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Physics⚡️ > Chapter 15 - Electromagnetism > Flashcards

Chapter 15 - Electromagnetism Flashcards

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1
Q

General rule of mgnetism

A

like poles repel, opposite poles attract

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2
Q

what are permanent magnets made from and why?

A

steel, it doesn’t lose its magnetism easily

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3
Q

what is a magnetic field?

A

the region around the magnet

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4
Q

why are magnetic field lines concentrated at the poles?

A

the magnetic fields are strongest at the poles

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5
Q

where will the line of direction of a force from a magnet always go?

A

from north to south

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6
Q

why does a magnet have a smaller effect on a compass when it is further away?

A

the magnetic force is weaker

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7
Q

what is induced magnetism?

A

when an unmagnetised material becomes magnetised by placing it in a magnetic field

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8
Q

what happens when a induced magnet is held near a bar magnet?

A

there is always an attractive force, no matter what end of the magnet it is

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9
Q

what is a solenoid?

A

a long coil of insulated wire

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10
Q

what happens to the magnetic field in a solenoid when the current is increased?

A

the strength of the magnetism increases

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11
Q

what happens to the magnetic field in a solenoid when the current is reversed?

A

the direction of the magnetic field reverses

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12
Q

what is an electromagnet and how is one formed?

A

a solenoid is wrapped around an iron bat which acts as the core
when the current is switched on, it travels along the solenoid and creates a magnetic field around the wire

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13
Q

what is the magnetic field of a solenoid like?

A

the magnetic field of a bar magnet, but this one is a complete loop

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14
Q

what is the magnetic field like inside a solenoid?

A

strong and uniform

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15
Q

which direction do the field lines of a solenoid go in and what are they like in comparison to the axis?

A

parallel to axis

all in same direction

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16
Q

what is a magnetic field?

A

a region surrounding a magnet where other magnetic objects experience a force

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17
Q

why do compasses always point north?

A

the earth generates its own magnetic field

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18
Q

how does a compass work?

A

there is a tiny magnet inside
the north pole is attracted to the south pole of any other magnet
the compass points in the direction of the magnetic field

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19
Q

what is the difference between a permanent and induced magnet?

A

a permanent magnet generates its own magnetic field

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20
Q

what do the magnetic field lines of a wire look like?

A

concentric circles perpendicular to the wire

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21
Q

how does the right hand thumb work?

A

point the thumb in the direction of the current flowing
curl fingers
direction of fingers is direction of field

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22
Q

what is an electromagnet?

A

a solenoid wrapped around an iron core which can be turned on and off

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23
Q

how do electromagnets work in a crane?

A

when current passes through the wire, it becomes magnetised
it sticks to the metal to pick it up
when the current is switched off, it drops it

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24
Q

4 examples of electromagnets

A

electric bell
circuit breaker
crane
relay

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25
Q

how do electromagnets work in an electric bell?

A

when the switch is pressed, the circuit is complete
current flows
this magnetises the iron core, attracting the iron armature
the arm hits the bell and it rings
the make-and-break switch is opened and the current and so electromagnet is switched off
the armature swings back, the make-and-break switch closes and the whole cycle is ready to repeat itself

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26
Q

how does an electromagnet work in a relay?

A

when the circuit is complete, current flows round the coil and the iron is magnetised
this pulls the armature onto the magnet
this action makes the pivot swing and hit the poles
this closes the gap between poles and the circuit is complete so current flows

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27
Q

what is the motor effect?

A

a current passes through a wire in an electric field and it moves

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28
Q

how can the size of the force between a wire and magnet be increased?

A

increasing the current

using a stronger magnet

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29
Q

what happens to the size of the force between a wire and magnet when the wire is perpendicular to the field?

A

it is greatest

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30
Q

what happens to the size of the force between a wire and magnet when the wire is parallel to the field lines?

A

it is 0

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31
Q

what does Fleming’s Left-hand Rule do?

A

shows how current, motion and field interact

32
Q

what does the first finger stand for in FLHR?

A

(magnetic) field

33
Q

what does the second finger stand for in FLHR?

A

current (of wire)

34
Q

what does the thumb in FLHR stand for?

A

motion

35
Q

what is the magnetic flux density of a magnetic field?

A

the strength of a magnetic field

36
Q

what does the force on a conductor (the wire) depend on?

A

current and length of conductor

magnetic flux density

37
Q

equation for force on a conductor (n)

A

magnetic flux density x length x current

38
Q

why is graphite used as brushes in a motor?

A

it conducts electricity but creates very little friction

39
Q

how does an electric motor work?

A

when the current flows, the negative side of the wire will rise (use FLHR)
the positive side will move down because of FLHR
when the contacts reach the gaps of the split ring commutator, no current flows and it continues turning out of its own inertia
the wires have now swapped sides
this process repeats and the commutator spins

40
Q

importance of split ring commutator

A

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

41
Q

how does a loudspeaker work?

A

when the alternating current is switched on, it flows around the wire and magnetises the middle pole, turning it into an electromagnet
because the current is constantly alternating, the magnetic field is constantly alternating
this means it alternatively attracts and repulses the cone
the cone vibrates because of this and this produces sound waves

42
Q

what is electromagnetic induction?

A

when coils of wire spinning in a magnetic field create a voltage by crossing through magnetic field lines

43
Q

what is the generator effect?

A

creates an induced p.d. in the conductor

if the conductor is part of a complete circuit, a current is induced

44
Q

what is the potential difference of mains electricity?

A

230 V

45
Q

what do transformers do?

A

they change the size of the potential difference in an alternating current

46
Q

what does a step-up transformer do?

A

increases the size of an alternating potential difference

47
Q

what does a step-down transformer do?

A

decreases the size of an alternating potential difference

48
Q

why are iron cores used in transformers?

A

it is easily magnetised

49
Q

where are step-up transformer used?

A

step pd up from power stations to the national grid

50
Q

what pd is found in power stations?

A

25 kV

51
Q

what pd is found on the national grid?

A

132,000 V

52
Q

where are step-down transformers used?

A

step pd down from national grid to consumers

53
Q

why don’t transformers work with a direct current?

A

there is no changing magnetic field so the secondary pd will always be zero

54
Q

what is the transformer equation?

A

potential difference across primary coil /potential difference across secondary coil

number of turns on primary coil / number of turns on secondary coil

55
Q

what is the efficiency of most transformers?

A

almost 100%

56
Q

equation for transformer efficiency

A

primary potential difference x primary current

secondary potential difference x secondary current

57
Q

what does a step-down transformer do to current and pd?

A

pd is raised, current is reduced

58
Q

why does a step up transformer increase the potential difference?

A

this means the current is reduced
so, the heating effect is smaller
this means less energy is wasted

59
Q

what is the heating effect in a wire proportional to?

A

the square of the current

60
Q

how does a transformer work? SEE REVISION CARDS

A

the primary coil is connected to an a.c. supply
when the a.c. passes through this coil, the core magnetises and demagnetises quickly
this results in a changing magnetic field
this induces an alternating p.d. in the secondary coil
if the secondary coil is part of a complete circuit, a current is induced

61
Q

how to create a generator?

A

move a magnet or a coil of wire in a magnetic field

62
Q

what is created if you keep moving aa magnet back and forth through a magnetic field?

A

an a.c. current

63
Q

why does a generator work?

A

the movement of the bar magnet induces a p.d. in the coil

this p.d. creates a current because the coil is part of a complete circuit

64
Q

how to make the current created by a generator stronger?

A

use a stronger magnet

65
Q

how does a microphone work?

A

like a loudspeaker in reverse
sound waves hit flexible diaphragm that is attached to a coil of wire wrapped around a magnet
this causes the coil of wire to move in and out of the magnetic field = current
louder sounds make the diaphragm move further
microphones can convert the pressure variations in sound waves into variations in current in the circuit

66
Q

what is an alternator?

A

an alternating current generator

67
Q

how to reverse the direction of the current created in a generator? 2

A

move the magnet in the opposite direction

reverse the polarity of the magnet

68
Q

how to increase the size of induced p.d.? 2

A

increase the speed at which magnetic field lines are cut

increase the strength of the magnetic field

69
Q

what is a dynamo?

A

a direct current generator

70
Q

difference between dynamo and generator?

A

dynamos have a split ring commutator instead of slip rings

71
Q

why does an induced current always oppose the charge that made it?

A

a magnetic field can induce a current in a wire
but when a current flows through a wire, a magnetic field is induced
the induced magnetic field is trying to return things to the way it was - it always acts against the charge that made it

72
Q

how does a microphone work?

A

sound wave hit a diaphragm
diaphragm attached to a coil of wire wrapped around a magnet
this causes coil of wire to move in the magnetic field
this induces a current
the generated current has the same properties as the sound wave

73
Q

how does a loudspeaker work?

A

when power on, a.c. current flows around circuit
current flows around coil, turning it into electromagnet
current alternates, so induced magnetic field does too
attracts then repulses cone
cone vibrates, creating sound waves

74
Q

how does a d.c. dynamo work?

A

a coil rotates inside a magnetic field
an electric current is induced
the output comes via split ring output
as the coil turns, the split-ring commutator reconnects the coil the opposite way around each the circuit each half turn
this means that the wire will only be connected to one half of the output
therefore, the current flows in one direction only

75
Q

how does an alternator work?

A

a coil rotates inside a magnetic field
an electrical current is induced
the current changes direction each half turn
the output comes via slip rings
this means the contacts don’t swap every half turn
this creates an alternating signal

Physics⚡️ (16 decks)

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