P3- Using magnetic fields to keep things moving

Question 1

What is an electromagnet?

Answer 1

An electromagnet consists of a coil of insulated wire wrapped around an iron core.

Question 2

What is a magnetic field?

Answer 2

The region around a magnet in which a piece of iron or steel will be attracted to it.

Question 3

What are electromagnets used for?

Answer 3

• Scrapyard cranes.
• Circuit Breakers.
• Electric Bells.
• Relays.

Question 4

What are lines of force or magnetic field lines?

Answer 4

Patterns of lines that loop from one pole to the other when iron filings are placed near a magnet.

Question 5

How can an electromagnet be made stronger?

Answer 5

• Wrapping the coil around an iron core.
• Adding more turns to the coil.
• Increasing the current flowing through the coil.

Question 6

What are the advantages and disadvantages to electromagnets?

Answer 6

✅Can be switched on and off.

✅Strength can be varied.

❌Uses a lot of energy.

Question 7

When we place a wire carrying an electric current in a magnetic field, it may experience a force.

What is this called?

Answer 7

The motor effect.

Question 8

In the motor effect, when is the force maximum?

Answer 8

The force is a maximum if the wire is at an angle of 90^o to the magnetic field, and zero if the wire is parallel to the magnetic field.

Question 9

In Fleming’s Left Hand Rule, what do the different fingers represent?

Answer 9

• The first finger represents the magnetic field.
• The second finger represents the current.
• The thumb represents the direction of the force.

Question 10

How can the direction of the force on the wire be reversed?

Answer 10

• If the direction of the current is reversed.
• If the direction of the magnetic field is reversed.

Question 11

How is the speed of the motor increased?

Answer 11

By increasing the size of the current.

Question 12

How can the direction of the motor be reversed?

Answer 12

By reversing the direction of the current.

Question 13

When a current passes through the coil, why does the coil spin?

Answer 13

• Because a force acts on each side of the coil due to the motor effect.
• Because the force on one side of the coil is in the opposite direction of the current.

Question 14

What does the ‘split-ring’ commutator do?

Answer 14

It reverses the direction of the current around the coil every half-turn so the coil is always pushed in the same direction.

Question 15

What are relays?

Answer 15

Relays are electrically operated switches.

Question 16

What is electromagnetic induction?

Answer 16

The process of creating a potential difference using a magnetic field.

Question 17

Why is there no potential difference induced when a bar magnet is held stationary inside a coil of wire?

Answer 17

The stationary bar magnet is not cutting any magnetic field lines.

Question 18

How can the potential difference induced be increased?

Answer 18

• Increasing the speed of movement.
• Increasing the strength of the magnetic field.
• Increasing the number of turns on the coil.

Question 19

Why is voltage increased in transformers?

Answer 19

Current travels faster to make it more efficient.

Question 20

Why does a transformer only work on alternating current?

Answer 20

Because a changing magnetic field is necessary to induce AC in the secondary coil.

Question 21

What core does a transformer have?

Answer 21

Most have an iron core, unless it is a switch mode transformer which has a ferrite core.

Question 22

What is the difference between a normal transformer and a switch mode?

Answer 22

A switch mode is lighter and smaller, operates at a high frequency and is more efficient.

Question 23

Why is the core of a transformer made of iron not copper?

Answer 23

Iron can be magnetised.

Question 24

How does a transformer work?

Answer 24

1) The alternating current in the primary coil makes the iron core into an electromagnet.
2) As the current is alternating, the magnetic field ‘moves’ and also changes direction.
3) This ‘moving’ magnetic field causes a current to be induced in the secondary coil.

Question 25

How does the number of coils and the potential difference relate to the type of transformer?

Answer 25

⬆️Number of coils=⬆️Potential difference= Step up transformer.

⬇️Number of coils=⬇️Potential difference= Step down transformer.

Question 26

What is the transformer equation?

Answer 26

V_p/V_s=n_p/n_s

V_p=Pd across primary coil (V)

V_s=Pd across secondary coil (V)

n_p=Number of turns on primary coil.

n_s=Number of turns on secondary coil.

Question 27

What is the equation for a 100% efficient transformer?

Answer 27

V_p x I_p= V_s x I_s

Question 28

In the motor effect why are graphite brushes used to connect the split-ring to the battery?

Answer 28

• Graphite is an excellent conductor.
• It causes very little friction on the conducting ring.

Question 29

How is physics used in hospitals?

Answer 29

• Measuring blood pressure.
• Using an ECG recording.
• Using an endoscope.
• Using a scanner.

Question 30

The loudspeaker cone vibrates when an alternating current flows through the coil.

Explain why (4 marks).

Answer 30

Current flowing through the coil creates a magnetic field (1). This magnetic field interacts with the permanent magnetic field (1) which produces a resultant force (1). When the direction of current changes, the direction of force changes to the opposite direction(1).

Question 31

The alternating current flowing through the primary coil of the transformer creates an alternating current in the secondary coil.

Explain how (3 marks).

Answer 31

The alternating current creates a changing magnetic field (1) in the iron core (1) causing a potential difference to be induced in the secondary coil (1).