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GCSE Physics > P15 - Electromagnetism > Flashcards

P15 - Electromagnetism Flashcards

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1
Q

What poles do magnets have? How can you think about the 2 poles in terms of positive or negative?

A

A North pole and a South pole.

North is usually thought of as positive, and south is negative:
North (+)
South (-)

*This is important when looking at electromagnetism.

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2
Q

How do poles work in terms of attraction and repulsion?

A

Like poles repel.

Unlike poles attract.

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3
Q

How can you classify a material in terms of magnetism?

A

1) Magnets
2) Magentic Material
3) NOT Magnetic

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4
Q

What are the 3 magentic materials?

A

1) Iron (or steel)
2) Cobalt
3) Nickel

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5
Q

What are permanent magnets made up of?

A

Steel as magnetised steel doesn’t lose its magnetism easily.

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6
Q

What is the region around a magnet called?

A

A magnetic field.

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7
Q

What are the lines in a magnetic field called?

A

Field lines.

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8
Q

What is the rule for where field lines should go?

A

Field lines always go from North to South.

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9
Q

What do field lines tell us?

A

They tell us at which points the magnetic field is strong or weak.

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10
Q

How does a compass work?

A

Has a tiny magnetic needle that that points north and south due to the earth’s magnetic field.

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11
Q

What do field lines tell us about a compass?

A

Which way it will point.

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12
Q

Why does an unmagnetised magnetic material (e.g paper clip,) attract to both poles of a magnet?

A

The magnetic field of the magnet is said to enduce magnetism.

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13
Q

How can you induce magnetism?

A

Place an unmagnetised magnetic material in the magnetic field of a magnet

The magnetic field will cause a force of attraction at both ends of the magnet.

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14
Q

What is the rule for electrons for a conventional current?

A

Electrons move from positive to negative.

*This contradicts C7 knowledge where electrons flow from negative to positive.

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15
Q

How can you create a magnetic field in a wire?

A

Pass an electric current through a wire.

A field will be set up around the wire.

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16
Q

What is a solenoid? What type of material is usually a solenoid?

A

A coil of insulated wire that can allow a current through it - creating a magnetic field.

*essentially a current-carrying wire that produces a magnetic field.

Coil of wire is usually copper (not magnetic - but interacts well with magnets)

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17
Q

What are solenoids used in?

A

In devices where a strong magnetic field needs to be produced.

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18
Q

How do you create an electromagnet?

A

You take a solenoid and wrap it around an iron bar (core).

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19
Q

How does an electromagnet turn on and off?

A

1) Current magnetises wire.
2) Magnetic field of wire magnetises the iron bar (core).
3) Current switched off = iron bar loses its magnetism

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20
Q

What are the 3 ways to strengthen the magnetic field of an electromagnet?

A

1) Increase current size
2) Increase the number of coils
3) Change the core material (e.g to iron)

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21
Q

How can the direction of the magnetic field for each direction of current be determined?

A

The Right Hand Grip Rule

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22
Q

How does the Right Hand Grip Rule work to find the direction of the magnetic field if the current is flowing OUT of the page?

A
  • Grip the wire with your right hand, with your thumb pointing upwards in the direction of the current.
  • The other fingers give the direction of the magnetic field around the wire.

So it flows anti-clockwise.

Out of the page symbol = . (Dot - arrow head)

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23
Q

How does the Right Hand Grip Rule work to find the direction of the magnetic field if the current is flowing INTO the page?

A

If the direction of current is reversed, the direction of the magnetic field is also reversed:

  • Grip the wire with the thumb pointing downwards along the direction of the current.
  • The other fingers give the direction of the magnetic field around the wire.

So it flows clockwise.

Into the page symbol: X - arrow feathers

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24
Q

How does the shape of a bar magnet’s magnetic field compare to a solenoid’s?

A

Same shape

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25
Q

How do electromagnets differ to normal magents?

A
  • Electromagnets need a current to work.
  • Electromagnets can be turned on/off by a current.
  • Electromagnets need a coil (solenoid) around a magnetic core (iron).
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26
Q

What 4 devices can electromagnets be used?

A

1) Scrapyard crane
2) Electric Bell
3) Circuit Breaker
4) Relay

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27
Q

How do electromagnets work on a scrapyard crane?

A
  • The end of the crane is a powerful electromagnet.
  • The steel frame of cars stick to the electromagnets when the current is on.
  • The car can be dropped when the current is switched off.
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28
Q

How do electromagnets work in an electric bell?

A
  • An iron armature (lever) is pulled on to the electromagnet.
  • The iron armature hits the bell and the electromagnet cuts out due to the ‘make or break switch’ being cut.
  • The iron armature is then magnetised again and springs back.
  • This repeats and causes an electric bell to ring.
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29
Q

How does an electromagnet work in a circuit breaker?

A
  • If current is too large, a switch in the circuit is magnetised and pulled open by an electromagnet.
  • This breaks the circuit until it is manually reset.
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30
Q

How are electromagnets used in relay circuits?

A
  • Relays are used to allow a small current in one circuit to control a large circuit in another.
  • An electromagnet allows a small current to switch on a machine with a bigger current.

-Used for safety reasons.

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31
Q

What is the function of a transformer? Why are they important?

A

Transformers are used to increase or decrease the size of the voltage in a current.

-Important for suiting the demands of the device.

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32
Q

What are step-up transformers?

A

Transformers that increase the size of potential difference.

33
Q

What are step-down transformers?

A

Transformers that decrease the size of the potential difference.

34
Q

When might step-up transformers be useful in the national grid? Why are they needed?

A

From the powerstation (25,000V) to the national grid (132,000V).

Voltage needs to be stepped up otherwise energy will be dissipated as heat energy.

35
Q

When might step-down transformers be useful in the national grid?

A

From the national grid (132,000V) to our homes (230V).

36
Q

What is the normal voltage in the UK (mains in the home) and what is the frequency?

A

230V & 50Hz

37
Q

How many coils does a transformer have?

A

A transformer has two coils of insulated wire around an iron core.

38
Q

Why is an iron core used as opposed to a steel core?

A

Iron is easily magnetised and demagnetised.

39
Q

What are the 2 coils that make up a transformer called?

A

The primary and secondary coil.

40
Q

How does a transformer work?

A

1) Alternating current (a.c.) passes through the primary coil, generating a magnetic field in the iron core.
2) This induces an alternating potential difference in the secondary coil.

41
Q

What is alternating current?

A

A current that reverses its direction many times a second.

42
Q

Why do transformers only work with alternating current and not direct current?

A

There needs to be a changing magnetic field to induce a secondary current and magnetic field.

With a direct current, there is no changing magnetic field.

43
Q

What factor impacts the voltage size of a transformer? How?

A

The number of coils.

More coils = higher voltage
Less coils = lower voltage

44
Q

How many coils does a step-up transformer have in its primary and secondary coils?

A

More coils in the secondary coil - so p.d goes from low to high.

45
Q

How many coils does a step-down transformer have in its primary and secondary coils?

A

Less coils in the secondary coil - so p.d goes from high to low.

46
Q

What is the transformer equation?

A

Voltage of primary coil / Voltage of secondary coil = Number of coils on primary coil / Number of coils on secondary coil

Vp / Vs = Np / Ns

47
Q

What re-arranging trick can you use to quickly make the secondary voltage and number of coils at the top?

A

Invert both equations:

Vp / Vs = Np / Ns goes to:

Vs / Vp = Ns / Np

48
Q

Why are transformers said to be 100% efficient (in theory)?

A

Because in theory, all the power (IxV) is supplied to the device.

49
Q

What equation is used to assume 100% power efficiency of transformers?

A

Vp x Ip = Vs x Is

Power (IxV) has to be the same on both sides (100%)

50
Q

How can power loss and the heating effect of a resistor be reduced by the national grid?

A

Raising the voltage to reduce the current (I = P/V)

This limits the heating effect of a resistor - whence step-up transformer needed.

51
Q

What does an electric motor contain?

A

A coil that turns when an electric current is passed through.

52
Q

What appliances use electric motors?

A

Hairdryers, electric shavers, computer hard drives, etc…

53
Q

Why does an electric motor work?

A

A force acts on a current-carrying wire in a magnetic field.

54
Q

What is the motor effect?

A

The force generated when a current-carrying wire is placed in a magnetic field.

55
Q

What does the motor effect do in terms of energy?

A

Electric Energy —> Kinetic Energy / Movement

E.g moving-coil headphones and Loudspeakers

56
Q

How can the size of the force produced by the motor effect be increased?

A
  • Increasing the current

- Using a stronger magnet

57
Q

What rule is used to help determine the direction of a force brought about by the motor effect?

A

Fleming’s Left-Hand Rule

58
Q

What do the 3 fingers in Fleming’s Left-Hand Rule represent the direction of?

A

ThuMb = Motion
First Finger = Field (magnetic)
SeCond Finger = current

*if you know the direction of the field and current. You know the force.

59
Q

What can we say about the direction of the force in relation to the current and field?

A

The direction of the force is always at right angles to the current and field.

60
Q

What happens to the force if the current is parallel to the field?

A

The force is 0.

Must be at right angles (perpendicular).

61
Q

What is Magnetic Flux Density?

A

The measure of the strength of a magnetic field.

62
Q

What is the symbol and unit of Magnetic Flux Density?

A

Magnetic Flux Density, B

Unit: tesla, T

63
Q

What equation, including Magnetic Flux Density, is used to calculate the force on a conductor (metal) as a result of the current?

*Think American federal police

A

Force = Magnetic Flux Density x Current x Length

F = B x I x L

(N) = (T) x (A) x (m)

64
Q

What is the generator effect?

A

The effect of inducing a voltage using a magnetic material (electromagnetic induction).

65
Q

What does the generator effect do in terms of energy?

A

Kinetic Energy / Movement —> Electrical Energy

OPPOSITE OF MOTOR EFFECT

E.g moving-coil microphone

66
Q

What rule is used to determine the direction of movement, current and field by the generator effect?

A

Right Hand Rule (NOT Grip Rule)

67
Q

What do the 3 fingers stand for in the right hand rule - generator effect?

A

ThuMb = Movement
First Finger = Field (Magnetic)
SeCond Finger = Current

Exactly the same as Motor Effect

68
Q

What does a generator contain? What does this induce?

A

Coils of wire that spin in a magnetic field. This induces a potential difference across the ends of the conductor.

69
Q

What is an alternating-current generator called?

A

An alternator (a.c generator).

70
Q

What is an a.c generator made up of? How does it work?

A

An a.c generator is made up of a coil that spins in a magnetic field.

How it works:

1) When the coil turns steadily, the meter pointer deflects one direction and then the other. This carries on as long as the coil keeps turning in the same direction.
2) The current in the circuit repeatedly changes direction through the meter because the induced potential difference in the coil repeatedly changes direction.
3) So the induced potential difference and the current alternate as they repeatedly change direction.

71
Q

How can the alternating potential difference be measured?

A

Displayed by an oscilloscope.

72
Q

When is the induced alternating potential difference at its maximum (positive) and minimum (negative) peak?

A
  • When the sides of coil cross directly through the magnetic field lines.
  • The faster the coil rotates.
73
Q

When is the induced alternating potential difference at zero?

A

When the sides of the coil move parallel to the field lines.

74
Q

What is a direct current generator called?

A

A dynamo (d.c generator)

75
Q

What is different about a d.c generator from an a.c generator?

A

A d.c generator has one split-ring commutator instead of 2.

76
Q

How does a d.c generator work?

A

1) As the coil spins, the split-ring commutator reconnects the coil the opposite way around in the circuit every half-turn.
2) This happens each time the coil is perpendicular to the field lines.
3) Because of this, the induced potential difference does not reverse its direction.

77
Q

What is the symbol for the positive and negative terminal in a circuit?

A 
| = Positive
- = Negative

Long Line = Positive
Short Fatter Line = Negative

78
Q

What is ac current?

A

Alternates between positive to negative.

79
Q

What are slip rings and brushes?

A

Slip rings are used so that the wires don’t get bunched up.

Brushes are used to keep the electricity connected.

GCSE Physics (18 decks)

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