Topic 7 - Magnetism and Electromagnetism

Question 1

Define

magnetic field.

Answer 1

a region where other magnets or magnetic materials experience a force

this is a non-contact force

Question 2

What is the

force between a magnet and a magnetic material?

Answer 2

always attractive

Question 3

Where do

compasses point?

(when they’re not near a magnet) and why?

Answer 3

the point north because the Earth generates its own magnetic field

Question 4

What are

permanent magnets?

Answer 4

magnets that produce their own magnetic field

Question 5

What are

induced magnets?

Answer 5

magnetic materials that turn into a magnet when they’re put into a magnetic field

when you take away the magnetic field, the induced magnets quickly lose their magnetism and stop producing a magnetic field

Question 6

What happens when

current flows through a wire?

Answer 6

a magnetic field is created around the wire

this field is made up of concentric circles perpendicular to the wire, with the wire in the centre

Question 7

What is the

righ-hand rule?

Answer 7

a rule that is used to determine the direction of the field around a wire

your thumb points in the direction of the current and the direction in which you curl your fingers is the direction of the field

Question 8

How can you

increase the strength of a magnetic field?

(of a wire) and how does this work?

Answer 8

by wrapping the wire into a coil (called a solenoid)

the field lines around each loop of wire line up with each other, resulting in lots of field lines pointing in the same direction that are very close to each other

Question 9

How can you

increase the field strength of a solenoid?

and how does this work?

Answer 9

add a block of iron in the centre of the coil

this becomes an induced magnet whenever current is flowing

Question 10

What is an

electromagnet?

Answer 10

a solenoid with an iron core

Question 11

Why are

electromagnets very useful?

Answer 11

they’re very quick to turn on and off so can create a varying force

Question 12

What are some

uses of electromagnets?

Answer 12

• cranes to attract and pick up things made from magnetic materials (and then drop it)
• within circuits as switches

Question 13

What is the

motor effect?

and how does this work?

Answer 13

the force that is exerted on a current-carrying wire that is put between magnetic poles

the magnetic field around the wire interacts with the magnetic field it has been placed in

Question 14

How can you

increase the amount of force produced by the motor effect?

(4)

Answer 14

• put the wire at 90° to the magnetic field
• increase the strength of the magnetic field
• increase the amount of current passing through the wire
• increase the length of the wire that’s in the field

Question 15

What equation allows you to calculate the

force created by the motor effect?

Answer 15

force = magnetic flux density x current x length of wire in the field

F = BIl

F: newtons, N
B: tesla, T
I: amps, A
l: metres, m

Question 16

What is

fleming’s left-hand rule?

Answer 16

a rule that is used to determine direction in which the force produced by the motor effect is going

your first finger points in the direction of the field
you second finger points in the direction of the current
your thumb will then point in the direction of the force

Question 17

How do

loudspeakers work?

(4 steps)

Answer 17

1. An alternating current is sent through a coil of wire attached to the base of a paper cone.
2. The coil surrounds one pole of a permanent magnet and is surrounded by the other pole, so the cuttent causes a force on the coil (which causes the cone to move).
3. When the current reverses, the force acts in the opposit direction, which causes the cone to move in the opposite direction too.
4. So variations in the current make the cone vibrate, which makes the air around the cone vibrate and creates variations in pressure that cause a sound wave.

the frequency of the sound wave is the same as the frequency of the alternating current, so by controlling the frequency you can alter the sound wave produced

Question 18

Define

generator effect.

Answer 18

the induction of a potential difference in a wire which is moving relative to a magnetic field, or experiencing a change in magnetic field

Question 19

How can you

change the size of the induced potential difference?

Answer 19

• increase the speed of the movement - cutting more magnetic field lines in a given time
• increase the strength of the magnetic field (so there are more field lines that can be cut)

Question 20

How do

alternators generate an alternating current?

Answer 20

1. They rotate a coil in a magnetic field.
2. As the coil spins, a current is induced in the coil. This current changes direction every half turn.
3. Instead of a split-ring commutator, ac generators have slip rings and brushes so the contacts don’t swap every half turn.

Question 21

How do

dynamos generate direct current?

Answer 21

1. They work in the same way as alternators but they have a split-ring commutator instead of slip rings.
2. This swaps the connection every half turn to keep the current flowing in the same direction.

Question 22

How do

microphones generate current from sound waves?

Answer 22

1. Sound waves hit a flexible diaphragm that is attached to a coil of wire, wrapped around a magnet.
2. This causes the coil of wire to move in the magnetic field, which generates a current.
3. The movement of the coil depends on the properties of the sound wave.

Question 23

What do

transformers do?

Answer 23

they change the size of the potential difference of an alternating current

Question 24

How do

transformers work?

Answer 24

1. When alternating potential difference is applied across the primary coil, the iron core magnetises and demagnetises quickly.
2. This induces an alternating potential difference in the secondary coil.
3. If the second col is part of a complete circuit, this causes a current to be induced.

The ratio between primary and secondary potential differences is the same as the ratio between teh number of turns on the primary and secondary coils.

Question 26

Why does the

national grid transmit at very high potential differences?

Answer 26

a low current means that less energy is being wasted heating the wires and the surroundings, for a given power, a high pd is needed for a low current, this makes the national grid an efficient way of transmitting power