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Physics GCSE Paper 2 > Topic 7 - Magnetism and Electromagnetism > Flashcards

Topic 7 - Magnetism and Electromagnetism Flashcards

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1
Q

What is a magnetic field?

A

A region where other magnets or magnetic materials experience a force. This is a non-contact force.

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2
Q

Describe magnetic field lines.

A

They always go from north to south and they show which way a force would act on a north pole if it was put at that point in the field. The closer together the lines, they stronger the magnetic field. The further away from the magnet, the weaker the field.

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3
Q

Where is the magnetic field the strongest?

A

At the poles of the magnet, so the magnetic forces are also strongest at the poles.

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4
Q

What happens when two poles of a magnet are put near each other?

A

They will exert a force on each other. This can be either attractive or repulsive.

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5
Q

Do to like poles attract or repel?

A

Repel.

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6
Q

Do to unlike poles attract or repel?

A

Attract.

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7
Q

How do compasses work?

A

Inside a compass is a tiny bar magnet. The north pole of this magnet is attracted to the south pole of any magnet near. So the compass points in the direction of this magnet field. When not near a magnet they always point north.

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8
Q

How do we know that the inside core of the Earth must be magnetic?

A

Compasses always point north when not near a magnet. This is becuase the earth generates its own magnetic field and therefore has a magnetic core.

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9
Q

What are the two types of magnet?

A

Permanent and induced.

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10
Q

What is a permanent magnet?

A

Produce their own magnetic field.

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11
Q

What is an induced magnet?

A

Magnetic materials that turn into a magnetic when they’re put into a magnetic field.

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12
Q

What is the force between permanent and induced magnets?

A

Attractive.

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13
Q

What happens when you take away the magnetic field from induced magnets?

A

They quickly lose their magnetism and stop producing a magnetic field.

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14
Q

What happens when current flows through a wire?

A

A magnetic field is created around the wire.

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15
Q

Describe the field made when current flows through a wire.

A

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

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16
Q

What can you use to work out which direction the magnetic field goes in?

A

The right hand thumb rule.

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17
Q

Describe the right hand thumb rule.

A

Using your right hand, point your thumb in the direction of current and curl your fingers. The direction of your fingers is the direction of the field.

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18
Q

What does the strength of the magnetic field produced depend on?

A

Changes with the current and the distance from the wire. The larger the current through the wire, or the closer to the wire you are, the stronger the field.

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19
Q

What is a solenoid?

A

A wire wrapped in a coil, this increases the strength of the magnetic field.

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20
Q

Why does a solenoid increase the strength of a magnetic field?

A

The field lines around each loop of the wire line up with each other. This results in lots of field lines pointing in the same direction that are very close to each other. As the field lines are close, it is strong.

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21
Q

Describe the field lines in a solenoid?

A

Strong and uniform.

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22
Q

What is meant by uniform?

A

Has the same strength and direction at every point in that region.

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23
Q

How can you further increase the field strength of a solenoid?

A

Putting a block of iron in the centre of the coil. The iron core becomes an induced magnet whenever current is flowing.

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24
Q

What is a solenoid with an iron core called?

A

Electromagnet. If the current stops, the magnetic field disappears.

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25
Q

Why are magnets that you can switch on and off useful?

A

They are quick to turn off/ on and can create a varying force.

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26
Q

What is one use of electromagnets?

A

Used in cranes to attract and pick up things made from magnetic materials like iron or steel, in scrap yards. Can be switch on to pick materials up, and turned off to drop them.

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27
Q

What is another use of electromagnets?

A

Used within other circuits to act as switches, eg. in electric starters of motors. When the switch om circuit one is closed, it turns on the electromagnet which attratcs the iron contact on the rocker. The rocker pivots and closes the contacts, completeing circuit two, and turning on the motor.

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28
Q

What is the motor effect?

A

When a current carrying wire is put between magnetic poles, the magnetic field around the wire interacts with the magnetic field it has been placed in. This causes the magnet and the conductor to exert a force on each other, causing the wire to move.

29
Q

Motor Effect: How does the wire have to be to experience a full force?

A

90 degrees to the magnetic field. If the wire runs parallel it won’t experience a force at all, angles in between will experience some force.

30
Q

Motor Effect: What is a good way of showing direction of the force?

A

Apply a current to a set of rails inside a horseshoe magnet. A bar is placed on the rails which completes the circuit. This generates a force that rolls the bar along the rails.

31
Q

What does the force acting on a conductor in a magnetic field depend on?

A

The magnetic flux density (how many field lines in a region).
Size of current through the conductor.
Length of the conductor in the magnetic field.

32
Q

How can you calculate the force acting on a conductor in a magnetic field when the current is at 90 degrees?

A

F=BIL, Force (N) = Magnetic flux density (T, tesla) x Current (A) x Length (m).

33
Q

How can you calculate the force acting on a conductor in a magnetic field when the current is at 90 degrees?

and How can you find the direction of the force?

A

With flemings left hand rule.

34
Q

Describe fleming’s left hand rule.

A

Using your left hand, point your First finger in the direction of the Field. Point your seCond finger in the direction of Current. Point your thuMb in the direction of the force (Motion).

35
Q

What does fleming’s left hand rule show you?

A

If either current or the magnetic field is reversed, then the direction of the force will also be reversed. Can be used for motors.

36
Q

Why does a basic dc motor rotate?

A

Because the coil is on a spindle and the forces act one up one down.

37
Q

What is a split ring commutator?

A

It swaps the contacts every half turn to keep the motor rotating in the same direction.

38
Q

How can the direction of the dc motor be reversed?

A

By swapping the polarity of the dc supply (reversing the current) or swapping the magnetic poles over (reversing the field).

39
Q

What can you do to work out which way the coil will turn in a dc motor?

A

Use fleming’s left hand rule.

40
Q

How do loudspeakers work?

A
  1. An alternating current is sent through a coil of wire attached to the base of a paper cone.
  2. The coil surrounds one pole or a permanent magnet, and is surrounded by another pole, so the current causes a force on the coil causing the cone to move.
  3. When the current reverses, the force acts in the opposite direction, causing the cone to move in the opposite direction too.
  4. Variations in the current cause the cone to vibrate, making the air around the cone vibrateand creates variations in pressure that cause a sound wave.
  5. The frequency of the sound wave is the same as the frequency of the ac so by controlling the frequency of the ac you can alter the sound wave produced.
41
Q

What is the generator effect?

A

The induction of a potential difference (and current if there’s a complete circuit) in a wire which is moving relative to a magnetic field, or experiencing a change in magnetic field.

42
Q

What does the generator effect do?

A

Creates a potential difference in a conductor, and a current if the conductor is part of a complete circuit.

43
Q

How can you create a generator effect.

A

Moving a magnet in a coil of wire or moving a conductor (wire) in a magnetic field (“cutting” magnetic field lines).

44
Q

How can you create current if the conductor is part of a complete circuit?

A

Shifting the magnet from side to side.

45
Q

The Generator Effect: How can you reverse the potential difference/ current?

A

Moving the magnet or conductor in the opposite direction.

46
Q

The Generator Effect: How can you produce an alternating current?

A

If you keep the magnet or coil moving backwards and forwards.

47
Q

How can you create an alternating current by turning a magnet end to end in coil?

A

As you turn the magnet, the magnetic field through the coil changes which induces a potential difference, which can make current flow in the wire.
When the magnet has been turned through a half turn, the direction of the magnet field through the coil reverses. This means the pd reverses to the current flows in the opposite direction.
If you keep the magnet in the same direction the pd will keep reversing every half turn.

48
Q

Why does induced current always oppose the change that made it?

A

The magnetic field created by an induced current always acts against the change that made it as it is try to return things to the way they were.

49
Q

How can you change the size of the induced pd?

A

Change the rate that the magnetic field is changing.

  1. Increase the speed of the movement - cutting more magnetic field lines in a given time.
  2. Increasing the strength of the magnetic field (so there is more field lines that can be cut.
50
Q

Describe how alternators generate alternating current.

A
  1. Generators rotate a coil in a magnetic field.
  2. As the coil spins, a current is induced in the coil. The current changes direction every half turn.
  3. Instead of a split ring commutator, ac generators habe slip rings and brushes so they contacts don’t swap every half turn.
  4. This means that they produce an alternating potential difference.
51
Q

Describe how dynamos generate direct current.

A
  1. Work in a similar way as altenators.
  2. They have a split-ring commutator instead of slip rings.
  3. This swaps the connection every half turn to keep the current flowing in the same direction.
52
Q

How can you see the generated potential difference?

A

By using an oscilloscope.

53
Q

What do oscilliscopes how you?

A

How the potential difference generated in the coil changes over time.

54
Q

What does the ac trace look like on an oscilliscope?

A

Goes up and down crossing the horizontal axis.

55
Q

What does the dc trace look like on an oscilliscope?

A

Not a straight line, stays above the axis (pd always positive).

56
Q

What happens when you increase the frequency of revolutions?

A

Increases overall pd and creates more peaks.

57
Q

How do microphones work?

A
  1. Sound waves hit a flexible diaphragm that is attahed to a coil of wire wrapped around a magnet. Causing the coil of wire to move in the magnetic field, generating a current.
  2. The movement of the coil depends on the properties of the sound wave (louder sounds make diaphragm move further).
  3. This is how microphones convert the pressure variations of a sound wave into variations in current in an electric circuit.
58
Q

What do transformers do?

A

Change the size of the potential difference of an alternating current.

59
Q

What is the structure of a transformer?

A

Two coils of wire, the primary and secondary, joined with an iron core.

60
Q

What happens when alternating pd is applied across the primary coil?

A

The iron core magnetises and demagnetises quickly. The changing magnetic field induces an alternating pd in the secondary coil. If the secondary coil is part of a complete circuit the current will also be induced.

61
Q

What is the ratio between the primary and secondary pd the same as?

A

The ratio between the number of turns on the primary and secondary coils.

62
Q

What do step-up transformers do?

A

Step the potential difference up (increase it). They have more turns on the secondary coil than the primary coil.

63
Q

What do step-down transformers do?

A

Step the potential difference down (decrease it), they have more turns on the primary coil than the secondary.

64
Q

What equation can you use to calculate the output pd from a transformer?

A

(Vp/Vs)=(np/ns), Input pd/ output pd = number of turns on primary coil/ number of turns on secondary coil.

65
Q

(Vp/Vs)=(np/ns), what can you do so there is less rearranging?

A

Have the unknown on the top.

66
Q

What equation can you make when assuming transformers are almost 100% efficient?

A

VsIs=VpIp, Pd across secondary coil x current through secondary coil = Pd across primary coil x current through primary coil. (input power = output power).

67
Q

Why is the national grid an efficient way to transmit power?

A

Low current means less energy is wasted heating the wires to the surroundings.

68
Q

VsIs=VpIp: What does this equation show?

A

For a given power, a high pd is needed for a low current.

69
Q

(Vp/Vs)=(np/ns): What can this equation be used to work out?

A

The number of turns needed to increase the pd (and decrease current) to the right levels.

Physics GCSE Paper 2 (4 decks)

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