Electricity and Magnetism Flashcards
What is magnetism?
the force exerted by magnets when they attract or repel each other
What happens to magnets when they are free to move?
they will line up between the Earth’s magnetic poles
What two materials are magnetic material?
Iron and steel
How do you call iron and steel?
ferrous
How do you call all other metals which are non-magnetic?
non-ferrous
Is IRON magnetically “soft” or “hard”?
magnetically “soft”
What does it mean for something to be magnetically “soft”?
to lose its magnetism easily: easy to magnetise/demagnetise
What does it mean for something to be magnetically “hard”?
to retain its magnetism: hard to magnetise/demagnetise
What are permanent magnets made from?
steel
Why are permanent magnets made from steel?
it is magnetically “hard”, meaning it retains its magnetism
How can you make a temporary magnet?
putting it close to a permanent magnet (induced magnetism)
What is induced magnetism?
making something into a magnet by putting it close to a permanent magnet
What are TWO ways in which a magnet may be demagnetised, without using a coil?
- heating
- hitting with a hammer
What will happen to a magnet if it is heated?
it will be demagnetised
What will happen to a magnet if it is hit with a hammer?
it will be demagnetised
What is another way a magnet can be demagnetised, using a.c current?
place magnet inside a coil, using a.c current, then slowly remove magnet
How can you test for a (permanent) magnet? Why is this the only sure test?
repulsion: temporary magnets (magnetic material) can attract only
What is a temporary magnet?
a magnet with no permanent N-S poles, only magnetised when there is a permanent magnet
What is a magnetic field?
a region where a magnetic pole experiences a force
How can you show a relatively stronger magnetic field in a drawing?
field lines closer together
How can you make a permanent magnet?
- place a steel rod inside a coil
- use high DC current for a time
- leave steel inside, turn off current
What are the differences between a permanent magnet and an electromagnet?
- permanent magnet has a constant magnetic field, electromagnet variable magnetic field strength
- permanent magnet cannot be switched on/off, electromagnet can quickly do so
- permanent magnet cannot switch N and S poles, electromagnet can change N and S
How can the direction of magnetic field in an electromagnet be reversed?
changing direction of current flow
What are uses of permanent magnets?
guitar pickups, speakers, cupboard latches
What are uses of electromagnets?
electric bells, loudspeakers, electric door locks, relays, MRI machines
How can you make an electromagnet?
wrap coil around an iron nail, connect to a cell and turn the current on
How can you test the strength of an electromagnet?
- set up circuit with coil surrounding iron nail and a variable resistor.
- change current using variable resistor, and change number of turns of wire
- see how many paper clips it picks up
How can you increase the strength of an electromagnet?
- use iron core
- more current
- more turns of wire (coil)
What material is used for an electromagnet?
a soft magnetic material
Are hard magnetic materials easy to magnetise/demagnetise or not?
difficult
What happens when a permanent magnet attracts a magnetic material?
it induces a magnetic field in the material
Can magnetic materials attract each other?
only when a permanent magnet is present
What is a permanent magnet?
a magnet that always has a magnetic field, can induce magnetic field in magnetic material
What causes magnetic forces?
interactions between magnetic fields
What is the direction of a magnetic field at a certain point?
direction of the force on the N pole of a magnet at that point
What are the differences between a temporary and a permanent magnet?
- a temporary magnet can ONLY attract, not repel
- a permanent magnet is harder to magnetize
- a temporary magnet is only a magnet when near a magnet
- a permanent magnet has its own, permanent magnetic field
What is the difference between magnetic and non-magnetic materials?
magnetic materials will be attracted to magnets, non-magnetic materials will not
How can you plot magnetic field lines?
using a compass or iron filings
How can you determine the direction of a magnetic field?
with a compass
What is a compass useful for?
determining the direction of a magnetic field
what does charging of solids by friction involve
transfer of negative charge (electrons)
Unit for charge?
Coulombs
What is an electric field?
a region in which an electric charge experiences a force
what is the direction of an electric field at a point
direction of force on a positive charge at that point
What is electric current related to?
flow of charge
Why are metals good electrical conductors?
movement of free electrons
What is electric current?
the charge passing a point per unit time
Formula for charge?
What is conventional current? How is this different from the actual flow of free electrons?
conventional current - positive to negative
flow of free electrons - negative to positive
Define ‘electromotive force’.
the electrical work done by a source in moving a unit charge around a complete circuit
What is e.m.f (electromotive force) measured in?
volts (V)
Define ‘potential difference’.
the work done by a unit charge passing through a component
What is the p.d between two points measured in?
volts (V)
What does an electric circuit do?
transfer energy from a source of electrical energy (eg electrical cell) to the circuit components and then into the surroundings
What always have magnetic fields around them?
current carrying conductors or moving electrons
What is the pattern of a magnetic field in straight wires?
circular and perpendicular to the wire, getting weaker with distance
How can you find the direction of a magnetic field in straight wires?
Right hand grip rule n1: your thumb is conventional current, and the rest of your fingers are the magnetic field direction
How do you know if the conventional current is going into or out of the paper?
cross: current into the page
dot: current out of the page
In a single wire, how can you increase the strength of the magnetic field?
greater current
What is the direction/pattern of the magnetic field in a solenoid?
the same as in a bar magnet
What is the relationship between current and direction of magnetic field on a solenoid?
the general direction of the current flow indicates the direction of the magnetic field
How can you find out the direction of the magnetic field in a solenoid?
right hand grip rule n2: your thumb points to the the north pole, the rest of your fingers are conventional current
How can you reverse magnetic field direction in a solenoid?
- reverse current/cell
- wrap wire in opposite direction
How can you increase the strength of the magnet in a solenoid?
- more turns of coil
- more current
- wrap wire on an iron core
Describe an experiment to identify the pattern/direction of a magnetic field in straight wires and solenoids.
- use a compass to determine the direction of a field at a single point
- draw an arrow in the direction of the field
- move the compass to different points and repeat
- you will have a diagram of the magnetic field
What is a relay?
electromagnetic switch, a circuit with a small current turns on another circuit with a large current
How does a relay work?
- switch is closed
- small current magnetised the coil
- iron reeds are attracted and close
- large current flows
What is the energy conversion in a loudspeaker?
it converts electrical energy into sound energy
How does a loudspeaker work?
- alternating current in coil creates an alternating magnetic field in coil
- this alternating magnetic field will interact with the permanent magnetic field
- this interaction will make the papercone move in/out producing sound waves
If you use high frequency ac in a loudspeaker, what will happen?
high frequency sound waves
If you use large voltage ac in a loudspeaker, what will happen?
loud sound waves
Why are relays useful?
the turn on larger current circuit by turning on a smaller current circuit: this way, we do not need to use thick, heavy, expensive cables to carry the large current
What happens if you supply a solenoid with AC?
the current will change, so the direction of the magnetic field will also change every half cycle
What is the variation of magnetic field strength in straight wires?
strongest closest to wire
What is the variation of magnetic field strength in solenoids?
uniform magnetic field inside coil
What happens if you increase the size of the current in a straight wire?
results in a stronger magnetic field
What happens if you increase the size of the current in a solenoid?
results in a stronger magnetic field
What happens if you reverse the current in a solenoid?
reverses magnetic field direction
What happens if you reverse the current in a straight wire?
reverses magnetic field direction
What happens whenever a current is at a right angle to a magnetic field?
it experiences a force, perpendicular to both the current and magnetic field
Why is a force experienced when a current carrying conductor is in a magnetic field?
interaction between the current’s magnetic field and the permanent magnetic field
How can we work out the direction of the force when there is a current carrying conductor in a magnetic field?
Fleming’s left hand rule:
First finger is magnetic field (N to S)
Second finger is conventional current
Thumb is the thrust/force
How can you reverse the force direction (current carrying conductor in a magnetic field)?
reverse current or reverse magnetic field direction
How can you increase the strength of the force (current carrying conductor in a magnetic field)?
stronger magnets, magnets closer together or increase current
What happens if you reverse the current in a current-carrying conductor in a magnetic field?
you reverse the force
What happens if you reverse the direction of the magnetic field in a current-carrying conductor in a magnetic field?
you reverse the force
How do you make a permanent magnet?
- place steel rod inside coil
- use high dc current for a time
- leave steel inside, turn off current
How can you demagnetise a magnet?
- place magnet inside coil
- use ac current
- slowly remove magnet with ac current still on
Describe an experiment to show the force in a current-carrying conductor in a magnetic field.
- Place a wire carrying current in between two permanent magnets
- turn on power supply
- observe force on wire
Will any moving charged particle in a magnetic field experience a force?
yes
Determine the direction of the force on charged particles in a magnetic field when the field is into the page.
positive particle: up
negative particle: down
Determine the direction of the current on charged particles in a magnetic field when the field is into the page.
positive particle: left to right
negative particle: right to left
What is the basic principle of a d.c motor?
a current + a magnetic field will create a force
How does a d.c motor work?
the sides of the coil have a magnetic field, which interacts with permanent magnetic field to produce a force
How can we identify the force direction in a d.c motor?
Fleming’s left hand rule: first finger is field, second finger is current and thumb is thrust
What causes the turning effect in a d.c motor?
a current-carrying coil in a magnetic field
Why do we have split-ring commutators on a d.c motor?
to reverse the direction of the current every half turn so that the coil rotates in the same direction
What would happen without split-rings?
the coil would flip backwards and forwards (the changing direction of current caused by the coil rotating through 180 would cause this)
Why do we need (carbon) brushes in d.c motors? Why are they good for this?
connect commutator to the circuit: low friction, good conductors
What do split rings do to the current?
reverse the direction of the current every half turn
What do brushes ensure?
that the wires don’t get twisted
How can the turning effect in a d/c motor be increased?
- increasing number of turns on coil
- increasing current
- increasing strength of magnetic field
What may happen if a conductor moves rapidly across a magnetic field?
creates induced emf or current or p.d
What is the basic principle of electromagnetic induction?
movement of a conductor + magnetism -> induced emf/current/pd
What would happen if a conductor cutting a magnetic field is not moving?
there will be no induced current
When is there a maximum of induced current in electromagnetic induction?
when the conductor is at 90 degrees to magnetic fields
What happens when a conductor is moving parallel to a magnetic field?
there will be no induced current
What are two situations in electromagnetic induction when there will be no induced current?
- when there is no movement
- when the conductor is moving parallel to the magnetic field
How can you reverse the current direction in electromagnetic induction?
- reverse wire movement direction
- reverse magnetic field direction
How can you increase the size of induced current in electromagnetic induction?
- stronger magnetic field
- move wire faster
- use a coil of wire
Describe an experiment to demonstrate electromagnetic induction.
- connect a wire to an ammeter or a galvanometer
- place two permanent magnets on either side of the wire
- move the wire
- watch what happens to ammeter/galvanometer
What is a galvanometer?
a very sensitive ammeter
Why does electromagnetic induction work?
the conductor cuts the magnetic field lines, forcing electrons in the wire to move as a current
How do we identify the induced current direction in electromagnetic induction?
Fleming’s right hand rule: first finger is field, second finger is current and thumb is thrust
What are two ways to induce an e.m.f (electromagnetic induction)?
- moving a magnet so its field lines are cut by a wire
- moving a wire across a magnetic field so that it cuts it
What is Lenz’s Law?
the direction of the induced emf (electromagnetic induction) opposes the change that creates it
How does the a.c generator work?
- coil made to rotate
- sides of coil cut magnetic field lines, inducing emf
- emf changes direction every half turn, because for half a turn one side moves up and for the other half turn the same side moves down
- the emf changes size because there is a max. induced emf when the coil is perpendicular
What is the simple idea behind ac generators?
movement of coil + magnetic field -> induced emf/current
How is the graph of induced emf vs time of an ac motor?
it goes up and down
What does a peak in an induced emf vs time graph of an ac motor mean?
long sides of coil are perpendicular to magnetic field, maximum emf is induced
What do the zeros in an induced emf vs time graph of an ac motor mean?
top and bottom sides of coil are parallel to magnetic field, so no emf is induced
What does a trough in an induced emf vs time graph of an ac motor mean?
long sides of coil move perpendicular to field in opposite direction: induced emf is maximum but in opposite direction
What is the function of slip rings in ac generators?
allow direction of induced emf to alternate, ergo causing an alternating current
connect coil to brush
What are slip rings?
cylindrical conductors that make constant contact with the coil during rotation
What is the function of carbon brushes in ac generators?
make electrical connection between rotating coil and circuit, avoiding wires becoming twisted
What does a transformer do?
increases or decreases alternating p.d
Describe the construction of a simple transformer.
a primary circuit, including the a.c power pack, with a primary coil attached to a soft iron core.
a secondary circuit with a secondary coil attached to the soft iron core.
How does a transformer work?
- alternating p.d/current in primary circuit
- produces an alternating magnetic field in primary coil
- alternating magnetic field travels in iron core
- secondary coil cuts the alternating magnetic field
- induces an alternating p/d/current in secondary circuit
What does a step up transformer do?
increases voltage (in secondary circuit), decreases secondary current
What does a step down transformer do?
decreases voltage (in secondary circuit), increases secondary current
What does Np mean?
number of turns on primary coil
What does Ns mean?
number of turns on secondary coil
What does Vp mean?
voltage applied to primary coil
What does Vs mean?
voltage induced on secondary coil
What does Ip mean?
current in primary coil
What does Is mean?
current in secondary coil
What is the physical difference between a step up and step down transformer?
step up: more turns on secondary coil
step up: less turns on secondary coil
How can you find out a missing number of turns in the primary/secondary coil?
Np/Ns = Vp/Vs
Vp/Vs =
Np/Ns
Primary power =
secondary power
Why is primary power = secondary power?
principle of conservation of energy
Power =
IV
Rate of heat energy loss from wires of resistance R:
P = Iˆ2 x R
How are transformers used in transmission of electricity?
- generator carries voltage to step up transformer
- wire will carry a high voltage and a low current, resulting in low heat loss
- it will then be brought to a step down transformer so the p.d can be used
State the advantages of high-voltage transmission.
- low heat loss (low current)
- most of electrical energy reaches destination
- thin, cheaper, less heavy cables can be used
Np/Ns =
Vp/Vs
IpVp =
IsVs (100% efficiency)
Power input = power output
IpVp = IsVs
Why are power losses in cable smaller when voltage is greater?
P = I^2 x R (where R is rate of heat energy loss)
Power input =
power output
Power =
current x voltage
