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GCSE - Physics > Section 5 - Motors, Generators and Transformers > Flashcards

Section 5 - Motors, Generators and Transformers Flashcards

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1
Q

What is a magnetic field?

A

A region where magnetic materials experience a force. Current-carrying wires also experience a force acting on them.

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2
Q

What are magnetic field lines?

A

Lines used to show the size and direction of magnetic fields.

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3
Q

Which direction do magnetic field lines always point?

A

From North to South (outside of the magnet).

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4
Q

What happens when the north and south poles of two permanent bar magnets are placed near each other?

A

A uniform magnetic field is created.

Diagram pg 81 of revision guide

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5
Q

What two things can be used to look at magnetic field lines?

A
  • Compasses

* Iron fillings

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6
Q

How can compasses be used to look at the magnetic field lines of a magnet?

A

They always point towards the south pole, so they can moved around and used to trace the field lines.

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7
Q

How can iron fillings be used to look at the magnetic field lines of a magnet?

A

The magnet is placed under a piece of paper and iron fillings are scattered on top. They arrange themselves along the field lines.

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8
Q

Like magnetic poles…

A

Repel

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9
Q

Opposite magnetic poles…

A

Attract

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10
Q

What does a diagram showing repelling magnets look like?

A

Pg 81 of revision guide

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11
Q

What does a diagram showing attracting magnets look like?

A

Pg 81 of revision guide

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12
Q

Do both poles of a magnet attract magnetic materials?

A

Yes (as long as they are not magnets)

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13
Q

What happens when a magnet is brought near a magnetic material?

A
  • Magnetism is induced in the material and it acts as a magnet.
  • The closer the magnet gets, the stronger the induced magnetism
  • A north pole will induce a south pole and vice versa
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14
Q

What happens in terms of magnetism when a current flows through a wire?

A

A magnetic field is generated.

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15
Q

What does the magnetic field around a straight wire look like?

A

Made up of concentric circles, with the wire in the centre.

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16
Q

What happens when the direction of the current in a wire is reversed?

A

The direction of a magnetic field is also reversed.

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17
Q

What is used to work out the direction of a magnetic field around a straight wire?

A

The Right-Hand Grip Rule

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18
Q

How does the Right-Hand Grip Rule work?

A

In a straight wire:
• Thumb -> Direction of current
• Fingers -> Direction of magnetic field

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19
Q

What happens when you bend a wire into a single flat circular coil?

A
  • Concentric circles of magnetic field lines are still generated at each part of the coil
  • In the centre, the magnetic field acts like a bar magnet
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20
Q

Give some examples of magnetic materials.

A
  • Iron
  • Nickel
  • Cobalt
  • Steel
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21
Q

What are hard magnetic materials?

A

Materials which retain their magnetisation once magnetised.

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22
Q

What are soft magnetic materials?

A

Materials which lose their magnetisation easily.

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23
Q

Give an example of a magnetically soft material.

A

Iron

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24
Q

Give an example of a magnetically hard material.

A

Steel

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25
Q

What is a solenoid?

A

A coil of wire carrying electric current.

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26
Q

What is the Right-Hand Grip Rule used for?

A

Working out the direction of the magnetic field around a straight wire when the current is given, or vice versa.

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27
Q

Describe the magnetic field in a solenoid.

A
  • Inside the coil, the magnetic field is strong and uniform

* Outside the coil, the field is like that of a bar magnet

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28
Q

How can the direction of a magnetic field in a solenoid be worked out?

A

The Right-Hand Grip Rule is used, except:
• The thumb represents the direction of the magnetic field (i.e. Thumb point to north pole)
• The fingers represent the coil -> The fingers point in the direction the current is flowing

(See diagram pg 82 of revision guide)

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29
Q

How can the magnetic field in a solenoid be increased?

A
  • Increasing current
  • Increasing the number of coils
  • Adding an iron core
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30
Q

What is an electromagnet?

A

A magnet whose electric field can be turned on and off with an electric current.

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31
Q

What are some uses of electromagnets?

A
  • Scrapyard cranes
  • Circuit breakers
  • Electric bells
  • Relays
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32
Q

When can the motor effect be observed?

A

When a current-carrying wire is put in a magnetic field.

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33
Q

What is the motor effect?

A

When a current-carrying wire is placed in a magnetic field, a force is exerted on the wire.

34
Q

What happens when a current-carrying wire is placed in a magnetic field?

A

A force is exerted on it.

35
Q

Why does the motor effect work?

A

The magnetic field produced by the wire and the magnetic field between the magnetic poles affect each other, exerting a force on the wire.

36
Q

How does the direction of the wire affect the motor effect?

A
  • Wire at 90* to the magnetic field -> Maximum force

* If the wire runs along the magnetic field, no force is exerted at all

37
Q

What factors will increase the force exerted by the motor effect?

A
  1. Increasing the current
  2. Increasing the magnetic field
  3. Wire at 90* to magnetic field
38
Q

Which rule is used for the motor effect?

A

Fleming’s Left-Hand Rule

39
Q

How does the Fleming’s Left-Hand Rule?

A
  • Thumb -> Force
  • First finger -> Magnetic field
  • Second finger -> Current
40
Q

What effect do electric motors use to make them work?

A

The motor effect

41
Q

What factors speed up an electric motor?

A
  1. Increasing current
  2. Increasing magnetic field
  3. Placing wire at 90* to magnetic field
42
Q

How does an electric motor work?

A
  1. A coil of wire is placed in a magnetic field
  2. A current is passed through this
  3. Due to the motor effect, one side of the wire moves up, while the other side moves down, causing the motor to spin
  4. A SPLIT RING COMMUTATOR is used to swap the contacts every half turn to keep the motor turning in the same turn
    (See diagram pg 84)
43
Q

How can the direction that an electric motor spins be reversed?

A
  1. Swapping the polarity of the DC supply

2. Reversing the magnetic field

44
Q

How are electric motors used inside appliances?

A

The coil is linked to an axle, making it spin round. A fan, blade, etc. can be attached to this to make it spin.

45
Q

What is the generator effect?

A

The induction of a potential difference (and maybe current) in a wire which is intersecting a magnetic field.

46
Q

What is another name for the generator effect?

A

The dynamo effect.

47
Q

When does the generator effect create a current?

A

When the wire is connected in a circuit.

48
Q

What happens when a wire is moved around inside a magnetic field?

A

If it is interesting the magnetic field lines, a potential difference is induced in the wire.

49
Q

What are the two ways the generator effect can be used to generate an AC current in a wire?

A
  1. Moving a magnet back and forth in a coil of wire

2. Rotating a magnet end to end in a coil of wire

50
Q

How can the direction of the potential difference generated by the generator effect be reversed?

A
  1. Moving the magnet in the opposite direction
  2. Moving the wire in the opposite direction
  3. Reversing magnetic field
51
Q

What factors increase the potential difference generated by the generator effect in a coil?

A
  1. Strength of magnetic field
  2. Area of the coil
  3. Number of turns on the coil
  4. Speed of movement
52
Q

What are the two types of generator?

A
  • AC generator

* DC generator

53
Q

How does an AC generator work?

A

• The construction is like a motor
• A coil rotates in a magnetic field
• As the coil spins, a current is induced in the coil. The direction of the current changes every half turn.
• The AC generator has SLIP RINGS AND BRUSHES instead of a split ring commutator.
• This produces an AC current.
(Diagram Pg 86)

54
Q

How does a DC generator work?

A

• The construction is like a motor
• A coil rotates in a magnetic field
• As the coil spins, a current is induced in the coil. The direction of the current changes every half turn.
• The DC generator has a SPLIT-RING COMMUTATOR -> Contacts swap every half turn
• This produces a DC current.
(Diagram Pg 86)

55
Q

Describe the display of a CRO connected to an AC generator.

A

The wave goes up and down, above and below the axis.

56
Q

Describe the display of a CRO connected to a DC generator.

A

The wave ‘bounces’ up and down above the time axis.

57
Q

What happens to the output of the generator if the speed of the rotating coil is increased?

A
  • Increased frequency

* Higher peaks (current or voltage)

58
Q

Which rule can be used for working out the direction of current in a generator?

A

Fleming’s RIGHT-hand rule

This is like Fleming’s left-hand rule, except on the other hand

59
Q

What is the difference between AC and DC generators?

A
  • AC generators -> Slip rings and brushes

* DC generators -> Split-ring commutator

60
Q

Describe the contacts in motors and generators.

A
  • AC generators -> Slip rings and brushes
  • DC generators -> Split-ring commutator
  • Motor -> Split-ring commutator
61
Q

What do slip rings do?

A

Nothing - they allow the coil to spin without reversing the direction of the current.

62
Q

What do split-ring commutators do in motors?

A

Reverse the direction of the current in the coil every half turn to keep the coil spinning in the same direction.

63
Q

What do split-ring commutators do in DC generators?

A

Swap the contacts every half turn, keeping the output current flowing in the same direction.

64
Q

What are the two types of transformer?

A
  • Step-up transformer

* Step-down transformer

65
Q

What does a transformer look like?

A

2 coils (primary and secondary) joined by a soft iron core.

66
Q

What do step-up transformers do?

A

Step voltage up.

67
Q

What do step-down transformers do?

A

Step voltage down.

68
Q

Describe the number of turns on the primary and secondary coil of a step-up transformer.

A

More turns on the secondary than primary.

69
Q

Describe the number of turns on the primary and secondary coil of a step-down transformer.

A

More turns on the primary than secondary.

70
Q

Which effect do transformers use?

A

Generator effect.

71
Q

What type of current is used in transformers and why?

A

AC - this produces a constantly changing magnetic field, so a potential difference can be induced in the secondary coil.

72
Q

How does a transformer work?

A
  • AC passes through the primary coil, producing a magnetic field that stays within the iron core
  • Nearly all of it passes from the primary to secondary coil due to the iron core
  • The changing magnetic field induced a potential difference in the secondary coil
  • This in turn causes a current
73
Q

What is the soft iron core used for in transformers?

A

Transferring the changing magnetic field from the primary coil to the secondary coil.
No electricity flows round the iron core!

74
Q

What is the transformer equation (in terms of coils and potential difference)?

A

P.D. across Primary Coil / P.D. across Secondary Coil = No. of Turns on Primary Coil / No. of Turns on Secondary Coil

Vp / Vs = np / ns

(This equation can be written either way up)

75
Q

How efficient are transformers and what does this mean?

A

Nearly 100%, so the power on each side of the transformer is equal.

76
Q

What is the transformer equation (in terms of power)?

A

Power Input = Power Output

Vp x Ip = Vs x Is

77
Q

What is a switch mode transformer?

A
  • A transformer that operates at a higher frequency than traditional transformers.
  • Used in chargers and power supplies (e.g. for laptops)
78
Q

What is the frequency for normal transformers?

A

50Hz

79
Q

What is the frequency for switch-mode transformers?

A

Between 50 kHz and 200 kHz.

80
Q

What makes a switch-mode transformer different to a normal transformer?

A
  • Lighter
  • Smaller
  • Operate at at higher frequency (50 kHz - 200kHz)
  • More efficient
81
Q

Why are switch-mode transformers efficient?

A

They use very little power when no load is applied but they are switched on.

e.g. If your charger is on, but not connected to a phone, it uses little power.

GCSE - Physics (6 decks)

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