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Physics > P7 - Magnetism and Electromagnetism > Flashcards

P7 - Magnetism and Electromagnetism Flashcards

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1
Q

What is an alternator?

A

A device that makes use of the generator effect to generate alternating current.

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2
Q

What is attraction?

A

Opposite poles experiencing a force towards each other.

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3
Q

What is a dynamo?

A

A device that makes use of the generator effect to generate direct current.

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4
Q

What is an electromagnet?

A

A solenoid with an iron core. (iron core increases the strength of the magnetic field)

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5
Q

What is the generator effect?

A

When there is a relative motion between an electrical conductor and a magnetic field, so a p.d. is induced across the ends of the conductor, causing a current to flow.

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6
Q

What is a magnetic compass?

A

A device containing a small bar magnet that points in the direction of the Earth’s magnetic field.

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7
Q

What are magnetic field lines?

A

Lines representing the strength and direction of a magnetic field.

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8
Q

What do you know about the field line direction?

A

It is in the direction that a force would act on another north pole if placed at that point.

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9
Q

What is a magnetic field?

A

The region around a magnet in which another magnet or magnetic material will experience a force.

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10
Q

What are magnetic poles?

A

The regions of a magnet where the magnetic forces are at their strongest.

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11
Q

What is the motor effect?

A

When a force is experienced between a current carrying wire placed in a magnetic field and the magnet responsible for the field, creating motion.

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12
Q

What is a permanent magnet?

A

A magnet that produces its own magnetic field.

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13
Q

What is repulsion?

A

When like-poles experience forces in opposite directions.

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14
Q

What is a solenoid?

A

A wire wrapped into the shape of a coil which when current passes through creates a strong magnetic field.

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15
Q

How can you increase a solenoid’s magnetic field strength?

A

Adding an iron core.
More turns.

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16
Q

What is a step-down transformer?

A

A transformer which has a smaller p.d. in the secondary coil than in the primary coil.

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17
Q

What is a step-up transformer?

A

A transformer that has a larger p.d. in the secondary coil than in the primary coil.

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18
Q

What is tesla?

A

The unit of magnetic flux density.

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19
Q

What is a transformer?

A

An iron core with a primary and secondary coil of wire wound in opposite ends.

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20
Q

At what part of the magnet are the magnetic forces the strongest?

A

The poles.

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21
Q

What happens when 2 magnets are brought close together?

A

They exert a force on each other.

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22
Q

What type of force is exerted is 2 like poles of a magnet are brought close together?

A

A repulsive - non-contact force.

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23
Q

What type of force is exerted is 2 unlike poles of a magnet are brought close together?

A

An attractive - non-contact force.

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24
Q

What is a permanent magnet?

A

Produces it’s own magnetic field.

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25
Q

What is an induced magnet?

A

A material that becomes magnetised when placed in a magnetic field.

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26
Q

What type of force does induced magnetism always cause?

A

A force of attraction.

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27
Q

What happens when an induced magnet is removed from a magnetic field?

A

It removes all/most of it’s magnetism

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28
Q

Give 4 examples of magnetic materials.

A

Iron.
Cobalt.
Nickel.
Steel.

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29
Q

How does the strength of a magnetic field alter as you move further away from the magent?

A

Magnetic field strength decreases.

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30
Q

In what direction does a magnetic field point?

A

From the north seeking pole to the south seeking pole.

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31
Q

What is produced when current flows through a conducting wire?

A

A magnetic field is produced around the wire.

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32
Q

What determines the strength of a magnetic field around a current-carrying wire?

A

The magnitude of the current.
The distance from the wire.
Number of coil turns.

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33
Q

Describe the magnetic field found inside a solenoid.

A

Strong and uniform.

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34
Q

What rule is used to determine the force experienced by the motor effect?

A

Fleming’s left-hand rule

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35
Q

What does the forefinger represent (left-hand rule)?

A

Direction of magnetic field.

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36
Q

What does the second finger represent (left-hand rule)?

A

The direction of current flow.

37
Q

What does the thumb represent (left-hand rule)?

A

Direction of the force/movement.

38
Q

What factors affect the size of the force on a current-carrying wire placed in a magnetic field?

A

The magnitude of the current.
The strength of the magnetic field.

39
Q

If the direction of current in a current-carrying wire is reversed what happens to the direction of force?

A

Is reversed aswell.

40
Q

If the strength of the current in a wire placed in a magnetic field is increased, what happens to the force?

A

Is increased.

41
Q

What is the criteria for the equation linking force, magnetic flux density, length and current to hold?

A

Conductor must be perpendicular to the magnetic field it is placed in.

42
Q

How does an electric motor work?

A

A coil of wire, carrying a current, is placed in a magnetic field.
The wire experiences opposite forces on each side of it causing a rotational effect.

43
Q

What equation links force, magnetic flux density, current and length of wire in field?

A

force = magnetic flux density x current x length of wire in field

44
Q

What is the unit for current?

A

Amps (A)

45
Q

What is the unit for length?

A

metres

46
Q

What is the force involved in the motor effect called?

A

Lorentz force.

47
Q

What happens when an electric conductor moves relative to a magnetic field?

A

A potential difference is induced across the ends of the conductor.

48
Q

What is the requirement for an induced p.d. to cause current flow?

A

The conductor must be part of a complete circuit.

49
Q

What can be said about the direction of the magnetic field produced by an induced current?

A

The direction of the field is such that it opposes the change that induced the current.
The change is either the movement of the conductor or a change to the field.

50
Q

2 ways that the generator effect is used to generate different types of current.

A

In an alternator to produce alternating current.
In a dynamo to produce direct current.

51
Q

What electromagnetic effect does a microphone take advantage of?

A

Generator effect.

52
Q

How does a microphone use the generator effect?

A

It converts the pressure variations in sound waves into alternating current in a circuit.

53
Q

Describe the make-up of a basic transformer.

A

A primary coil and a secondary coil wrapped around an iron core.

54
Q

Why is an iron core used for a transformer?

A

It is easily magnetised.

55
Q

How does a transformer work?

A
  1. AC flows through the primary coil.
  2. Primary coil current induces a changing magnetic filed.
  3. Iron core increases strength of the magnetic field.
  4. Changing magnetic field induces a changing potential difference in the secondary coil
  5. the induced potential difference produces an alternating current in the secondary coil.
56
Q

Why must the current flowing through a primary coil be alternating?

A

For current to be induced in the secondary coil, the magnetic field must be constantly changing.

57
Q

What can be said about the electrical power output and input of a 100% efficient transformer?

A

Electrical power output = electrical power input

58
Q

How do loudspeakers use the motor effect?

A

Converts variations in the current of an electrical circuit into the pressure variations which produce audible sound.

59
Q

How does a loudspeaker work?

A
  1. A cone with a wire wrapped around it is connected to an a.c. power supply and placed in a magnetic field.
  2. When current flows through a wire it produces a second magnetic field which interacts with the permanent magnetic field.
  3. Which produces a force causing the cone to vibrate.
60
Q

How is the pitch of the sound from a loudspeaker changed?

A

The frequency of the a.c. is altered.
Creating a different frequency of vibration in the cone.

61
Q

What does the direction of the magnetic field depend on in a current-carrying wire?

A

The direction of current.

62
Q

How can you determine the direction of the magnetic field by knowing the direction of current?

A

Right-thumb rule.

63
Q

What 2 factors increase the strength of the magnetic field strength of a solenoid?

A

Higher current.
Greater number of coils.

64
Q

What is a relay made up of?

A

An electromagnet cell and a magnetic switch.

65
Q

How does a solenoid produce a strong magnetic field?

A

The magnetic field lines at each loop of the wire result in lots of magnetic field lines pointing in the same direction.

66
Q

What is the opposite of the generator effect and explain why?

A

Motor effect. Generator effect you put electricity in and get motion out. Motor effect you put motion in and electricity out.

67
Q

How is an alternating current produced in the generator effect?

A

When a magnet is moved into a coil a current is generated. When it is moved out of the coil the current reverses. So therefore, moving it in and out generates an alternating current.

68
Q

What 3 factors increase the size of the induced p.d in the generator effect?

A
  1. Speed of movement of the magnet or coil is increased.
  2. Number of turns on the coil is increased.
  3. Magnetic field strength is increased.
69
Q

What creates the p.d. in the generator effect?

A

The change in the magnetic field that the wire experiences, as it moves through the field.

70
Q

What happens to the current produced when the wire stops moving in the generator effect?

A

No current is produced as there is no change to the magnetic field.

71
Q

2 ways we can change the direction of the current in the generator effect.

A
  1. swapping poles
  2. moving magnet in and out of coil
72
Q

Why is iron a good material for a core?

A

It is easily magnetised.

73
Q

Describe the steps in which an alternating p.d. in the primary coil creates an alternating p.d. in the secondary.

A
  1. Alternating p.d. applied to primary
  2. Produces a magnetic field, which changes as the current changes.
  3. Changing magnetic field induces an alternating p.d. in the secondary coil.
74
Q

What is true about the induced voltage compared to the applied voltage in a step-down transformer?

A

Induced voltage is lower than applied voltage.

75
Q

What is true about the induced voltage compared to the applied voltage in a step-up transformer?

A

Induced voltage is larger than applied voltage.

76
Q

In a step-down transformer, what is true about the number of turns in the primary coil and the secondary coil?

A

Primary coil has more turns than secondary coil.

77
Q

In a step-up transformer, what is true about the number of turns in the primary coil and the secondary coil?

A

Primary coil has less turns than secondary coil.

78
Q

What does a high amount of turns mean in a transformer?

A

A high p.d.

79
Q

What is the equation for the ratio of p.d. and no. turns?

A

V(p) / V(s) = N(p) / N(s)
Voltage of primary / Voltage of secondary = Number of turns of primary / Number of turns of secondary.

80
Q

What is the efficiency of a tranformer?

A

100% - meaning power in = power out.

81
Q

What is the equation using the concept that power in = power out?

A

V(s) x I(s) = V(p) x I(p)
Because P=IV

82
Q

What does V(p) x I(p) determine?

A

The power input.

83
Q

What does V(s) x I(s) determine?

A

The power output.

84
Q

Why is the iron core important in transformers?

A

It carries magnetic fields from the primary coil to the secondary coil.

85
Q

How is a p.d. produced in the generator effect?

A

Magnetic field around the coil changes.

86
Q

Why is the p.d. produced in the generator effect alternating?

A

Because the magnet changes direction.

87
Q

When drawing the magnetic filed lines around a current, how many do you have to draw?

A

3

88
Q

How does a commutator work in the motor effect?

A

Every half-revolution the commutator swaps form one carbon brush to another - which reverses the current - but ensures the forces work in the same direction.

Physics (9 decks)

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