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magnetic fields Flashcards

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1
Q

what way do the field lines go

A

north to south

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2
Q

if the field lines are closer what does this mean

A

the field is stronger

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3
Q

what happens when current flows in a wire or any straight conductor

A

a magnetic field is induced around the wire

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4
Q

what shape are the field lines around a wire

A

concentric circles

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5
Q

what is the motor effect

A

When a current passes along the wire in a magnetic field, a force is exerted on the wire

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6
Q

when is the force greatest during motor effect

A

*when wire is at a right angle to the magnetic field
*zero when the wire is parallel to the magnetic field

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7
Q

what are concentric circles

A

circles with the same centre but different radii

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8
Q

how do you work out the direction of the field lines around a current carrying wire

A

the right hand rule

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9
Q

when using the right hand rule what direction does your thumb point

A

in the direction of conventional current

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10
Q

if you loop a wire into a coil what shape is the field

A

doughnut shaped

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11
Q

if you get a solenoid what shape is the field

A

has a field like a bar magnet

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12
Q

what is a solenoid

A

coil with a length

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13
Q

if you put a current carrying wire into a magnetic field what happens

A

external field and the field from the wire are added together resulting in a resultant field causing a force on the wire

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14
Q

the resultant force experienced from the fields is always in what direction to the magnetic field and current

A

perpendicular to the direction of the magnetic field and the direction of current

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15
Q

if the current is parallel to the field lines the size of the force is

A

0N

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16
Q

in Flemings left hand rule what direction do the fingers point

A

thumb points in the direction of the force first finger points in direction magnetic field and second finger points in direction of current

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17
Q

magnetic flux definition

A

the force on one metre of wire carrying current of one amp at right angles to the magnetic field

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18
Q

what is the force on a wire

A

F=BIL

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19
Q

1 Tesla =

A

Wb/m²

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20
Q

what equation do you use to work out the force acting on a wire when it is at an angle to the magnetic field

A

F=BILsinθ

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21
Q

what is the force acting on a charged particle

A

BQv
B = magnetic flux density in T,
Q = charge on the particle in C,
v = velocity of the particle in ms–1

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22
Q

what path do charged particle in a magnetic field make

A

Charged Particles in a Magnetic Field are Deflected in a Circular Path

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23
Q

why do charged particles deflect into a circular path

A

Fleming’s left-hand rule says that the force on a moving charge in a magnetic field is always perpendicular to its direction of travel.

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24
Q

how do you work out the radius of a charged particle moving in a circular path

A

r=mv/BQ

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25
Q

where are circular deflections used

A

cyclotrons

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26
Q

what are cyclotrons used for

A

Cyclotrons have many uses, for example in medicine. Cyclotrons are used to produce radioactive tracers or high-energy beams of radiation for use in radiotherapy.

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27
Q

what is a cyclotron made of

A

A cyclotron is made up of two hollow semicircular electrodes

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28
Q

what qualities do cyclotrons have

A

a uniform magnetic field applied perpendicular to the plane of the electrodes, and an alternating potential difference applied between the electrodes:

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29
Q

how does the cyclotron work

A

Charged particles are fired into one of the electrodes. The magnetic field makes them follow a (semi)circular path and then leave the electrode. An applied potential difference between the electrodes accelerates the particles across the gap until they enter the next electrode. Because the particle’s speed is slightly higher, it will follow a circular path with a larger radius before leaving the electrode again. The potential difference is reversed so the particle is accelerated again before entering the next electrode. This process repeats as the particle spirals outwards, increasing in speed, before eventually exiting the cyclotron.

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30
Q

what is magnetic flux density a measure of

A

Magnetic flux density, B, is a measure of the strength of a magnetic field. It helps to think of it as the number of field lines per unit area.

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31
Q

what is magnetic flux

A

total number of field lines

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32
Q

how do you work out magnetic flux

A

Φ = BA

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33
Q

when are electromotive forces induced in conductors

A

Electromotive Forces are Induced in Conductors when they move through Magnetic Field

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34
Q

If there is relative motion between a conducting rod and a magnetic field what happens

A

If there is relative motion between a conducting rod and a magnetic field, the electrons in the rod will experience a force which causes them to accumulate at one end of the rod.

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35
Q

what is electromotive induction

A

If there is relative motion between a conducting rod and a magnetic field, the electrons in the rod will experience a force, which causes them to accumulate at one end of the rod This induces an electromotive force (e.m.f.) across the ends of the rod

36
Q

how can you induce an e.m.f. in a flat coil or solenoid

A
  • moving the coil towards or away from the poles of a magnet.
  • moving a magnet towards or away from the coil.
37
Q

what is needed for emf to be induced

A

magnetic field or magnetic flux that passes through the coil has to change

38
Q

what happens if a coil is part of a circuit and emf is induced

A

an induced current will flow through it

39
Q

what does more turns mean for the induced emf

A

More Turns in a Coil of Wire Mean a Bigger e.m.f. will be Induced

40
Q

what is the flux linkage equation

A

NΦ = BAN

41
Q

what does the rate of change in flux linkage tell you

A

The rate of change in flux linkage tells you how strong the electromotive force will be in volts

42
Q

A change in flux linkage of one weber per second will

A

A change in flux linkage of one weber per second will
induce an electromotive force of 1 volt in a loop of wire.

43
Q

what is faradays law

A

The induced e.m.f. is directly proportional to the rate of change of flux linkage.

44
Q

faradays law

A

ε = flux linkage change/time taken = NΦ/t

45
Q

for a graph of NΦ against time what is the gradient

A

the induced emf

46
Q

for a graph of emf against time what is the area under the graph

A

the flux linkage change

47
Q

what is Lenz law

A

the induced emf is always in such a direction as to oppose the change that caused it

48
Q

Lenz law equation

A

emf= -flux linkage change/ time taken = -NΦ/t

49
Q

how does the idea that a induced e.m.f. ina conductor will oppose the change that caused it, agree with the principle of the conservation of energy

A

the energy used to pull a conductor through a magnetic field, against the resistance caused by magnetic attraction, is what produces the induced current.

50
Q

what is a alternator

A

An Alternator is a Generator of Alternating Current

51
Q

what do generators do

A

Generators, or dynamos, convert kinetic energy into electrical energy —
they induce an electric current by rotating a coil in a magnetic field.

52
Q

how many degrees are flux linkage and induced voltage out of phase

A

90 degrees

53
Q

write flux linkage in terms of angular speed

A

flux linkage = NF = BAN cos wt

54
Q

write induced emf in term of angular speed and time

A

e = BAN w sin wt

55
Q

what does alternating current or voltage change with

A

time

56
Q

what can you use oscilloscope for

A

You can use an oscilloscope to display the voltage of an alternating current

57
Q

what does the vertical height on oscilloscope show

A

input voltage at that point

58
Q

what pattern does alternating current give you on a oscilloscope

A

An alternating current (ac) source gives a regularly repeating sinusoidal waveform.

59
Q

what pattern does direct current give you on a oscilloscope

A

horsizontal line

60
Q

if you turn the time base on a oscilloscope what pattern does it give you for alternating current

A

vertical line

61
Q

if you turn the time base on a oscilloscope what pattern does it give you for direct current

A

a dot

62
Q

what is the root mean square voltage useful for

A

can compare direct and alternating current supply

63
Q

what is the root mean square voltage equation

A

vrms= v0/√2

64
Q

what is root mean square current

A

irms=I0/√2

65
Q

how to work out average power for ac supply

A

irms x vrms

66
Q

is mains electricity as or dc

A

ac

67
Q

what is the uk’s mains electricity supply

A

230v

68
Q

what do transformers do

A

Transformers are devices that make use of electromagnetic induction
to change the size of the voltage for an alternating current.

69
Q

what does alternating current flowing in the primary coil do in a transformer

A

produce magnetic flux

70
Q

when magnetic field is changed because of the first coil what happens next

A

The changing magnetic field is passed through the iron core to the secondary (or output) coil, where it induces an alternating voltage of the same frequency as the input voltage.

71
Q

what is the transformer equation for a ideal transformer

A

Ns/Np = Vs/Vp

72
Q

what o step up tarnsformers do

A

Step-up transformers increase the voltage by having more turns on the secondary coil than the primary.

73
Q

what do step down transformers do

A

Step-down transformers reduce the voltage by having fewer turns on the secondary coil.

74
Q

if a transformer was 100% efficient what would be true

A

power in = power out

75
Q

why will there be a loss in power in transformers

A

because of eddy currents in the transformers core

76
Q

what are eddy currents

A

Eddy currents are looping currents induced by the changing magnetic flux in the core. They create a magnetic field that acts against the field that induced them, reducing the field strength. They also dissipate energy by generating heat.

77
Q

how can you reduce the effect of eddy currents

A

laminating the core with layers of insulation

78
Q

heat is also generated by resistance in the coils how can you minimise this

A

Heat is also generated by resistance in the coils — to minimise this,
thick copper wire is used, which has a low resistance.

79
Q

how do you work out the efficiency

A

the ratio of power out to power in
IsVs/IpVp

80
Q

how is electricity from power stations sent round the county

A

sent round the country by the national grid

81
Q

why is electricity sent round thew country at the lowest possible current

A

because the power losses due to the resistance of the cables is equal to P = I²R
— so if you double the transmitted current, you quadruple the power lost.

82
Q

Transformers allow us to step up the voltage to around 400 000 V for transmission through the national grid and then

A

and then reduce it again to 230 V for domestic use.

83
Q

What is a generator

A

turns kinetic enrgy into electrical energy

84
Q

What does a alternator contain

A

A rectangular coil that spins in a uniform magnetic field, slip rings and brushes to connect the coil to an external circuit

85
Q

What happens when the coil in a alternator spins

A

The flux linkage changes continuously

86
Q

In a alternator why is there alternating current

A

Because the output voltage and current change direction with every half rotation of the coil producing an alternating current

physics (11 decks)

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