P3- Using magnetic fields to keep things moving Flashcards Preview

Physics GCSE AQA 2016 > P3- Using magnetic fields to keep things moving > Flashcards

Flashcards in P3- Using magnetic fields to keep things moving Deck (31)
Loading flashcards...
1
Q

What is an electromagnet?

A

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

2
Q

What is a magnetic field?

A

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

3
Q

What are electromagnets used for?

A
  • Scrapyard cranes.
  • Circuit Breakers.
  • Electric Bells.
  • Relays.
4
Q

What are lines of force or magnetic field lines?

A

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

5
Q

How can an electromagnet be made stronger?

A
  • Wrapping the coil around an iron core.
  • Adding more turns to the coil.
  • Increasing the current flowing through the coil.
6
Q

What are the advantages and disadvantages to electromagnets?

A

✅Can be switched on and off.

✅Strength can be varied.

❌Uses a lot of energy.

7
Q

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

What is this called?

A

The motor effect.

8
Q

In the motor effect, when is the force maximum?

A

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

9
Q

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

A
  • The first finger represents the magnetic field.
  • The second finger represents the current.
  • The thumb represents the direction of the force.
10
Q

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

A
  • If the direction of the current is reversed.
  • If the direction of the magnetic field is reversed.
11
Q

How is the speed of the motor increased?

A

By increasing the size of the current.

12
Q

How can the direction of the motor be reversed?

A

By reversing the direction of the current.

13
Q

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

A
  • 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.
14
Q

What does the ‘split-ring’ commutator do?

A

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

15
Q

What are relays?

A

Relays are electrically operated switches.

16
Q

What is electromagnetic induction?

A

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

17
Q

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

A

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

18
Q

How can the potential difference induced be increased?

A
  • Increasing the speed of movement.
  • Increasing the strength of the magnetic field.
  • Increasing the number of turns on the coil.
19
Q

Why is voltage increased in transformers?

A

Current travels faster to make it more efficient.

20
Q

Why does a transformer only work on alternating current?

A

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

21
Q

What core does a transformer have?

A

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

22
Q

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

A

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

23
Q

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

A

Iron can be magnetised.

24
Q

How does a transformer work?

A

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.

25
Q

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

A

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

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

26
Q

What is the transformer equation?

A

Vp/Vs=np/ns

Vp=Pd across primary coil (V)

Vs=Pd across secondary coil (V)

np=Number of turns on primary coil.

ns=Number of turns on secondary coil.

27
Q

What is the equation for a 100% efficient transformer?

A

Vp x Ip= Vs x Is

28
Q

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

A
  • Graphite is an excellent conductor.
  • It causes very little friction on the conducting ring.
29
Q

How is physics used in hospitals?

A
  • Measuring blood pressure.
  • Using an ECG recording.
  • Using an endoscope.
  • Using a scanner.
30
Q

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

Explain why (4 marks).

A

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).

31
Q

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

Explain how (3 marks).

A

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).