P15 Electromagnetism Flashcards
not including transformers
what is a permanent magnet?
a material that produces its own magnetic field
what is an induced magnet?
a material that has become a magnet after being placed in a magnetic field
why are magnetic field lines concentrated at the poles of a magnet?
at the poles, magnetic field is strongest
what is the magnetic field of an object?
the region around a magnet in which a force acts on another magnet or magnetic material
what does the strength of a magnetic field depend on?
distance to the magnet
what is the direction of magnetic field lines of any magnet?
from the north to south pole
why is steel used instead of iron to make permanent magnets?
steel does not lose its magnetism so easily
describe the magnetic field around a wire through which electric current flows
concentric circles
centred on the wire
on a plane perpendicular to the wire
what is a solenoid?
a long coil of insulated wire
where are solenoids used?
where a strong magnetic field needs to be produced
when is there a magnetic field around a wire?
when electric current flows through it
if the current increases in a solenoid
then the magnetic field…
if the current increases in a solenoid
then the magnetic field INCREASES IN STRENGTH
if the current in a solenoid is reversed
then the magnetic field…
if the current in a solenoid is reversed
then the magnetic field REVERSES ITS DIRECTION
describe the magnetic field inside a solenoid with electric current flowing through it
strong and uniform magnetic field
with field lines running parallel to the solenoid’s axis
describe the magnetic field outside a solenoid with electric current flowing through it
field lines bend in a complete loop from one end to another
passing through the inside of the solenoid itself
what is an electromagnet?
a solenoid in which the insulated wire is wrapped around an iron bar (the core_.
what happens to the core of an electromagnet when current flows through its wire?
current flows through wire
creating a magnetic field around the wire
which magnetises the iron bar/core
what happens to the core of an electromagnet when current stops flowing through its wire?
the iron bar (core) loses most of its magnetism
why does an electromagnet contain an iron bar?
it increases the strength of the solenoid’s magnetic field once magnetised.
give 3 examples of items which use electromagnets
electric bell
scrapyard crane
circuit breaker
what is the motor effect?
when a conductor carrying an electric current
is placed in the magnetic field produced by a magnet
causing the magnet and conductor to exert a force onto each other.
how to increase the size of a force caused by the motor effect
- increase the current,
- increase the strength of the magnet.
what angle between the wire and magnetic field make the force greatest?
- force is greatest when wire is perpendicular to magnetic field,
- force is zero when wire is parallel to magnetic field lines.
what happens to the direction of force due to the motor effect when the direction of:
- current is reversed,
- magnetic field is reversed
when the magnetic field or current’s direction reverses
the direction of force also reverses
what does each letter stand for:
F = BIL
force = magnetic flux density x current x length of wire
N tesla, T A m
what does magnetic flux density measure
the strength of a magnetic field
describe the basis of an electric motor
a coil of wire rotates when current passes through it, in a magnetic field.
why does the coil turn in an electric motor
- current passes through rectangular coil of insulated wire,
- causing a force on each side of the coil due to the motor effect,
- in opposite directions to each other,
- so the coil rotates.
how is the coil connected to the battery
and split ring communtator
in an electric motor
- coil connected to battery by metal/graphite brushes,
- which conducts electricity,
- brushes press onto metal split-ring commutator,
- fixed to the coil,
- with little friction due to the metal/graphite material of brushes.
purpose of the split-ring commutator in an electric motor
- reverses the direction of the current around the coil every half turn,
- pushing the coil in the same direction for every half turn.
what causes electromagnetic induction?
when the ends of a conductor (e.g. wire) pass into a magnetic field
due to the conductor or the magnetic field being moved
inducing potential difference.
what is the generator effect?
when the ends of a wire WHICH IS PART OF A COMPLETE CIRCUIT pass into magnetic field lines,
due to the conductor or magnetic field being moved,
inducing potential difference and inducing ELECTRIC CURRENT which passes through the circuit.
what is the motor effect?
when a conductor carrying an electric current passes into the magnetic field produced by a magnet, causing the conductor and magnet to exert a force onto each other.
how to increase the induced potential difference due to the generator effect
make the conductor pass through the magnetic field lines of the magnet faster
what is the direction of potential difference induced due to the generator effect
potential difference induced due to the generator effect is in the opposite direction of the change that caused
what is the purpose of an alternator
to generate alternating current using the generator effect
describe how an alternator works
- there is a rectangular coil of wire,
- connected to 2 commutators which allow current to pass out the coil
- the coil rotates in a magnetic field,
- which, due to the generator effect, induces a potential difference and current,
- both of which are alternating because the 2 sides of the coil are attached to 2 different rings.
what is the purpose of a dynamo
to generate direct current using the generator effect
describe how a dynamo works
- coil of wire rotates, passing through magnetic field lines twice per full rotation,
- inducing a potential difference and current due to the generator effect,
- the direction of current and potential difference do not reverse as the coil rotates therefore they are direct.
describe how a moving coil microphone works
- coil of wire rests on a diaphragm,
- sound waves hit the diaphragm, making it vibrate,
- making the coil of wire move in and out of the magnetic field,
- inducing potential difference across the ends of the wire due to the generator effect,
- the changing pattern of potential difference passes through an amplifier,
- then into a moving coil loudspeaker which increases the volume of the sound waves.
