Topic 7: Magnetism and Electromagnetism

Question 1

What do two unlike poles do?

Answer 1

Attract each other

Question 2

What do two like poles do?

Answer 2

Repel each other

Question 3

What is a permanent magnet?

Answer 3

A magnet that produces its own magnetic field

Question 4

What is an induced magnet?

Answer 4

A material that becomes a magnet when placed in a magnetic field

Question 5

What are 2 rules of induced magnets?

Answer 5

1. Always cause a force of attraction
2. Loses its magnetism quickly when removed from a magnetic field

Question 6

Which way do field lines go?

Answer 6

From north to south

Question 7

How can you plot a magnetic field pattern using a compass? [4]

Answer 7

1. Inside a compass is a tiny bar magnet
2. North pole of this magnet is attracted to the south pole of any other magnet
3. So the compass points in the direction of the magnetic field it is in
4. You can move a compass around a magnet and trace its position to plot a magnetic field pattern

Question 8

How does the behaviour of a magnetic compass show that the core of the Earth must be magnetic?

Answer 8

When not near a magnet, compasses always point north

Question 9

What happens when a current flows through a wire?

Answer 9

A magnetic field is created

Question 10

What does shaping a wire to form a solenoid do?

Answer 10

Increases the strength of the magnetic field

Question 11

How can you increase the strength of an electromagnet?

Answer 11

1. Increase the current
2. Add an iron core
3. Increase the number of turns of the solenoid (whilst keeping the length of wire the same)

Question 12

When does the motor effect occur?

Answer 12

When a current-carrying wire is placed in a magnetic field

Question 13

How does the wire have to be orientated to feel the full force in the magnetic field?

Answer 13

At 90° to the magnetic field

Question 14

In Fleming’s left-hand rule, what does your first finger represent?

Answer 14

Direction of field (magentic field)

Question 15

In Fleming’s left-hand rule, what does your second finger represent?

Answer 15

Direction of current

Question 16

In Fleming’s left-hand rule, what does your thumb represent?

Answer 16

Direction of motion (force)

Question 17

Explain how a motor turns in terms of forces

Answer 17

One side of the coil acts up and one acts down and because the coil is on a spindle it rotates

Question 18

Explain how a split-ring commutator makes an electric motor spin [4]

Answer 18

1. It swaps the positive and negative connections every half turn
2. This swaps the direction of the current every half turn
3. Therefore, the forces acting on the coil will always be in the same direction
4. The coil will rotate in the same direction and spin

Question 19

How can the speed of a motor be increased? [3]

Answer 19

1. Increasing the current
2. Adding more turns to the coil
3. Using stronger magnets

Question 20

State the ‘generator effect’

Answer 20

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 21

How can you increase the strength of the induced current in a generator? [3]

Answer 21

1. Increase strength of magnetic field
2. Increase speed wire is moved at
3. Add more turns to coil

Question 22

Describe how dynamos work [4]

Answer 22

1. A coil of wire is turned inside of a magnet
2. This induces a magnetic field and a current in the coil of the wire
3. The split-ring commutator switches the connections every half turn
4. This causes the current to be a direct current

Question 23

What type of current do dynamos produce?

Answer 23

Direct current

Question 24

Describe how alternators work [4]

Answer 24

1. A coil of wire is turned inside of a magnet
2. This induces a magentic field and a current in the coil of the wire
3. The slip rings and brushes mean that the connections don’t change every half turn
4. This causes the current to be an alternating current

Question 25

What type of current do **alternators** produce?

Answer 25

Alternating current

Question 26

Explain how **loudspeakers** work? [6]

Answer 26

1. An alternating current flows through wire and creates a magnetic field in the coil 2. The magnetic field of the coil interacts with the magnetic field of the permanent magnet 3. The interacting magnetic fields will exert a force on the coil, causing it to move back and forth along the magnet 4. This causes the cone to change shape 5. As the current is alternating, its magnetic field and force it experiences will also alternate. This means that the coil of wire and cone rapidly move back and forth, meaning they vibrate 6. These vibrations are so fast that they cause pressure variations in the air - which are sound waves

Question 27

Explain how **microphones** work? [4]

Answer 27

1. Sound waves hit the diaphragm 2. This causes the diaphragm and coil of wire to move 3. As the wire is moving within the magnetic field of the permanent magnet, it will generate a current 4. The frequency and amplitude of the sound waves will determine how much the diaphragm vibrates, and therefore determine the frequency and amplitude of the current

Question 28

What do transformers do?

Answer 28

Step-up and step-down the p.d

Question 29

How do step-up transformers increase the p.d (number of coils)?

Answer 29

They have more turns on the secondary coil than the primary coil

Question 30

How do step-down transformers decrease the p.d (number of coils)?

Answer 30

They have more turns on the primary coil than the secondary coil

Question 31

Explain how a **transformer** works [5]

Answer 31

1. An alternating p.d is applied across the primary coil 2. This causes a current to flow and generates a magnetic field 3. This alternating current then induces an alternating magnetic field in the iron core 4. This induces a p.d across the secondary coil 5. This causes a current to flow

Question 32

Why is iron used in transformers?

Answer 32

It is easily magnetised

Question 33

What technique is used to determine the **direction** of the field lines in a wire?

Answer 33

Right hand rule

Question 34

In an electromagnet, what happens to the magnetic field when the current is turned off?

Answer 34

Magnetic field dissapears

Question 35

What effect do **loudspeakers** use?

Answer 35

Motor effect

Question 36

What effect do **microphones** use?

Answer 36

Generator effect

Question 37

What type of **current** do loudspeakers and microphones create?

Answer 37

Alternating current