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Physics > Electromagnetism > Flashcards

Electromagnetism Flashcards

1
Q

what is magnetism caused by?

A

movement of electric charges

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2
Q

what is a stationary charge surrounded by

A

an electric field

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3
Q

what is a moving charge surrounded by

A

a magnetic field

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4
Q

what is the cause of magnetism in a permanent magnet

A

electrons orbiting and spinning around the nucleus

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5
Q

what does the medium surrounding the moving charge have an influence on?

A

has an influence on the effects of the magnetism

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6
Q

what is the magnetism caused by in a non-magnetic object

A

the different electrons in each atom cancel each other out

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7
Q

does the cancellation happen in a permanent magnet?

A

no

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8
Q

what forms magnetic domains

A

magnetic fields due to moving electrons align themselves

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9
Q

what happens when the magnetic domains are aligned?

A

you have a permanent magnet

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10
Q

poles in magnets

A

permanent magnets have a north and south pole

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11
Q

if a permanent magnet is broken in two

A

each piece has a north and south pole

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12
Q

earths magnetic field is partially due to

A

currents circulating in molten lava within the earth

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13
Q

what does the earths magnetic field resemble

A

as if there was a large bar magnet within the middle of the earth

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14
Q

do the earths magnetic poles vary

A

they vary from time to time

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15
Q

how to plot the magnetic field of a bar magnet

A

place it on a piece of paper and sprinkle iron filings around it. tap the paper gently and the iron filings align themselves

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16
Q

what is a solenoid

A

a coil of wire used as an electromagnet

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17
Q

describe the apparatus used to plot the magnetic field of a solenoid

A

it consists of a solenoid with a piece of plastic passing midway through it

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18
Q

how to obtain the magnetic field of a solenoid

A

sprinkle iron filings on the sheet of plastic and tap gently

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19
Q

looking end on, a solenoid, current flowing clockwise

A

south pole

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20
Q

looking end on, a solenoid, current flowing anti clockwise

A

north pole

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21
Q

describe set up of apparatus when plotting the magnetic field of a straight wire

A

a straight conducting wire is passed through a piece of paper held horizontally

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22
Q

how do you find the magnetic field of a straight wire

A

sprinkle iron filings on the piece of paper and tap gently, they will arrange themselves in the magnetic field

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23
Q

how to find the direction of magnetic field lines of a straight wire

A

the right hand thumb rule

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24
Q

describe the right hand thumb rule

A

when you hold the wire in your right hand, the conventional current is flowing in the direction of your thumb, then the electric field is flowing in the direction of your fingers

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25
Q

how do you show that a current carrying conductor in a magnetic field experiences a force

A

put a wire connected to a switch and battery inside a folded magnet. close the switch. if the force it experiences is big enough, it will move.

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26
Q

the magnitude of a magnetic force equation

A

F = I l B sinθ

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27
Q

I (letter i)

A

Current

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28
Q

l (letter L)

A

length of a conductor in a magnetic field

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29
Q

B

A

magnetic flux density

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30
Q

θ

A

angle between direction of current and magnetic field

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31
Q

if there is no angle given and you want to find the magnetic force

A

F = I l B

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32
Q

Definition of magnetic flux density

A

the force acting on 1 m of a conductor carrying 1 A of current, perpendicular to the direction of the magnetic field

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33
Q

symbol of magnetic flux density

A

B

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34
Q

unit of magnetic flux density

A

T

tesla

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35
Q

is magnetic flux density a vector or scalar quantity

A

scalar

36
Q

how is the force obtained

A

flemings left hand rule

37
Q

flemings left hand rule is the principle of what

A

main principle behind electric motors, loudspeakers and electric meters (analog)

38
Q

force is perpendicular to what? (2)

A

force is perpendicular to the direction of the magnetic field and perpendiculoar to the direction of motion of a charged particle

39
Q

what does the fact that “force is perpendicular to the direction of the magnetic field and perpendiculoar to the direction of motion of a charged particle” cause?

A

circular motion

40
Q

wht is perpendicular in circular motion?

A

force and velocity

41
Q

force on a charged particle moving in a magnetic field equation

A

F = q v B sinθ

42
Q

F

A

force

43
Q

q

A

charge

44
Q

v

A

velocity

45
Q

B

A

magnetic flux density

46
Q

θ

A

angle between direction of moving charge and direction of magnetic field

47
Q

how can two parallel conductors with current flowing the the same direction, attract each other?

A

the magnetic fields cancel due to the fact that they are opposite in this region. weak magnetic field between them, strong magnetic fields on the other sides. they get pushed into the weak magnetic field. They seem like they attract

48
Q

how can two parallel conductors with current flowing the opposite direction, repel each other?

A

the magnetic fields add due to the fact that they are the same in this region. strong magnetic field between them, weak magnetic fields on the other sides. they get pushed into the weak magnetic fields. They seem like they repel

49
Q

2 equations for magnetic force

A 
F = I l B
F = q v B
50
Q

if a magnetic force causes circular movemet, what do we do in maths questions?

A

let magnetic force equal centripetal force

51
Q

magnetic force

A

q v B

52
Q

centripetal force

A

r

53
Q

set up to show how a motor works

A

iron cylinder in the middle
bent magnet around it
wire with current flowing in it around top and bottom of cylinder

54
Q

what happens when you close the switch on a motor

A

left hand side of the coil is forced inwards and right hand side of the coil is forced outwards

55
Q

if the magnetic field is uniform for a motor

A

then the two forces are equal in magnitude, opposite in direction and do not have a common line of action
ie. a couple

56
Q

what happens when you move a coil relative to a magnet

A

current starts to flow

57
Q

what causes the current to flow when you move a coil relative to a magnet

A

the change in magnetic flux passing through the coil made a voltage which caused the current to flow

58
Q

how does alternating current work with a lamp?

A

current flow from terminal A of the generator, through the lamp and to terminal B of the generator and then from terminal B to terminal A. it switches 50 times a second

59
Q

explain the graphs of both voltage and current over time with ac

A

the magnitude of both current and voltage starts at zero, rises rapidly to a peak value, decreases rapidly to zero. then the direction changes and magnitude of current and voltage start at zero, rise rapidly to a peak value, decrease rapidly to zero

60
Q

how long does the ac cycle usually take

A

0.02 seconds

61
Q

equation for root mean square voltage of ac

A

Vo
Vrms = —-
√2

62
Q

equation for root mean square current of ac

A

Io
Irms = —-
√2

63
Q

Vrms

A

root mean sqaure voltage

64
Q

Vo

A

peak voltage

65
Q

Io

A

peak current

66
Q

Irms

A

room mean square current

67
Q

what is the primary circuit of a conductor connect to?

A

an ac source which give a changing magnetic flux in the iron frame

68
Q

what’s happening to the north and south poles on both sides of the iron fram

A

they are switching places at the same frequency as the a.c source

69
Q

what does the changing magnetic flux cause

A

it causes an emf in the secondary circuit

70
Q

what happens in the second circuit?

A

the bulb lights/ammeter detects current

71
Q

what is the value of the voltage of the secondary circuit compared to the first?

A

it could be smaller, the same or larger

72
Q

what does the voltage of the secondary circuit depend on?

A

the ration of number of turns of wire in the two circuits

73
Q

step up transformer (turns in wires)

A

larger number of turns in the secondary circuit

74
Q

step down transformer (turns in wires)

A

smaller number of turns in the secondary circuit

75
Q

to obey the law of conservation of energy, a larger voltage in the secondary circuit results in?

A

an smaller current in the secondary circuit

76
Q

to obey the law of conservation of energy, a smaller voltage in the secondary circuit results in?

A

an larger current in the secondary circuit

77
Q

Vi
—- = (2)
Vo

A

Np Io
—– = —–
Ns Ii

78
Q

Vi

A

input voltage

79
Q

Vo

A

output voltage

80
Q

Np

A

number of turns in pri

81
Q

Ii

A

input current

82
Q

Io

A

output current

83
Q

2 energy losses in transformer

A

laminated iron slices get loose and vibrate and generate the sound of a hum
ohmic resistance in the coils of a wire

84
Q

1 use of step down transformers

A

mobile phone rechargers

85
Q

1 use of step up transformers

A

older TV sets

86
Q

2 reasons why ESB uses tranformers

A
  1. energy transmitted at very high voltages and low currents results in reduction in heat energy loss
  2. ac supply very versatile because you can change the voltage of the ac

Physics (11 decks)

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