P15 Electromagnetism

Question 1

what is a permanent magnet?

Answer 1

a material that produces its own magnetic field

Question 2

what is an induced magnet?

Answer 2

a material that has become a magnet after being placed in a magnetic field

Question 3

why are magnetic field lines concentrated at the poles of a magnet?

Answer 3

at the poles, magnetic field is strongest

Question 4

what is the magnetic field of an object?

Answer 4

the region around a magnet in which a force acts on another magnet or magnetic material

Question 5

what does the strength of a magnetic field depend on?

Answer 5

distance to the magnet

Question 6

what is the direction of magnetic field lines of any magnet?

Answer 6

from the north to south pole

Question 6

why is steel used instead of iron to make permanent magnets?

Answer 6

steel does not lose its magnetism so easily

Question 7

describe the magnetic field around a wire through which electric current flows

Answer 7

concentric circles
centred on the wire
on a plane perpendicular to the wire

Question 8

what is a solenoid?

Answer 8

a long coil of insulated wire

Question 9

where are solenoids used?

Answer 9

where a strong magnetic field needs to be produced

Question 10

when is there a magnetic field around a wire?

Answer 10

when electric current flows through it

Question 11

if the current increases in a solenoid
then the magnetic field…

Answer 11

if the current increases in a solenoid
then the magnetic field INCREASES IN STRENGTH

Question 12

if the current in a solenoid is reversed
then the magnetic field…

Answer 12

if the current in a solenoid is reversed
then the magnetic field REVERSES ITS DIRECTION

Question 13

describe the magnetic field inside a solenoid with electric current flowing through it

Answer 13

strong and uniform magnetic field
with field lines running parallel to the solenoid’s axis

Question 14

describe the magnetic field outside a solenoid with electric current flowing through it

Answer 14

field lines bend in a complete loop from one end to another
passing through the inside of the solenoid itself

Question 15

what is an electromagnet?

Answer 15

a solenoid in which the insulated wire is wrapped around an iron bar (the core_.

Question 16

what happens to the core of an electromagnet when current flows through its wire?

Answer 16

current flows through wire
creating a magnetic field around the wire
which magnetises the iron bar/core

Question 17

what happens to the core of an electromagnet when current stops flowing through its wire?

Answer 17

the iron bar (core) loses most of its magnetism

Question 18

why does an electromagnet contain an iron bar?

Answer 18

it increases the strength of the solenoid’s magnetic field once magnetised.

Question 19

give 3 examples of items which use electromagnets

Answer 19

electric bell
scrapyard crane
circuit breaker

Question 20

what is the motor effect?

Answer 20

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.

Question 21

how to increase the size of a force caused by the motor effect

Answer 21

• increase the current,
• increase the strength of the magnet.

Question 22

what angle between the wire and magnetic field make the force greatest?

Answer 22

• force is greatest when wire is perpendicular to magnetic field,
• force is zero when wire is parallel to magnetic field lines.

Question 23

what happens to the direction of force due to the motor effect when the direction of:
- current is reversed,
- magnetic field is reversed

Answer 23

when the magnetic field or current’s direction reverses
the direction of force also reverses

Question 24

what does each letter stand for:

F = BIL

Answer 24

force = magnetic flux density x current x length of wire
N tesla, T A m

Question 25

what does magnetic flux density measure

Answer 25

the strength of a magnetic field

Question 26

describe the basis of an electric motor

Answer 26

a coil of wire rotates when current passes through it, in a magnetic field.

Question 27

why does the coil turn in an electric motor

Answer 27

• 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.

Question 28

how is the coil connected to the battery
and split ring communtator
in an electric motor

Answer 28

• 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.

Question 29

purpose of the split-ring commutator in an electric motor

Answer 29

• reverses the direction of the current around the coil every half turn,
• pushing the coil in the same direction for every half turn.

Question 30

what causes electromagnetic induction?

Answer 30

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.

Question 31

what is the generator effect?

Answer 31

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.

Question 32

what is the motor effect?

Answer 32

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.

Question 33

how to increase the induced potential difference due to the generator effect

Answer 33

make the conductor pass through the magnetic field lines of the magnet faster

Question 34

what is the direction of potential difference induced due to the generator effect

Answer 34

potential difference induced due to the generator effect is in the opposite direction of the change that caused

Question 35

what is the purpose of an alternator

Answer 35

to generate alternating current using the generator effect

Question 36

describe how an alternator works

Answer 36

• 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.

Question 37

what is the purpose of a dynamo

Answer 37

to generate direct current using the generator effect

Question 38

describe how a dynamo works

Answer 38

• 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.

Question 39

describe how a moving coil microphone works

Answer 39

• 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.