Electromagnetic induction Flashcards Preview

Physics 1 - Particles, quantum phenomena and electricity > Electromagnetic induction > Flashcards

Flashcards in Electromagnetic induction Deck (100)
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1
Q

Why, when transporting electricity on the National Grid, are high voltages and low currents used

A

The heat losses are reduced

2
Q

In an experiment to illustrate electromagnetic induction, a permanent magnet is moved
towards a coil causing an emf to be induced across the coil.
Using Faraday’s law, explain why a larger emf would be induced in this experiment if a
stronger magnet were moved at the same speed.

A

greater flux (linkage) or more flux lines (at same distance)
[or stronger magnet produces flux lines closer together] (1)
greater rate of change of flux (linkage)
[or more flux lines cut per unit time] (1)
emf rate of change of flux (linkage) (1)

3
Q

State four factors which if increased would increase the magnitude of the induced electromotive force when coils are removed quickly from magnetic fields

A

flux density of magnetic field
speed of movement
area of coil
initial angle between plane of coil and magnetic field

4
Q

Magnet dropped into solenoid what would happen on ammeter as magnet falls through coil

what happens as it enters the coil and as it leaves the coil and what would happen if circuit was incomplete

A

deflects on way then the other

acceleration is less than g (due to Lenz’ law)
if incomplete then magnet falls at acceleration g, emf induced but no current, no energy lost from circuit (no opposing force on magnet from force from produced magnetic field)

5
Q

Magnetic flux units

A

Nm/A
Wb
Tm^2

6
Q

Three identical magnets P Q R are released simultaneously from rest and fall to the ground from the same height. P falls directly to the ground, Q falls through the centre of a thick conducting ring and R falls through a ring which is identical except for a gap cut into it. What sequence do they arrive in?

A

P and R arrive together followed by Q

Higher resistivity slows down journey to bottom

7
Q

Magnetic flux unit

A

Wb

8
Q

Cauases of inefficiency

A

Eddy currents, resistance in coils, not all of magnetic flux through coil one passes through coil two

9
Q

Farady’s law of induction

A

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

10
Q

Len’z law

A

the direction of the induced e.m.f is such as to oppose the change that induces it

(generates magnetic field that generates a repulsive, opposing reaction force against the change that caused it)

= Newton’s third law

11
Q

High voltage is used in the National Grid to

A

reduce power loss to resistance of cables

12
Q

Unit for magnetic flux. BA = Φ

A

Wb or Tm^2

13
Q

BA = magnetic flux =

A

Flux linkage through a closed loop of wire is the total flux Φ enclosed by the wire, for a single loop this is the average B * A of the coil

14
Q

If a wire cuts through a magnetic field lines then

A

an emf is induced across it

15
Q

Left hand rule for

A

motor effect

16
Q

Right hand rule for

A

Generators/induction

17
Q

The relative motion of wires and field lines which induces the enf so:

A

field lines cutting the turns of a coil induces an emf, a coil cutting field lines also induces an emf

Since the turns are in series, each additional turn adds to the emf generated. If we complete the circuit then the emf will cause a current to flow

18
Q

The flux linkage through a coil can only change if

A

some of the flux crosses the area

19
Q

An emf in a coil will generate if

A

there is a net charge in flux linkage

20
Q

NΦ = BAN and the induced emf =

A

NΔΦ/Δt

21
Q

Fixed coil in a solenoid with changing B =

A

ΔBAN/Δt

22
Q

If there is a complete circuit then an induced emf causes … so there is a _____ due to the _____

A

a current to flow

So there is a force on the wire due to the motor effect

23
Q

The direction of force is always in what direction

A

opposite direction to the motion which caused the emf

24
Q

Work done by falling magnet =

A

mgd - Ek = dissipated energy as heat by induced current in the pipe

25
Q

If the electric current does work then the work must be supplied by

A

the force generating the current

26
Q

Emf = W/Q =

A

BLV = BA/t

27
Q

BNLV =

A

NΦ/t

28
Q

Time taken for coil to enter field completely =

A

w/v

29
Q

An alternating current is

A

a current that repeatedly reverses direction.

30
Q

Mains electricity has a frequency of

A

50 Hz

31
Q

The peak value of an alternating current is the

A

maximum current in either direction

32
Q

The peak current in a circuit depends on

A

the peak pd of the alternating current source and on the components in the circuit.

33
Q

The peak to peak value is

A

the difference between the peak value one way and the peak value in the opposite direction (i.e. twice the peak value)

34
Q

Sinusoidal variation is when

A

the shape of the voltage/current time graph is a sine wave.

35
Q

we can change the peak pd of an alternating current using _____ but not _____

A

transformers but not its frequency

36
Q

For a sinusoidal current, the mean power over a full cycle is

A

half the peak power

37
Q

The maximum power is achieved at

A

Maximum current

38
Q

mean power is

A

0.5rI(0)^2

39
Q

The direct current that would give the same as the mean power is called

A

the root mean square value of the alternating current

The root mean square of an alternating current is the value of direct current that would give the same heating effect as the alternating current in the same resistor.

40
Q

Irms =

Vrms =

A

1/(√2)*I0

1/(√2)*V0

41
Q

Electromagnetic induction - what is it and explain

A

To generate electricity all you need is a magnet and some wire connected to a sensitive meter. When the magnet is moved near the wire, a small current passes through the meter. This happens because an electromotive force is induced in the wire. This effect is known as electromagnetic induction and occurs whenever a wire cuts across the lines of a magnetic field. If the wire is part of a complete circuit, the induced emf forces electrons round the circuit.

42
Q

The induced emf can be increased by

A

moving the wire faster, using a strong magnet, making the wire into a coil and pushing the magnet in or out of the coil.

43
Q

No emf is induced in a wire if

A

a wire is parallel to the magnetic field lines as it moves through the field

44
Q

For an emf to be induced in a wire

A

The wire must cut across the lines of the magnetic field for an emf to be induced in the wire.

45
Q

Other methods of generating an induced emf

A

• Using an electric motor in reverse. A falling weight attached to the motor makes the motor coil turn between the poles of the magnet in the motor. The emf induced in the coil forces a current round the circuit and so causes the lamp to light. The faster the coil turns, the brighter the lamp is.

Using a cycle dynamo. When the magnet in the dynamo spins, an emf is induced in the coil. If the coil is connected to a lamp, the lamp lights because the emf forces a current round the circuit.

In both examples, an emf is induced because there is relative motion between coil and the magnet. In the electric motor in reverse, the coil spins and the magnet is fixed. In the dynamo, the magnet spins and the coil is fixed.

46
Q

When a magnet is moved relative to a conductor (e.g. a wire or coil)… explain this

A

an emf is induced in the ciruit
If the conductor is part of a complete circuit which has no other sources of emf, a current passes round the circuit just as if the circuit included a battery. However, unlike the emf of a battery which is a constant, the induced emf becomes zero when the relative motion between the magnet and the wire ceases.

47
Q

For conductor/generator use what rule

A

Dynamo rule = Fleming’s right-hand rule = for a conductor/generator, riGht hand rule = Generators

for straight conductor which is moving at right angles to the magnetic field in order to induce an electric current

48
Q

A transformer does what

A

changes an alternating pd to a different peak value

49
Q

Any transformer consists of

A

two coils: the primary coil and the secondary coil

50
Q

The two coils in the transformer share the same

A

iron core

51
Q

How does a transformer work

A

When the primary coil is connected to a source of alternating pd, an alternating magnetic field produced in the core. The field passes through the secondary coil. So an alternating emf is induced in the secondary coil by the changing magnetic field.

52
Q

Circuit symbol for transformer

A

look in text book

and magnetic field what it looks like for solenoid

53
Q

A transformer is designed so that

A

all the magnetic flux produced by the primary coil passes through the secondary coil

54
Q

Transformer rule/equation

A

Vs / Vp = Ns / Np = Ip / Is

55
Q

Step up transformer

A

A step up transformer has more turns on the secondary coil than on the primary coil so the secondary voltage is stepped up compared with the primary voltage i.e. Ns > Np so Vs > Vp

In a step up transformer, the voltage is stepped up and the current is stepped down.

56
Q

Step down transformer

A

A step down transformer has fewer turns on the secondary coil than on the primary coil. So the secondary voltage is stepped down compared with the primary voltage i.e. Ns < Np so Vs < Vp

In a step down transformer, the voltage is stepped down and the current is stepped up

57
Q

How are transformers made efficient (3)

A

Transformers are almost 100% efficient because they are designed with: 1) low resistance windings to reduce power wasted due to the heating effect of the current. 2) a liminated core which consists of layers of iron separated by layers of insulator. Induced currents in the core itself, referred to as eddy currents, are also reduced in this way so the magnetic flux is as high as possible. Also, the heating effect of the induced currents in the core is reduced. 3) a soft iron core which is easily magnetised and demagnetised. This reduces power wasted through repeated magnetisation and demagnetisation of the core

58
Q

Efficiency of transformer equation

A

(Power delivered by the secondary coil) / (Power delivered to the primary coil) = IsVs /IpVp *100%

59
Q

Transformers only work if

A

the magnetic flux through them is changing, they won’t t work with steady dc

60
Q

Power stations generate alternating current at x Hz and yV

A

50Hz 25kV
Step up transformers at the power station increase the alternating voltage to 400kV or more for long distance transmission via the grid system. Step down transformers operate in stages.

61
Q

How are electrical power transmissions kept efficient

A

Transmission of electrical power over long distances is much more efficient at high voltage than at low voltage which is why the current needed to deliver a certain amount of power is reduced if the voltage is increased. So power wasted due to the heating effect of the current through the cables id reduced. The higher the voltage is, the smaller the ratio of the wasted power to the power transmitted is.

Superconducting cables would be even more efficient. At present, materials that are superconducting at sufficiently higher temperatures do not exist.

62
Q

A dc generator can be made by

Why does the emf not reverse its polarity

A

replacing the two slip rings of the ac generator with a split ring. The emf does not reverse its polarity because the connections between the split ring and the brushes reverse every half cycle

63
Q

For a rotating coil, the rate of change of flux is greatest when

A

the flux through it is zero

64
Q

The rate of change of flux is zero when

A

the flux through it is greatest

65
Q

An emf is induced in the spinning coil of an electric motor because

A

the flux linkage through the coil changes

66
Q

The emf induced in the spinning coil of an electric motor is known as … because…

A

An emf is induced in the spinning coil of an electric motor because the flux linkage through the coil changes. The induced emf is referred to as a back emf because it acts against the pd. V applied to the motor in accordance with Lenz’s law. At any instant, V – e = IR.

67
Q

Electrical power supplied by the source IV = Electrical power transferred to mechanical power Ie =

A

Electrical power wasted due to circuit resistance I^2R.

68
Q

When a device is connected to the secondary coil, because the efficiency of a transformer is almost equal to 100% then

A

the electrical power supplied to the primary coil = the electrical power supplied to the secondary coil therefore the current ratio Is/Ip = Vp/Vs = Np/Ns

69
Q

The flux linkage in the secondary coil =

A

NsΦ

70
Q

the induced emf in the secondary coil

A

Vs = Ns ΔΦ/Δt

71
Q

Peak emf, e0 =

A

2NBlv = 2pifNBA = BANw

72
Q

epsilon =

A

BANwsinwt

73
Q

Observing alternating current

A

• use an oscilloscope to display the waveform (i.e. variation with time) of the alternating pd from from a signal generator; (increasing the output pd from the signal generator makes the oscilloscope trace taller. This shows the peak value of the alternating pd has been made larger. Increasing the frequency of the signal generator increases the number of cycles on the screen. This is because the number of cycles per second of the alternating pd has increased)

74
Q

Len’z law states that

A

the direction of the induced current is always such as to opposite the change that causes it.

Conservation of energy states that the induced current must be in the opposite direction to what causes it

75
Q

Induced emf, epsilon =

A

BA/t

76
Q

Magnetic flux =

A

Magnetic flux density * Area

77
Q

Magnetic flux linkage =

A

magnetic flux * number of turns = magnetic flux linkage where B is perpendicular to area

78
Q

Flux density is

A

the flux per unit area passing at right angles through the area

79
Q

1 Tesla =

1Wb =

A

1Wb/m^2

1V/s

80
Q

Whenever the flux linkage through a circuit changes

A

an emf is induced in the circuit

81
Q

Two things that can cause flux are … explain each one

A

permanent magnet or current carrying wire

A) if the flux is due to a permanent magnet, motion of them magnet relative to the circuit is necessary to cause an induced emf. This is how an emf is generated in an ac generator or a dynamo.

B) if the flux is due to a current-carrying wire, changing the current in the wire causes an induced emf in the circuit. This is how an emf is generated in a transformer or induction coil.

82
Q

When the magnetic field is perpendicular to the coil face, the flux linkage =

A

N*phi = BAN

83
Q

When the coil is turned through 180 degrees, the flux linkage =

A

-BAN

84
Q

When the magnetic field is parallel to the coil area, the flux linkage =

A

0 as no field lines pass through the coil area

85
Q

When the magnetic field is at angle X to the normal at the coil face, the flux linkage through the coil N*phi =

A

BANcosX

86
Q

Faraday’s law of electromagnetic induction states that

A

the induced emf in a circuit is equal to the rate of chage of flux linkage through the circuit

87
Q

Induced emf, epsilon =

A

= -N * delta phi / delta t, the minus sign represents the fact that the induced emf acts in such a direction as to oppose the change that caused it

88
Q

Induced emf in a moving conductor in a magnetic field =

A

Blv

89
Q

Induced emf in a fixed coil in a changing magnetic field

A

Induced emf in a fixed coil in a changing magnetic field = N*delta phi/delta t = AN * delta B/delta t As B is proportional to the current I in the solenoid. The magnitude of the induced emf is proportional to the rate of change of current

90
Q

A rectangular coil moving into a uniform magnetic field

A

the time taken by the coil to enter the field completely = coil width / speed , during this time the flux linkage increases steadily from 0 to BNlw therefore the change of flux limkage per second = BNlv and so a graph of flux linkage, BNlw against time, w/x is y=x. When the coil is completely in the field, the flux linkage through it does not change so the induced emf is zero/. The induced emf, BNlv, against time, width/velocity, is a graph of y=c

91
Q

A simple ac generator consists of

A

simple ac generator consists of a rectangular coil that spins in a uniform magnetic field.

92
Q

When the coil spins at a steady rate in an ac generator

A

the flux linkage changes continuously

93
Q

The induced emf in an ac generator =

A

epsiolon(0)sin(2ftpi) where f is frequency of rotation and e0 is the peak emf
(2fpi = w)
(V = piwf)

94
Q

The induced emf is a maximum when

A

the sides of the coil cut at right angles across the field lines. At this position, the emf induced in each wire of each side = Blv, so for N turns and two sides, the induced emf at this position e0= 2NBlv. The equation shows that the peak emf can be increased by increases the speed (frequency of rotation) or using a stronger magnet or bigger coil or coil with more turns

95
Q

A magnetic field is produced in and around a coil when it is connected to a bateery and a current is passed through it. In a solenoid, if each end in turn is viewed from the outside, current passes

A

aNticlockwise round the North pole end and clockwise at the south pole end

96
Q

The rate of transfer of energy from the source of emf to the other components of the circuit is equal to

A

the product of the induced emf and the current. This is because: the induced emf is the energy transferred from the source per unit charge that passes through the source, the current is the charge flow per second.

97
Q

induced emf * current = power =

A

energy transferred per second from the source

98
Q

In transformer calculations, remember that the currents are determined by

A

the load in the secondary coil

99
Q

Because induced emf is proportional to the speed of rotation of the motor…

A

low speed = low emf = high current

high speed = high emf = low current

100
Q

The changing magnetic flux in the core induces a back emf in the primary coil as well as an emf in the secondary coil. The back emf acts against the primary voltage, making the primary current very small when the secondary current is ‘off’. When the secondary current is on, the magnetic field it creates is in the opposite direction to the magnetic field of the primary current. In this situation, the back emf in the primary coil is reduced so the primary current is larger than when the secondary current is off.

A

When a beam of electrons is directed across a magnetic field, each electron experiences a force at right angles to its direction of motion and to the field direction. A metal rod is a tube containing lots of free electrons. If the rod is moved across a magnetic field, the magnetic field forces the free electrons in the rod to one end away from the other end. So one end of the rod becomes negative and the other end positive. In this way, an emf is induced in the rod. The same effect happens if the magnetic field is moved and the rod is stationary. As long as there is relative motion between the rod and the magnetic field, an emf is induced in the rod. If the relative motion ceases, the induced emf becomes zero because the magnetic field no longer exerts a force on the electrons in the rod. Note that when the rod is part of a complete circuit, the electrons are forced round the circuit. In other words, the induced emf drives a current round the circuit.