, graph shows a flat line where current is zero at all voltages
BECAUSE ;;
A diode is a non-ohmic conductor that allows current to flow in one direction only
The direction is shown by the triangular arrow of the diode symbol = forward bias
In the reverse direction, the diode has very high resistance, and therefore no current flows = reverse bias
length & effect on R - directly proportional
cross-sectional area & its effect on R - inversely proportional
LENGTH - If the wire is longer, each electron will collide with more ions and so there will be more resistance:
SO: The longer a wire, the greater its resistance
AREA - If the wire is thicker (greater diameter) there is more space for the electrons and so more electrons can flow:
THEREFORE: The thicker a wire, the smaller its resistance
electric circuits transferβ¦
transfer energy
from a source of electrical energy, such as an
electrical cell or mains supply, to the circuit
components and then into the surroundings
eqn for electrical power
P = IV
eqn for electrical energy
E = IVt
joules = amps x volts x seconds = ELECTRICAL energy
The amount of energy an appliance transfers depends on:
- e.g.,
A 1 kW iron uses the same amount of energy in 1 hour as a 2 kW iron would use in 30 minutes
A 100 W heater uses the same amount of energy in 30 hours as a 3000 W heater does in 1 hour
π how long the appliance is switched on for
π the power of the appliance
Define the kilowatt-hour (kWh)
eqn?
1kW = ___W
A unit of energy equivalent to one kilowatt of power expended for one hour
E = Pt
Where [e & p are Γ 10^3]
E = energy (kWh)
P = power (kW)
t = time (h)
[kilo (k) means 1000, so 1 kW = 1000 W]
POWER ratings tell
The amount of energy transferred (by electrical work) to the device every second
Since the usual unit of energy is joules (J), this is the 1 W in 1 s
Therefore:
1kWh =1000W x 3600s = 3.6 x 10^6 J
Since 1 kW = 1000 W and 1 h = 3600 s
π HOW TO CONVERT btwn Joules & kWh
kWh x (3.6 x 10^6) = J
J / (3.6x10^6) = kWh
Energy Transfer in Electrical Circuits
- domestic appliances transfer energy from batteries
- household appliances transfer energy from the AC mains [mostly]
-[electric] to the kinetic energy of an electric motor [motors in vacuums, fridges, washmachin.)
electric to thermal ;; or heating devices: toasters, kettles, radiators
circuit w/ closed-loop, i.e. a SERIES CIRCUIT [!!]
the CURRENT is the same value at any point/all components in a closed-loop have the same current
because β¦
& amount of current flowing around a series circuit depends on:
- The voltage of the power source
- The resistance of the components in the circuit
bc the number of electrons per second that passes through one part of the circuit is the same number that passes through any other part
&& & inc. voltage of power source drives more current around the circuit
So, decreasing the voltage of the power source reduces the current
- INC. the number of COMPONENT in the circuit INC. the total RESISTANCE
Hence LESS CURRENT flows through the circuit
SERIES CIRCUIT - POTENTIAL DIFFERENCE.
- combined EMF is equal to the sum of their individual EMFs; sum of potential differences across the components is equal to the total EMF of the power supply
Current in Parallel Circuits [consists of two or more components attached along separate branches of the circuit]
advantages of parallel;;
what happens to current in parallel?
- the components can be individually controlled, using their own switches
- if one component stops working the others will continue to function
;; the current splits up, so the current in each branch will be smaller than the current from the power supply
Potential Difference in Parallel Circuits?
potential difference across each component connected in parallel is the same
At a JUNCTION in a PARALLEL circuit (where two or more wires meet), the current is conserved, MEANINGβ¦
the amount of current flowing into the junction is equal to the amount of current flowing out of it
bc CHARGE is conserved
current does this bc it is the flow of electrons:
π electrons are physical matter β they cannot be created or destroyed.
π the total number of electrons (and hence current) going around a circuit must remain the same.
π when the electrons reach a junction, however, some of them will go one way and the rest will go the other
is current always split equally in parallel circuits
the current does not always split equally β often there will be more current in some branches than in others
The current in each branch will only be identical if the REISTANCE of the components along each branch are IDENTICAL
resistors in series -
When two or more components are connected in series:
The combined resistance of the components is equal to the sum of individual resistances
I.E.;; add them together
combined resistance
of 2 resistors
in parallel
is⦠____ than that of either one alone.
eqn:
the combined resistance of two
resistors in parallel is less than that of either resistor by itself
[If two resistors of equal resistance are connected in parallel, then the combined resistance will halve]
;;; EQN! 1/R = 1/R1 + 1/R2
Determining Resistance in Parallel
To calculate the resistance:
First find the value of 1/R (by adding 1/R1 + 1/R2)
Next find the value of R by using the reciprocal button on your calculator (labelled either x-1 or 1/x, depending on your calculator)
Recall and use in calculations, the fact that:
3 facts
(a) the sum of the currents entering a junction in a parallel circuit is equal to the sum of the currents that leave the junction
b)
(b) the total p.d. across the components in
a series circuit is equal to the sum of the
individual p.d.s across each component
c)
c) the p.d. across an arrangement of parallel resistances is the same as the p.d. across one branch in the arrangement of the parallel resistances
Explain that the sum of the currents into a
junction isβ¦
the same as the sum of the currents
out of the junction
p.d. across conductorβ¦
the p.d. across an electrical conductor
increases as its resistance increases for a constant current
the equation for two resistors used
as a potential divider
π potential divider eqn
- potential dividerβ¦,
[used widely in volume controls and sensory circuits using LDRs and thermistors,]
- potential dividerβ¦,
R1/R2 = V1/V2
π V out = R2 / (R1+R2) x V in
[numerator has to be the resistance of the resistor over/next to/in series with V out]
V in = power supply voltage
+ - are circuits which produce an output voltage as a fraction of its input voltage
- have two main purposes:
To provide a variable potential difference
To enable a specific potential difference to be chosen
To split the potential difference of a power source between two or more components
Variable Potential Dividers
When two resistors are connected in series,β¦
the potential difference across the power source is shared between them
potential difference across each resistor dependsβ¦
β¦ depends upon its resistance:
- The resistor with the LARGEST RESISTANCE will have a GREATER POTENTIAL DIFFERENCE than the other one [from V=IR]
- If the resistance of one of the resistors is increased, it will get a greater share of the potential difference, whilst the other resistor will get a smaller share
what is a potentiometer
- a kind of variable resistor
- a single component that (in its simplest form) consists of a coil of wire with a sliding contact, midway along it
w/ a terminal A and terminal B
sliding contact
has the effect of separating the potentiometer into two parts β an upper part and a lower part β both of which have different resistances
Moving the slider (as seen in variable resistor symbol) changes the resistances (& β΄ potential differences) of the upper and lower parts of the potentiometer
If slider moved upwards, the resistance of the lower part will increase
& β΄ the potential difference across it will also increase
Resistors as Potential Dividers
- Kirchhoffβs Second Law β¦
When two resistors are connected in series, the potential difference across the power source is divided between them
π input voltage V in is applied to the top and bottom of the series resistors
π The output voltage V out is measured from the centre to the bottom of resistor R2
π In potential divider circuits, the p.d across a component is proportional to its resistance from V = IR
When thinking about potential dividers, remember that the higher the resistance the more energy it will take to βpush the current throughβ and therefore the higher the potential difference.
This means that if a component (often shown as a voltmeter in questions) needs to be switched on by a change such as increased light or temperature, then the resistor it is in parallel with needs to become larger compared to the other resistor.
a mains circuit consists of
explain why a switch must be connected to the
live wire for the circuit to be switched off safely
consists of a live wire
(line wire), a neutral wire and an earth wire
magnets
magnets have two poles: a north and a south
- objects which experience attraction and repulsion
explain attraction & repulsion
Like poles repel (push each other apart)
Unlike poles attract (move towards each other)
what are magnetic materials
- experience a force when placed in a magnetic field
- are attracted to a magnet when unmagnetised
- can be magnetised to form a magnet
- only a magnet can repel another magnet
Series
Rt = r1 + r2 + r3 β¦
Parallel
1/Rt = 1/r1 + 1/r2 + 1/r3β¦
1/Rt = 1/4
Rt = 4
add in series
4 + 5 + 3 = 12 ohms
so then I = V/R = 24/12 = 2 amps
Kirchoffβs second law
- applies to voltage drops across components in a circuit
- states that around any closed loop in a circuit, the directed sum of potential differences across components is 0
the magnetic field is always in the direction from North to South and current is always in the direction of a positive terminal to a negative terminal.
mag field - N to S
- current pos to neg
uses of permanent magnets (steel, stay magnetised)
π Compasses: humans use for navigation, since needle always points north
π School lab experiments: magnets used in school science demos - permanent magnets
π Toys: toy trains and trucks often have magnets which attach the carriages or trailers to the engine or cab
π Fridge magnets: made either of flexible magnetic material or by sticking a magnet to the back of something
uses of Electromagnets (use electricity to create magnet from a current-carrying wire; have adva. of being magnetised and demagnetised at the flick of a switch; can be switched on and off; soft iron; easily become a temporary magnet)
π MRI scanners: hospitals; large, cylindrical machine using powerful electromagnets to produce diagnostic images of the organs of the body
π Speakers (loudspeakers), microphones and earphones: used in phones and laptops use electromagnets to sense or send soundwaves
π Recycling: bc steel is a magnetic material it can be easily separated from other metals and materials using electromagnets. Once recovered the steel is re-used and recycled, reducing mining for iron ore and processing ore into steel
π Mag-Lev Trains: the ability of Mag-Lev trains to hover above the rails is due to them being repelled by large electromagnets on the train and track. This reduces friction and allows speeds of nearly 400 miles per hour
magnetic elements
iron, steel (iron alloy), cobalt, nickel
test whether a material is a magnet
- material brought close to a known magnet
repelled by the known magnet then the material itself is a magnet
If it can only be attracted and not repelled then it is a magnetic material
define: magnetic field
(which all magnets are surrounded by)
The region around a magnet where a force acts on another magnet or on a magnetic material
(such as iron, steel, cobalt and nickel)
define uniform magnetic field
- one that has the same strength and direction at all points
to show mag. fie. has same strength at all points - MUST: equal spacing between all magnetic field lines; same distance apart between the gaps of the poles
& acting in the same direction at all points - MUST: an arrow on each magnetic field line going from the north pole to the south pole
This field can be determined by using plotting compasses that will point from north to south or by using iron filings
uniform magnetic field - NORTH to south
2 bar magnets can be used to produce this;
Point opposite poles (north and south) of the two magnets a few centimetres apart
A uniform magnetic field will be produced in the gaps between opposite poles
[Outside that gap, the field will not be uniform]
Magnetic Field Lines
- used to represent the strength and direction of a magnetic field; direction of the magnetic field is shown using arrows
RULES - Magnetic field lines:
π Always go from north to south (indicated by an arrow midway along the line)
π Must never touch or cross other field lines
Magnetic Field Around a Bar Magnet
- magnetic field is strongest at the poles
This is where the magnetic field lines are closest together - The magnetic field becomes weaker as the distance from the magnet increases
This is because the magnetic field lines are getting further apart
Magnetic Field Strength
- strength of the magnetic field is shown by the spacing of the magnetic field lines,
as in⦠;;
If the magnetic field lines are CLOSE together then the magnetic field will be STRONG
If the magnetic field lines are far apart then the magnetic field will be weak
describe a method of plotting the magnetic field around a bar magnet
1st way: Using Iron Filings
Place a piece of paper on top of the magnet
Gently sprinkle iron filings on top of the paper
Now carefully tap the paper to allow the iron filings to settle on the field lines
- surrounds, circles
2nd: compass
Place the magnet on top of a piece of paper; draw around magnet
Draw a dot at one end of the magnet (near its corner)
Place a plotting compass next to the dot, so that one end of the needle of the compass points towards the dot
Use a pencil to draw a new dot at the other side of the compass needle
Now move the compass so that it points towards the new dot, and repeat the above process
Keep repeating until you have a chain of dots going from one end of the magnet to the other. Then remove the compass, and link the dots using a smooth curve β the magnetic field line
π The direction of the field line is the same as the direction of the plotting compass
You can now repeat the whole process several times to create several other magnetic field lines
obj to use to know which end of magnet is north pole; explain
While both the spoon and horseshoe will be attracted to the magnet, only the compass will be able to identify which of the two poles is the north pole
[!!] AS the north arrow in the compass will align with the magnetic field of the magnet.
Demonstrating Electrostatic Charges
- Electrostatic repulsion is caused by the _____ between charges
FORCE btwn charges
simple exp. showing the production of electrostatic charges by FRICTION, insulating solids (e.g. plastics) are given a charge
- done using friction to transfer electrons from the surface. Removing electrons
which meansβ¦
by REMOVING electrons, which have negative charge, the insulator is left with a POSITIVE charge
Method
Suspend one of the insulating materials using a CRADLE and a length of string so that the material can rotate freely
Rub one end of the material using a cloth (in order to GIVE it a CHARGE)
Now take a second piece of insulating material and charge that by rubbing with a cloth
Hold the CHARGED end of the second piece close to the charged end of the first piece:
- If the first piece rotates away (is repelled) from the second piece then the materials have the same charge
- If the first piece moved towards (is attracted to) the second piece then they have opposite charges
results: observations
when desc a demo what should you do
should state a conclusion β in other words, explain what you expect to happen and what it means.
what rods used
there will be friction between the cloth and the rods
only insulating materials can be charged by friction
glass and plastic rods should be chosen as they are insulating materials & copper and steel rods are made of conductive materials
Investigating Conductors & Insulators
key difference
Conductors allow charge carriers to freely move. Insulators do not allow charge carriers to move
- reasons for this are to do with their internal structure
Conductors
a material that allows charge (usually electrons) to flow through it easily
e.g., Silver, Copper, Aluminium, Steel (metals)
- made up of positively charged metal ions with their outermost electrons delocalised, β΄ the electrons are free to move
- Current is the rate of flow of charged particles. So, the more easily electrons are able to flow, the better the conductor
insulators (rubber, plastic, glass, wood ALTHOUGH wood allows some charge to pass through them - conduct a little in the form of static electricity)
a material that has no free charges, hence does not allow the flow of charge through them very easily
Investigating Electrical Conductors & Insulators
- Gold-leaf Electroscope (GLE); to distinguish btwn cond. and insul., used to demonstrate charge - CONSISTS OFβ¦
[or as alt: electronic charge detector]
- A metal plate attached to one end of a metal rod
- At the other end of the rod a very thin leaf of gold foil is attached
- The rod is held by an insulating collar inside a box with glass sides, allowing the gold leaf to both be seen and protected from draughts
GLE CHARGED,
rod and leafβ¦
When the GLE is charged, the plate, rod and gold leaf have the same charge (either positive or negative)
Since the rod and leaf have the same charge, they repel, and the leaf sticks out to the side
When the rod and leaf are discharged (are neutral) the leaf hangs down
To Test Electrical Conductors and insulators
Charge the plate of the GLE so that the gold leaf stands clear of the rod
Carefully touch the plate of the GLE with the items being tested, for example:
Metals (wire, paperclip, scissor blades),
Non-metals (paper, fingers, glass, graphite), Plastics (plastic ruler, the handles of the scissors, finger in a plastic sandwich bag), Comparisons (wet cloth, dry cloth; finger and finger in a plastic sandwich bag)
Record the observations each time
Leaf falls: material is a good conductor
Leaf remains in place: object is a poor conductor (good insulator)
Leaf falls slowly: material is a poor conductor
power
p.d.
current
SO electrical power: rate of change of work done; energy transferred per second in an electrical component
rate of doing work
work done per unit charge
rate of flow of charge
eqns
P = E or W / t
P = IV
P = I^2R. P = V^2 /R.
βTwinkle Twinkle Little Star, Power equals I squared Rβ
Circuit Components
- Power supplies
Cells, batteries, power supplies and generators all
SUPPLY CURRENT
to the circuit
Resistors
Potential dividers, fixed and variable resistors, thermistors and light-dependent resistors (LDRs) are all used to
CONTROL CURRENT
Meters
Ammeters and voltmeters are used to MEASURE the current and potential difference
- ammeters are always connected in series
- voltmeters are always connected in parallel
Electromagnetic Components
Magnetising coils, relays and transformers USE ELECTROMAGNETIC EFFECTS
Relays use a SMALL current in ONE CIRCUIT to SWITCH on a much LARGER CURRENT in ANOTHER
Transformers βstep upβ and βstep downβ current and potential difference
Fuses
- line right thru rectangle
Protect expensive components from current surges and act as a safety measure against fire
Thermistors - [ _/ thru rectangle]
a non-ohmic conductor and a temperature-dependent resistor!
[opposite]
the resistance of a thermistor changes depending on its temperature
- as temperature increases the resistance of a thermistor decreases - high temp, low resistance
temp dec., resistance inc. - low temp, high resistance
Light-dependent Resistors (LDR) -
a non-ohmic conductor and sensory resistor
Its resistance automatically changes depending on the light energy falling onto it (illumination)
As the light intensity increases, the resistance of an LDR decreases - more light, lower resistance
less light, higher resistance
Diodes
[triangle arrow with line at tip, and line NOT going thru it but sometimes does]
a component that only allows a current in one direction
If a diode is connected to an a.c. (alternating current) power supplyβ¦
it will only allow a current half of the time
- called rectification
using 2 diodes, lamp & power supply, draw a circuit diagram that will provide the lamp with a rectified current
a.c. at top, diode with line at bottom, lamp, then diode with line at top
when pos. terminal is on the left, the diode stops the flow of current.
rectified current/circuit. lamp receives current every 2nd cycle or half the time
using 2 diodes, lamp & power supply, draw a circuit diagram that will provide the lamp with a rectified current
a.c. at top, diode with line at bottom, lamp, then diode with line at top
when pos. terminal is on the left, the diode stops the flow of current.
rectified current/circuit. lamp receives current every 2nd cycle or half the time
[card 72]; potentially lethal β¦
Mains electricity - 50 volts can be serious hazard. COMMON HAZARDS;;
π Damaged Insulation β If someone touches an exposed piece of wire, they could be subjected to a lethal shock
π Overheating of cables β Passing too much current through too small a wire (or leaving a long length of wire tightly coiled) can lead to the wire overheating. This could cause a fire or melt the insulations, exposing live wires
π Damp conditions β If moisture comes into contact with live wires, the moisture could conduct electricity either causing a short circuit within a device (which could cause a fire) or posing an electrocution risk
π Excess current from overloading of plugs, extension leads, single and multiple sockets when using a mains supply - If plugs or sockets become overloaded due to plugging in too many components the heat created can cause fires
what is Mains electricity
the electricity generated by power stations and transported around the country through the National Grid
- we connect to the mains when plugging in an appliance such as a phone charger or kettle
- is an alternating current (a.c.) supply ;; does not have positive and negative sides to the power source - equivalent to positive and negative are called live and neutral and these form either end of the electrical circuit
- has a frequency of 50 Hz and a potential difference of about 230 V [vary 50-60Hz]
A frequency of 50 Hz means the direction of the current changes back and forth 50 times every second
neutral, earth, live wires, fuse, cable grip, wires leading to metal cased appliance
wires purpose?
live and neutral wires deliver the electricity to the device.
The Earth wire is for safety
Insulation & Double Insulation
conducting part of a wire is usually made of copper or some other metal
If this comes into contact with a person, this poses a risk of electrocution
For this reason, wires are covered with an insulating material, such as rubber
- Insulation around the wires themselves
A non-metallic case that acts as a second layer of insulation = double insulation if thereβs a metal case
Double insulated appliances do not requireβ¦
an earth wire or have been designed so that the earth wire cannot touch the metal casing
Earthing;
earth wire is an additional safety wire that can reduce risk of electrocution
Many electrical appliances have metal cases = potential safety hazard: If a live wire (inside appliance) came into contact with the case, the case would become electrified and anyone who touched it would risk being electrocuted
If this happens:
π The earth wire provides a low resistance path to the earth
π It causes a surge of current in the earth wire and hence also in the live wire
π The high current through the fuse causes it to melt and break
π This cuts off the supply of electricity to the appliance, making it safe
fuse - a glass cylinder which contains a thin metal wire.
(typically 3A, 5A and 13A)
safety device designed to cut off the flow of electricity to an appliance if the current becomes too large (due to a fault or a surge)
- come in a variety of sizes so know how much current an appliance needs. always choose the next size up [e.g. 3.1 amp appliance - 3 amp too small, 13 amp too big, 5 amp fuse good]
If the current in the wire becomes too largeβ¦
The wire heats up and melts
This causes the wire to break, breaking the circuit and stopping the current
trip switch [found in the Consumer Box (where the electricity enters the building)]
how it works?
When the current is too high the switch βtripsβ (automatically flicks to the off position)
This stops current flowing in that circuit
Induced EMF
A conductor, such as a wire, cuts through a magnetic fieldβs lines
The direction of a magnetic field through a coil changes and conductor is stationary in it
used in:
Electrical generators which convert mechanical energy to electrical energy
Transformers which are used in electrical power transmission
an EMF will be induced in a conductor [wire] if there is
- RELATIVE MOVEMENT between the conductor [wire] and the magnetic field.
π For an electrical conductor moving in a fixed magnetic field, the conductor (e.g wire) cuts through the fields lines, inducing an EMF in the wire.
- also be induced if the conductor is stationary in a changing magnetic field
e.g - When the magnet enters the coil, the field lines cut through the turns/wire, inducing an EMF
For a fixed conductor in a changing magnetic filed
π As the magnet moved through the conductor (e.g. a coil), the field lines cut through the turns on the conductor (each individual wire)
π This induces an EMF in the coil
how to measure emf
A sensitive voltmeter can be used to measure the size of the induced EMF
If the conductor is part of a complete circuit then a current is induced in the conductor
This can be detected by an ammeter
no deflection of needle in voltmeter
C is correct because there the magnet is stationary
This means there is no relative movement between the coil and the magnetic field, therefore there are no magnetic field lines being cut
If the magnetic field lines are not being cut then there will not be a potential difference induced
Lenzβs Law -
The direction of an induced potential difference always opposes the change that produces it
- means that any magnetic field created by the potential difference will act so that it tries to stop the wire or magnet from moving
Demonstrating Lenzβs Law
If a magnet is pushed north end first into a coil of wire then the end of the coil closest to the magnet will become a north pole
π Explanation
Due to the generator effect, a potential difference will be induced in the coil
The induced potential difference always opposes the change that produces it
The coil will apply a force to oppose the magnet being pushed into the coil
Therefore, the end of the coil closest to the magnet will become a north pole
This means it will repel the north pole of the magnet
thenβ¦
If a magnet is now pulled away from the coil of wire then the end of the coil closest to the magnet will become a south pole
π Explanation:
Due to the generator effect, a potential difference will be induced in the coil
The induced potential difference always opposes the change that produces it
The coil will apply a force to oppose the magnet being pulled away from the coil
Therefore, the end of the coil closest to the magnet will become a south pole
This means it will attract the north pole of the magnet
Right-Hand Dynamo Rule
^ When moving a wire through a magnetic field, the direction of the induced EMF can be worked out by using THIS RULE.
THUMB - THRUST [direction wire]
FIRST FINGER - FIELD
SECOND FINGER - CURRENT
Start by pointing the first finger (on the right hand) in the direction of the field
Next, point the thumb in the direction that the wire is moving in
The Second finger will now be pointing in the direction of the current (or, strictly speaking, the EMF)
Demonstrating Induction
Experiment 1: Moving a magnet through a coil
When a coil is connected to a sensitive voltmeter, a bar magnet can be moved in and out of the coil to induce an EMF
- expected results are:
π When the bar magnet is NOT MOVING, the voltmeter shows a ZERO reading
π When the bar magnet is held STILL inside, or OUTSIDE , the coil, there is NO CUTTING of magnetic FIELD LINES, so, there is NO EMF induced
π When the bar magnet begins to move inside the coil, there is a READING on the voltmeter. As the bar magnet moves, its magnetic field lines βcut throughβ the coil. This induces an EMF within the coil, shown momentarily by the reading on the voltmeter
π When the bar magnet is taken back out of the coil, an e.m.f is induced in the opposite direction (a result of Lenzβs law)
As the magnet changes direction, the direction of the current changes
The voltmeter will momentarily show a READING with the OPPOSITE SIGN
π Increasing the SPEED of the magnet induces an e.m.f with a HIGHER MAGNITUDE
The direction of the electric current, and e.m.f, induced in the conductor is such that it opposes the change that produces it = Lenzβs law
Factors that will increase the induced EMF are
- when moving a magnet through a coil
- when moving a wire through a magnet
- Moving the magnet faster through the coil
ππ [USE THIS WORDING:] Adding more turns to the coil
Increasing the strength of the bar magnet
π 2.
Increasing the length of the wire
Moving the wire between the magnets faster
Increasing the STRENGTH of the magnets
Experiment 2: Moving a wire through a magnet
When a long wire is connected to a voltmeter and moved between two magnets, an EMF is induced
The pattern of a magnetic field in a wire can be investigated using this set up
[no current flowing through the wire to start with]
expected results are:
π When the wire is not moving, the voltmeter shows a zero reading. When the wire is held still inside, or outside, the magnets, the rate of change of flux is zero, so, there is no EMF induced
π As the wire is moved through between the magnets, an EMF is induced within the wire, shown momentarily by the reading on the voltmeter. As the wire moves, it βcuts throughβ the magnetic field lines of the magnet, generating a change in magnetic flux
π When the wire is taken back out of the magnet, an EMF is induced in the opposite direction. As the wire changes direction, the direction of the current changes
The voltmeter will momentarily show a reading with the opposite sign
π As before, the direction of the electric current, and e.m.f, induced in the conductor is such that it opposes the change that produces it
magnitude (size) of the induced EMF is determined by:
the speed at which the wire, coil or magnet is moved
the number of turns on the coils of wire
the size of the coils
the strength of the magnetic field
direction of the induced potential difference is determined by:
The orientation of the poles of the magnet [Reversing the direction in which the wire, coil or magnet is moved]
speed at which the wire, coil or magnet is moved:
Increasing the speed will increase the rate at which the magnetic field lines are cut
This will increase the induced potential difference
number of turns on the coils in the wire:
INC. the number of turns on the COILS in the wire will INC. the potential difference induced
- bc each coil will cut through the magnetic field lines and the total potential difference induced will be the result of all of the coils cutting the magnetic field lines
size of the coils:
Increasing the area of the coils will increase the potential difference induced
This is because there will be more wire to cut through the magnetic field lines
strength of the magnetic field:
Increasing the strength of the magnetic field will increase the potential difference induced
Simple A.C Generators;
generator effect can be used to generate ALTERNATING CURRENT in an alternator, varying in size and direction as the coil rotates & emf induced when coil rotates in the external magnetic field
[simple alternator: type of generator that produces an alternating current; a rotating coil in a magnetic field connected to slip rings]
π a rectangular coil is forced to spin in a uniform magnetic field; coil is connected to a centre-reading meter by metal brushes that press on two metal slip rings
π The slip rings and brushes provide a continuous connection between the coil and the meter
π The coil turns in one direction:
1) The pointer defects first one way, then the opposite way, and then back again
This is because the coil cuts through the magnetic field lines and an EMF, and therefore current, is induced in the coil
π The pointer deflects in both directions because the current in the circuit repeatedly changes direction as the coil spins. This is because the induced EMF in the coil repeatedly changes its direction. This continues on as long as the coil keeps turning in the same direction
The induced EMF and the current alternate because they repeatedly change direction
difference btwn motor and generator
A motor takes in electricity and turns it into motion.
A generator takes in motion and generates electricity.
Graphs for A.C. Generators
When the number of field lines through the coil is at a maximum, induced e.m.f. is at a minimum; number of field lines through the coil is at a maximum and induced e.m.f. is zero in this position
When the number of field lines through the coil is at a minimum, induced e.m.f. is at a maximum; no field lines pass through the centre of the coil
an alternating current can be produced by:
A coil rotating in a magnetic field
A magnet rotating within a coil
Both will induce an e.m.f. in the coil as they both ensure the coil will experience a changing magnetic field.
Magnetic Field Around Wires & Solenoids -
occurs whenβ¦
When a current flows through a conducting wire [any wire that has current flowing through it] a magnetic field is produced around the wire
no current = no magnetic field
-The shape and direction of the magnetic field can be investigated using plotting compasses. The compasses would produce a magnetic field lines pattern that would like look circles anticlockwise - made up of concentric circles
circular field pattern indicates thatβ¦
the magnetic field around a current-carrying wire has no poles
As the distance from the wire increasesβ¦
the circles get further apart
shows that the magnetic field is strongest closest to the wire and gets weaker as the distance from the wire increases
- Increasing the amount of current flowing through the wire = increases the strength of the magnetic field = field lines will become closer together
right-hand thumb rule can be used to work out the direction of the magnetic field around the wire
dot = current out, cross = current in
Reversing the direction in which the current flows through the wire will β¦
reverse the direction of the magnetic field
Magnetic Field Around a Solenoid
When a wire is looped into a coil, the magnetic field lines CIRCLE AROUND each part of the COIL , passing through the centre of it
- To INC. the STRENGTH of the magnetic field around the wire it should be COILED to form a SOLENOID
The magnetic field around the solenoid is similar to that of a bar magnet
magnetic field inside the solenoid is strong and uniform
HOW DO the ends of the solenoid behave?
[Look for a coil / solenoid - this is going to act as an electromagnet
Look for a piece of iron - this will be attracted to the solenoid]
One end of the solenoid behaves like the north pole of a magnet; the other side behaves like the south pole
To work out the polarity of each end of the solenoid it needs to be viewed from the end
If the current is travelling around in a clockwise direction then it is the south pole
If the current is travelling around in an anticlockwise direction then it is the north pole
If the current changes direction then the north and south poles will be reversed
If there is no current flowing through the wire then there will be no magnetic field produced around or through the solenoid
Magnetic Effects of Changing Current -
solenoid can be used as an electromagnet byβ¦
adding a soft iron core ;; solenoid wrapped around a soft iron core
iron core will become an induced magnet when current is flowing through the coils
magnetic field produced from the solenoid and the iron core will create a much stronger magnet overall
magnetic field produced by the electromagnet can be switched on and off
π When the current is flowing there will be a magnetic field produced around the electromagnet
π When the current is switched off there will be no magnetic field produced around the electromagnet
as alwaysβ¦ changing the direction of the current also changes β¦
the direction of the magnetic field produced by the iron core
Factors Affecting Magnetic Field Strength
π The strength of the magnetic field produced around a solenoid can be increased by:
Increasing the size of the current which is flowing through the wire
Increasing the number of coils
Adding an iron core through the centre of the coils
π The strength of an electromagnet can be changed by:
Increasing the current will increase the magnetic field produced around the electromagnet
Decreasing the current will decrease the magnetic field produced around the electromagnet
Applications of the Magnetic Effect
π Relay circuits (utilised in electric bells, electronic locks, scrapyard cranes etc)
π Loudspeakers & headphones
Electromagnets are commonly used in-
RELAY CIRCUITS
what are relays? what do relay circuits consist of?
relays are switches that open and close via the action of an electromagnet
- An electrical circuit containing an electromagnet
- A second circuit with a switch which is near to the electromagnet in the first circuit
how they are used
When a current flows through Circuit 1, a magnetic field is induced around the coil
The magnetic field attracts the switch, causing it to pivot and close the contacts in Circuit 2. This allows a current to flow in Circuit 2
When no current flows through Circuit 1, the magnetic force stops. The electromagnet stops attracting the switch. The current in Circuit 2 stops flowing
real life use
Scrapyard cranes utilise relay circuits to function:
When the electromagnet is switched on it will attract magnetic materials
When the electromagnet is switched off it will drop the magnetic materials
Electric bells also utilise relay circuits to function:
As the current alternates, the metal arm strikes the bell and drops repeatedly to produce the ringing effect
When the button K [right button] is pressed:
A current passes through the electromagnet E creating a magnetic field
This attracted the iron armature A, causing the hammer to strike the bell B
The movement of the armature breaks the circuit at T
This stops the current, destroying the magnetic field and so the armature returns to its previous position
This re-establishes the circuit, and the whole process starts again
Loudspeakers & Headphones
- convert electrical signals into sound
- work due to the motor effect
- loudspeaker consists of a coil of wire which is wrapped around one pole of a permanent magnet
- An alternating current passes through the coil of the loudspeaker, creates a changing magnetic field around the coil
As the current is constantly changing direction, the direction of the magnetic field will be constantly changing
The magnetic field produced around the coil interacts with the field from the permanent magnet
The interacting magnetic fields will exert a force on the coil. The direction of the force at any instant can be determined using Flemingβs left-hand rule
As the magnetic field is constantly changing direction, the force exerted on the coil will constantly change direction. This makes the coil oscillate
The oscillating coil causes the speaker cone to oscillate. This makes the air oscillate, creating sound waves
Investigating the Field Around a Wire - The magnetic field patterns due to currents in straight wires and in solenoids can be investigated using:
A thick wire
A solenoid (a wire wrapped into a coil) - for example, a metal slinky
Cell, ammeter, variable resistor and connecting wires
Cardboard with holes (the holes must be large enough for the wire to fit through)
Clamp stand
Iron filings or a compass
- Spread the iron filings uniformly on the cardboard and place the magnetic needle on the board
Tap the cardboard slightly and observe the orientation of iron filings
When the current direction is reversed, the compasses pointβ¦
point in the opposite direction showing that the direction of the ο¬eld reverses when the current reverses
Experiment 1: Plotting the magnetic field around a wire
π Attach the thick wire through a hole in the middle of the cardboard and secure it to the clamp stand
Secure the wire vertically so it sits perpendicularly to the cardboard
Attach the ends of the wire to a series circuit containing the variable resistor and ammeter on either side of the cell
ππ Using plotting compasses:
Place plotting compasses on the card and draw dots at each end of the needle once it settles
Make sure to draw an arrow to show the direction of the ο¬eld at different points
Move the compass so that it points away from the new dot, and repeat the process above
Keep repeating the previous process until there is a chain of dots on the card
Then remove the compass, or compasses, and link the dots using a smooth curve β this will be the magnetic field line
Repeat the whole process several times to create several other magnetic field lines
π ππ Using iron filings:
If using iron filings, simply pour the filings onto the cards and gently shake the card until the filings settle in the pattern of the magnetic field around the wire
Experiment 2: Plotting the magnetic field around a solenoid
π Attach the thick wire through a hole on one side of the cardboard and loop it through a hole on the other side of the cardboard and secure it to the clamp stand
Secure the wire so it forms a circular loop around the cardboard
Attach the ends of the wire to a series circuit containing the variable resistor and ammeter on either side of the cell
π π Using plotting compasses:
Follow the procedure outlined in Experiment 1
Note: this can be carried out using a solenoid, but since a solenoid is essentially many circular loops, the pattern around a circular loop can be extended to give the pattern around a solenoid
π π π Using iron filings and a solenoid:
Take a solenoid (a metal slinky works well for this) and thread it through pre-made holes in a piece of card
Pour the filings onto the card and gently shake the card until the filings settle in the pattern of the magnetic field around the solenoid
MOTOR EFFECT;; Force on a Current-Carrying Conductor (that produces its own magnetic field)
it will only experience a force whenβ¦
only experience a force if the current through it is perpendicular to the direction of the magnetic field lines
- current-carrying wire -> in a magnetic field -> experience force if wire perpendicular
direction of the FORCE is determined by FLEMINGβS LEFT HAND RULE
Two ways to reverse the direction of the force (and therefore, the copper rod) are by reversingβ¦
The direction of the current
The direction of the magnetic field
Left Hand Rule
direction of the force (aka the thrust) on a current carrying wire DEPENDS on the direction of the current and the direction of the magnetic fieldβ¦
all three PERPENDICULAR to each other
- point to field first
- find direction of current
- determine direction of force
π RMBR: magnetic field is always in the direction from North to South AND current is always in the direction of a positive terminal to a negative terminal.
Charged Particles in a Magnetic Field
current-carrying wire is placed in a magnetic field, it will experience a force if the wire is perpendicular BECAUSEβ¦
bc the magnetic field exerts a force on each individual electron flowing through the wire [THE ELECTRON; in the case of a electron in a magnetic field the second finger points in the opposite direction to the direction of motion
when a charged particle (an electron) passes through a magnetic field, the field can exert a force on the particle, causing it to be DEFLECTED BY THE FIELD
The force is always at 90 degrees to both the direction of travel and the magnetic field lines
particle is travelling _____, it experiences a _____ force
π If the particle is travelling perpendicular to the field lines:
It will experience the maximum force
π If the particle is travelling parallel to the field lines:
It will experience no force
π If the particle is travelling at an angle to the field lines:
It will experience a small force
Electric Motorsβ¦
motor effect can be used toβ¦
to create a simple d.c. electric motor. Simple dc motor consists: of a coil of wire (which is free to rotate) positioned in a uniform magnetic field
force on a current-carrying coil is used to make it rotate in a single direction
current flows through coilβ¦
As current flows through the coil, it produces a magnetic field which interacts with the uniform external magnetic field, so a force is exerted on the wire
Forces act in opposite directions on each side of the coil, causing a turning effect so it rotates [On the blue side of the coil, current travels towards the cell so the force acts upwards (using Flemingβs left-hand rule). On the black side, current flows away from the cell so the force acts downwards]
The greater the turning effect on the coil, the faster it will turn
The turning effect is increased by increasing:
The number of turns on the coil
The current in the coil
The strength of the magnetic field
when in a dc motor is a complete circuit with a cell formed
when the coil of wire is horizontal,
desc the setup
The coil is attached to a split ring (a circular tube of metal split in two) ;; split ring connects the coil to the flow of current
This split ring is connected in a circuit with the cell via contact with conducting carbon brushes
what happens when the coil has rotated 90 degrees
Once the coil has rotated 90Β°, the split ring is no longer in contact with the brushes
No current flows through the coil so no forces act
- No force acts on the coil when vertical, as the split ring is not in contact with the brushes
Even though no force acts,β¦
π the momentum of the coil causes the coil to continue to rotate slightly; current still flows anticlockwise and the forces still cause rotation in the same direction
π The split ring reconnects with the carbon brushes and current flows through the coil again; Now the blue side is on the right and the black side is on the left
π Current still flows toward the cell on the left and away from the cell on the right, even though the coil has flipped
π The black side of the coil experiences an upward force on the left and the blue side experiences a downward force on the right [FLIPS]
π The coil continues to rotate in the same direction, forming a continuously spinning motor
Factors Affecting the D.C Motor
π The speed at which the coil rotates can be increased by:
Increasing the current
Use a stronger magnet
π The direction of rotation of coil in the d.c. motor can be changed by:
Reversing the direction of the current supply
Reversing the direction of the magnetic field by reversing the poles of the magnet
π The force supplied by the motor can be increased by:
Increasing the current in the coil
Increasing the strength of the magnetic field
Adding more turns to the coil
dc motor set up - Determine whether the coil will be rotating clockwise or anticlockwise.
STEPS:
- Draw arrows to show the direction of the magnetic field lines - from the north pole of the magnet to the south pole of the magnet
- Draw arrows to show the direction the current is flowing in the coils. Current will flow from the positive terminal [LONG LINE ON LEFT] of the battery to the negative terminal
- Use Flemingβs left hand rule to determine the direction of the force on each side of the coil ; direction of the force on each side of the coil, these should be in opposite directions because the directions of the current through each side are opposite.
Start by pointing your First Finger in the direction of the (magnetic) Field
Now rotate your hand around the first finger so that the seCond finger points in the direction of the Current
The THumb will now be pointing in the direction of the THrust (the force)
- Use the force arrows to determine the direction of rotation
The coil will be turning clockwise
transformer
a device used to change the value of an alternating potential difference or current
using the generator effect
structure - a basic transformer consists of:
A primary coil [first] - copper (conductive, cost efficient)
A secondary coil
A soft IRON core [arrow pointing at top]
why is iron used
easily magnetised
Operation of a Transformer
π An alternating current is supplied to the primary coil.
π The current is continually changing direction. This means it will produce a changing magnetic field around the primary coil
π The iron core is easily magnetised, so the changing magnetic field passes through it
π As a result, there is now a changing magnetic field inside the secondary coil
π This changing field cuts through the secondary coil and induces a potential difference
π As the magnetic field is continually changing the potential difference induced will be alternating
π The alternating potential difference will have the same frequency as the alternating current supplied to the primary coil
π If the secondary coil is part of a complete circuit it will cause an alternating current to flow
step-up transformer does what?
π increases the potential difference of a power source
A step-up transformer has more turns on the secondary coil than on the primary coil (Ns > Np)
step-down transformerβ¦
π decreases the potential difference of a power source
A step-down transformer has fewer turns on the secondary coil than on the primary coil (Ns < Np)
output potential difference (voltage) of a transformer depends on:
number of turns on the primary and secondary coils
The input potential difference (voltage)
OUTPUT POTENTIAL DIFFERENCE of a TRANSFORMER
π EQUATION
potential difference across primary coil / potential difference across secondary coil = number of turns on primary coil / number of turns on secondary coil
Vp / Vs = Np / Ns
- can be flipped to have secondary on top
- shows that ratio of the potential differences across the primary and secondary coils of a transformer is equal to the ratio of the number of turns on each coil
example ques
an input voltage of 10V supplied to primary coil. output voltage of 40V produced across secondary coil. 10V supply at primary coil now replaced with 40V.
new output voltage across secondary coil =?
ratio of primary to secondary turns
10/40 = 0.25.
input voltage changes but number of turns on primary coil/secondary stays the SAME
40 / Vs = 0.25
Vs = 40/0.25 = 160V
2nd example ques
output current =?
power =?
input current =0.050A
input power = I x V = 0.050 x 230 = 11.5W
input power = output power
11.5W = output power.
P out = Is x Vs
Vs = 6.0V output.
11.5 = Is x 6
Is = 1.9A
NOTE to rmbr
individual loops of wire going around each side of the transformer should be referred to as turns and not coils.
Transformer Efficiency - ideally, 100% efficient, meaning input power = output power
limitation of them isβ¦ bcβ¦
although transformers can increase the voltage of a power source,
due to the law of conservation of energy, they cannot increase the power output
if a transformer is 100% efficient then:
EQN
Vp Γ Ip = Vs Γ Is
ALSO: eqn
Ps = Vp Γ Ip
[Ps = output power (power produced in secondary coil) in Watts (W)]
HOW TO PARSE QUES:
A transformer in a travel adapter steps up a 115 V ac mains electricity supply to the 230 V needed for a hair dryer. A current of 5 A flows through the hairdryer.
Voltage in primary coil, Vp = 115 V
Voltage in secondary coil, Vs = 230 V
Current in secondary coil, Is = 5 A
advantages of High Voltage Transmission
;; Electricity is transmitted at high voltage, reducing the current and hence power loss in the cables
makes the transfer of electrical energy through the wires more efficient
When electricity is transmitted over large distances, the current in the wires heats them, resulting in energy loss
To transmit the same amount of power as the input power, the potential difference at which the electricity is transmitted should be increased
so a smaller current is being transmitted through the power lines
bc P = IV, so if V increases, I must decrease to transmit the same power
π A smaller current flowing through the power lines results in less heat being produced in the wire so REDUCES THE ENERGY LOSS IN THE POWER LINES
Calculating Power Losses
eqns
P = I^2R
E = P x t [[the power is the energy lost per second, the total energy lost in a time t ]]
Vs/Vp = Ns/Np
IpVp = IsVs
P = I2R.
The power (energy per second) lost
When a current passes through a wire, the current creates a heating effect which means the wires warm up
This means they lose electrical energy as heat which reduced the efficiency of the transformer
This is due to electrical resistance
βfaradayβs lawβ
- Whenever a conductor is placed in a varying magnetic field, an electromotive force is induced. If the conductor circuit is closed, a current is induced, which is called induced current.
-
simple summary of
- generator effect
- motor effect
- generator effect: the creation of a potential difference (voltage), therefore an induced current (flowing electrons) when a wire experiences a changing magnetic field; occurs when a coiled wire is wrapped around a magnet.
- motor effect: A wire carrying a current creates a magnetic field. This can interact with another magnetic field, causing a force that pushes the wire at right angles;
putting a current-carrying wire between the poles of a horseshoe magnet; deflect
relay
A relay is made of an electromagnet coil and a magnetic switch. I
If a current flows through a coil, a magnetic field will be produced. This can attract a switch in a separate circuit, causing it to flow and allowing a current to pass through the circuit.
in a generator, MECHANICAL energy is used to rotate a magnet inside a set of stationary windings of wire. As the magnet spins, it changes the magnetic field inside the wires.
Changing magnetic field induces a flow of electric current in the wire
electromagnetic induction
a current produced because of voltage production (electromotive force) due to a changing magnetic field.
magnetic flux
a measure of the number of field lines passing through a region of space.
State and explain the effect of this change [resistance increasing] on the power of the lamp.
Power (of lamp) decreases
P = IV and current in lamp decreases. OR P = V2 /R
State the name and purpose of component X.
variable resistor AND
control or limit the voltage across heater
Some time after the heater is switched on, the ammeter reading is seen to have decreased.
Suggest why this happens. [2]
π βEnergy of battery used up. Resistance (of heater) increases. Current decreases.β
[battery going flat / energy of battery used up OR V less OR more/increased resistance (of heater)
use of relationship between I and V or R OR the current decreases]
The temperature of the thermistor is increased so that its resistance decreases.
State the effect of this change in resistance on the current through the 500Ξ© resistor.
Explain your answer. [2]
βCurrent increases. Total resistance (of circuit) decreases.β
π (more current in circuit so) current (in 500β¦ resistor) increases
π resistance of PARALLEL COMBINATION decreases
OR total resistance (of circuit) decreases
A variable resistor is included so that the current in the circuit may be changed.
Suggest an advantage of being able to change the current [1]
different results OR graph can be plotted
OR to ensure wire does not overheat
- aluminium cable replaced with a new aluminium cable of the same length. Current remains at 250 A.
- diameter of the new cable is double the diameter of the original cable.
[DIAMETER DOUBLES = POWER LOST DECREASES TO 1/4 OF POWER in kW]
State and explain how the power loss is affected by this change. [3]
- POWER loss reduced
- resistance decreases/REDUCED
- power lost decreases to a quarter OR (P =)19kW / 18.75 kW [75 x 0.25 = 18.75kW]
π WRITE IN kW AND WATTS IN SIMPLE ANSWER QUES. π
thermistor - heat detector
transistor - switch
In Fig. 9.1, lamp A is not glowing brightly.
Suggest and explain what could be done to component B to make lamp A glow brightly. [4]
π βIncrease light intensity of B. Resistance of B decreases. Greater share of voltage. More current flows through lamp.β π
increase light intensity / brightness of B
resistance (of B) decreases [cao]
voltage at mid-point increases OR greater (share of) voltage
(more) current flows (through lamp)
State what is meant by the direction of an electric field.
direction of the force on a positive charge
- by an electric field.
A region in which a force acts upon an (electric) charge/charged object
Suggest, in terms of forces, why the oil drop does not move up or down. [2]
Upward force (on drop) due to electric field/ charge on plates = weight of drop
Upward force on drop = downward force on drop. no resultant force on drop
Without losing any of its charge, the oil drop begins to evaporate.
State and explain what happens to the oil drop. [2]
Drop moves upwards
Weight/mass of drop decreases OR downward force decreases
Describe what happens to the charges in the metal paint on the ball as the positively charged
rod is brought close to the ball.
electrons / negative charges move towards the rod
The ball is attracted towards the charged rod.
Explain why this happens, given that the ball is uncharged.
negative charges (are) close(r) (to the rod)
attraction between opposite charges greater than repulsion between like charges
e.m.f what is meant
energy transferred per coulomb
drawing -
Straight parallel lines from upper to lower plate B1
B1
At least 3 lines drawn. All lines drawn equally spaced,
approximately symmetrical with respect to plates
Arrows downwards [3]
++ at least three horizontal, parallel lines evenly spaced (ignore edge effects) B1
arrows pointing left to right
+++ right hand half of ball has more + signs than β signs
AND left hand half of ball has more β signs than + signs
equal numbers of + and β signs
πππ negative charge(s) on left [bcoppositecharges] AND positive charge(s) on right M1
equal number of positive and negative charges AND number of each β€ 7
An earthed metal wire is touched against the smaller metal sphere.
State and explain what happens to the charge on the smaller sphere.
electrons (/ negative charges) flow from Earth/ on to sphere
total charge negative OR (some) protons / positive charges cancelled
Explain, in terms of their structures, why the metal wire is an electrical conductor but the
plastic stand is an electrical insulator.
metal contains free (delocalised) electrons
electrons in plastic not free to move
Fig. 7.2 shows a metal sphere on an insulating support.
A student has available two rods, one charged positively and one charged negatively. Using one of these rods, she gives the sphere a uniform negative charge by induction.
State which rod she chooses, and describe the procedure she follows. [4]
π Use positively charged rod
π Place rod close to surface of sphere
[Touch sphere (briefly) with finger /] π Briefly connect sphere to earth
π Remove charged rod
State the direction of the force exerted on the vertical rod. [2]
to the right or left
to the right
State what happens to the ammeter reading if the 1 Ξ© resistor is replaced by a 3 Ξ© resistor.
ammeter reading reduces
) Suggest why B is connected to a relay, rather than directly to the lamp.
B cannot provide enough power / current for lamp, or equivalent
OR allows remote lamp
π The safety expert recommends that normal domestic light switches, as shown in Fig. 9.1,
are replaced.
Explain why this recommendation is made.
π Suggest how the lights should be switched on and off.
π current could flow through switch due to dampness. water (good) conductor
danger of electrocution
π pull switch with long cord of insulating material
OR normal switch outside workroom
Suggest how fuel flowing through the hose can cause a large build-up of electric
charge on the aircraft.
friction with hose
charge moved to aircraft from hose
[or rubber insulates]
The aircraft is refuelled on a particular day when the tyres and wheels are wet.
Explain why there will be no large build-up of charge in this case.
(water conducts) charge away/to ground ;
OR owtte: earthing ; [of case/outside]
Explain what the hazard might be if the heater is not earthed.
electrocution
live wire touches case
A change to the environment around component B (thermistor) causes component A (light-emitting diode) to emit light.
State the environmental change.
ii) Explain your answer to (i)
temperature (decreases)
ii)
change of resistance causes a change of temperature, they both increase
voltage of left of LED increases πOR πhigher voltage across thermistor
current flows through to light LED, enough voltage
wire (current) downwards, into the page
direction of magnetic field? at right wire
force on right wire?
MAGNETIC field -> DOWN, βtowards the bottom of the pageβ
force - left
RMBR HOW TO USE LEFT HAND RULE FOR FORCE.
State and explain whether there is also a force on wire X.
There is a force on X
due to the (magnetic) field of Y
OR the (magnetic) fields due to X and Y interacting
When switch S is first closed, the needle of the galvanometer deflects briefly, then returns to
zero.
Explain why the brief deflection occurs
Change in current is brief, occurs as switch closes
Changing magnetic field links with secondary coil
field lines cut coil
Causes induced voltage and thus current
i) Explain the final position of the brass pointer.
(ii) Explain the final position of the magnet.
i) no forces or effect on needle
ii) needle aligns with field
[N or S pole attracted along field line]
Using your knowledge of investigating the magnetic field around a bar magnet,
suggest an experiment or experiments to confirm that you have drawn the correct pattern and direction in (a). [4]
use of compass/suspended small magnet
observe needle/magnet on one field line
observe needle/magnet on another field line
mark on card OR needle/magnet shows direction of field
πOORRR;;
(sprinkle) iron filings
tap card
alignment of iron filings show field
use compass to show field direction
A second current-carrying wire is inserted vertically through the card at Y.
Suggest why there is now a force on the wire at X. [2]
wire X is in a magnetic field
current carrying conductor in field interact
On Fig. 10.1, draw lines to show the pattern of the magnetic field due to the current.
Fig shows a variable resistor (rheostat) and a solenoid (long coil) connected to a
battery. [2]
at least two vertical lines inside the coil
at least four lines (two left, two right) outside the coil of correct shape
State the feature of the pattern of the magnetic field lines that indicates the strength
of the magnetic field at particular points.
lines closer where field is stronger
State and explain the effect on the magnetic field of increasing the resistance of the
variable resistor.
reduces (strength of) field
(increasing the resistance) reduces the current
State and explain the effect on this path of reversing the current in the coils. [2]
curves in opposite direction to (c)(i)
magnetic field reversed
The student drops the magnet so that it falls through the solenoid.
State and explain what would be observed on the milliammeter
(i) as the magnet enters the solenoid, [2]
(ii) as the magnet speeds up inside the solenoid [2]
i) π (milliammeter) deflects/shows reading [there is a current]
π field lines cut / emf induced
ii) greater deflection/current
rate of change of flux (linkage) is greater. more magnetic field lines cutting coil (per second)
As the magnet passes into the coil in part (a), the coil exerts a force on the magnet even though there is no contact between them.
(i) State the direction of this force.
(ii) Explain how this force is caused. [3]
i) UPWARDS /opposite to magnetβs direction of travel
ii) current (in coil) causes a magnetic field
force caused by overlapping (magnetic) fields
When XY is moved slowly upwards the needle of the voltmeter shows a small deflection.
(i) State how XY must be moved to produce a larger deflection in the opposite
direction.
(ii) XY is now rotated about its central point by raising X and lowering Y. Explain why no deflection is observed.
i) In the opposite direction
Faster
ii) No voltage/current induced
Currents (induced) in each half of XY are equal and in opposite directions / oppose each other
lower end of the magnet is pushed down into the upper end of the coil and
held at rest.
During the movement, an e.m.f. is induced in the coil. The meter shows a deflection to the right and then returns to zero.
Explain why this e.m.f. is induced.
(magnetic) field (lines) of magnet cut by turns / coil
State what happens to the needle of the meter when
- the magnet is released from rest and is pulled up by the spring, [1]
- the magnet continues to oscillate up and down, moving in and out of the coil with each oscillation. [1]
- (needle of meter) deflects to the left (and returns to zero)
- deflects to right and left (alternately)
magnet is released and it falls through the solenoid. During the initial stage of the fall, the sensitive ammeter shows a small deflection to the left.
(a) Explain why the ammeter shows a deflection.
(magnetic) field (lines) of magnet cuts coils (of solenoid)
magnet passes the middle point of the solenoid and continues to fall. It reaches
position Y.
Describe and explain what is observed on the ammeter as the magnet falls from the middle point of the solenoid to position Y. [4]
meter deflects in opposite direction
deflection is greater (than initially) OR for shorter time
magnet moving faster
more field lines cut per second
Suggest two changes to the apparatus that would increase the initial deflection of the ammeter. [2]
stronger magnet
- use a solenoid (of same length) with more turns
Explain why there is a current in the lamp. [4]
(alternating current causes) magnetic field in core
magnetic field changes
field cuts secondary coil
e.m.f. induced (and current flows in lamp)
output of an a.c. generator in a power station is 5000 V. A transformer increases the voltage to 115 000 V before the electrical power is transmitted to a
distant town.
(a) State and explain, using a relevant equation, one advantage of transmitting electrical power
at a high voltage. [3]
P = I^2 R
less energy lost
smaller current
The transformer contains two coils, the primary coil and the secondary coil.
(i) State the other main component of a transformer and the material from which it is made.
ii) State the component in the transformer to which the a.c. generator is connected.
c) Transformers within the town reduce the voltage to 230 V.
Suggest one reason for this.
i) iron core
ii) connected to primary coil
c) safer OR less insulation needed
i) Explain why the wheel turns when the switch is closed.
ii) Describe how the split-ring commutator on an electric motor works
i) current in spoke in magnetic field
causes force on spoke/wheel
ii) brushes connect to other split ring every half turn/coil vertical
reverses direction of current every half turn/coil vertical
Explain what is meant by electromagnetic induction.
when magnetic field cuts by coil
current/e.m.f caused
A cathode ray is a beam of electrons.
Suggest one way of controlling the number of electrons in the beam.
change current in filament/cathode/heater
