Topic 7 - Magnetism and Electromagnestism

Question 1

What two poles do magnets have?

Answer 1

North pole and south pole

Question 2

What is a magnetic field?

Answer 2

A region where other magnets or magnetic materials experience a force.

Question 3

In a magnetic field diagram, what direction do the lines go?

Answer 3

The lines always go north to south

Question 4

What do the lines in a magnetic field diagram show?

Answer 4

They show which way a force would act on a north pole if it was put at that point in the field.

Question 5

What does how close the lines are mean?

Answer 5

The closer the lines are, the stronger the magnetic field.

Question 6

What are the two types of magnet?

Answer 6

Permanent and induced

Question 7

What are permanent magnets?

Answer 7

Permanent magnets produce their own magnetic fields.

Question 8

What are induced magnets?

Answer 8

Induced magnets are magnetic materials that turn into a magnet when they’re put into a magnetic field.

Question 9

What happens when a current flows through a wire?

Answer 9

When a curruent flows through a wire, a magnetic field is created around the wire.

Question 10

What is the magnetic field made up of in a wire?

Answer 10

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

Question 11

How to strengthen the magnetic field in a wire?

Answer 11

Increase the current or get closer to the wire.

Question 12

What is a solenoid?

Answer 12

A solenoid is a coil of wire.

Question 13

What is the purpose of a solenoid?

Answer 13

You can increase the strength of the magnetic field that a wire produces by wrapping the wire into a coil called a solenoid.

Question 14

Why does a solenoid icnrease the strength of a magnetic field?

Answer 14

It results in lots of field lines pointing in the same direction that are very close together.

Question 15

How to further increase the field strength of a solenoid?

Answer 15

1. Put a block of iron in the centre of the coil.
2. This iron core becomes an induced magnet whenever current is flowing.

Question 16

Why is a solenoid with an iron core an electromagnet?

Answer 16

1. If you stop the current, the magnetic field disappears.
2. It’s magnetic field can be turned on and off.

Question 17

What is the motor effect?

Answer 17

1. When a current -carrying wire is put between magnetic poles, the magnetic field around the wire interacts with the magnetic field it has been placed in.
2. This causes the magnet and the conductor to exert a force on each other.
3. This is called the motor effect and can cause the wire to move.

Question 18

What is the magnetic flux density?

Answer 18

1. How many field (flux) lines there are in a region.
2. This shows the strength of the magnetic field.

Question 19

What three factors changes the force acting on conductor in a magnetic field?

Answer 19

1. Magnetic flux denisity.
2. The size of the current.
3. The length of the cunductor.

Question 20

Formula to find the force

Answer 20

1. F = BIL
2. Force (N) = Magnetic flux denisty (T) x Current (A) x Length (L)

Question 21

What hand is the motor effect?

Answer 21

Left hand

Question 22

How does a loudspeaker work?

Answer 22

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 current causes a force on the coil which causes the cone to move.
3. When the current reverses, the force acts in the opposite direction, which causes the cone to move in the opposite direction.
4. Variations in the current make the cone vibrate, which makes the air around the coil vibrate and creates variations in pressure that causes sound waves.
5. The frequency of the sound wave is the same as the frequency of the ac, so by controlling the frequency of the ac you can alter the sound wave produced.

Question 23

How does an electric motor work?

Answer 23

1. The sides of the coil parallel to the magnet experience a force in opposite directions.
2. The forces cause moments that act in the same direction, causing the coil to rotate.
3. The split-ring commutator swaps the positive and negative connections every half term.
4. Each half-revolution the commutator reverses the current in the coil, keeping the forces in the same direction

Question 24

What is the generator effect?

Answer 24

1. The induction of a potential difference (and current if there is a complete circuit) in a wire which is moving relative to a magnetic field, or experiencing a change in magnetic field.
2. When a wire experiences a changing magnetic field, a potential difference will be induced across the wire.
3. If the ends of the wire are connected to make a closed circuit, then a current will flow around the circuit.

Question 25

What do alternaters generate?

Answer 25

An alternating current.

Question 26

How to increase the size of the potential difference generated in electromagnetic induction?

Answer 26

1. Move the coil through the magnetic field faster 2. Add more turns to the coil 3. Increase the magnetic field strength / magnetic flux density

Question 27

What do dynamos generate?

Answer 27

Direct current.

Question 28

Differences between structure of dynamo and alternator?

Answer 28

An alternator has slip rings and brushes whereas a dynamo has a split-ring commutator.

Question 29

How does an alternator work?

Answer 29

1. The coil rotates between the poles of a magnet 2.This causes it to cut through magnetic field lines 3.This causes a potential difference to be induced across the coil due to electromagnetic induction 4. There is a complete circuit, so a current is induced in the coil 5. The slip rings and brushes cause the potential difference to reverse direction every half turn 6. So, every half turn the current changes direction, producing AC

Question 30

How does a dynamo work?

Answer 30

1. The coil rotates between the poles of a magnet 2.This causes it to cut through magnetic field lines 3.This causes a potential difference to be induced across the coil due to electromagnetic induction 4. There is a complete circuit, so a current is induced in the coil 5. The split-ring commutator causes the connections to swap every half turn so the PD flows in same direction 6. So, the current flows in the same direction, producing DC

Question 31

How do microphones work?

Answer 31

1. Sound waves cause the diaphragm to vibrate. 2. This causes the coil to vibrate. 3. The coil / wire moves through the magnetic field 4. A potential difference is induced across the ends of the coil / wire 5. As the coil is part of complete circuit -> the induced voltage causes a current to flow that matches the soundwave pattern

Question 32

What do transformers do?

Answer 32

Transformers change the size of the potential difference of an alternating current.

Question 33

How does a transformer work?

Answer 33

1. When an alternating pd is applied across the primary coil, the iron core magnetises and demagnetises quickly. 2. This changing magnetic field induces an alternating pd in the secondary coil. 3. If the second coil is part of a complete circuit, this causes a current to be induced. 4. Ratio between P coil and S coil = ratio between number of turns on the P and S coils.