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A level Physics > Unit 4.5 - Electromagnetic Induction > Flashcards

Unit 4.5 - Electromagnetic Induction Flashcards

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1
Q

Induction

A

Cause something to happen

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2
Q

What is electromagnetic indication?

A

The phenomenon that electricity could be generated by moving a magnetic material relative to a conductor

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3
Q

What is used to generate electricity via electromagnetic induction?

A

Magnetic fields

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4
Q

Magnetic field strength (B)

A

This is a vector quantity. Its direction is that in which the North Pole of a freely-pivoted magnet points. Its magnitude is defined by B = F/Il, in which F is the force on a length l of a wire carrying a current I, placed perpendicular to the direction of the field. Unit T.

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5
Q

What is the model we can use to imagine a magnetic field?

A

A flow model

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6
Q

Magnetic flux unit

A

Wb

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7
Q

Magnetic flux symbol

A

φ

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8
Q

What is magnetic flux?

A

It described the “flow” of magnetism around a magnet or a wire carrying a current

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9
Q

What is the way we consider magnetic flux similar to?

A

The way we consider a flow of light from a lamp

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10
Q

How would we determine the intensity of a light source at a distance from it with a lamp?

A

By measuring the rate at which light energy passes through a given area and dividing this rate with the area

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11
Q

How do we get flux density?

A

Imagine an area A with a uniform magnetic field B passing through it
The total magnetic flux density passing through the area divided by the area gives us the flux density B at this point

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12
Q

How can we visually imagine flux density?

A

Judge how closely bunched up the lines of the field are

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13
Q

Flux density symbol

A

B

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14
Q

What is flux density, B?

A

The flux per m^2

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15
Q

Flux density (B) equation

A

B = φ/A

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16
Q

What is the total flux equation and how do we get to this?

A

φ = ABcostheta

B = φ/A
If this area is at an angle theta to the field then the formula is modified to only involve the perpendicular component of the flux density and becomes:

B = φ/Acostheta
Which is rearrange to calculate the total flux into the top equation

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17
Q

What does each turn in a coil have flowing through it?

A

A magnetic flux φ

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18
Q

Flux linkage

A

The total magnetic flux through a circuit

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19
Q

The total magnetic flux through a circuit

A

Flux linkage

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20
Q

Flux linkage symbol

A

N φ

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21
Q

N φ

A

Flux linkage

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22
Q

Flux linkage equation to learn

A

φ = BANcostheta

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23
Q

φ = BANcostheta
What does this equation calculate?

A

Flux linkage

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24
Q

What appends in a circuit if the flux linkage through the ciruit changes?

A

An emf is induced

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25
Q

When is an emf induced in a circuit?

A

If the flux linkage through the circuit changes

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26
Q

What are the ways in which the flux linkage of a circuit can be changed?

A
  1. By keeping the flux density constant and changing the area enclosed by the circuit
  2. By changing the flux density (B) and keeping the area enclosed by the circuit constant
  3. By a combination of both 1 and 2
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27
Q

What would be unrealistic to change in order to change flux linkage?

A

N

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28
Q

What is it called when the flux density is kept constant and the area enclosed in the circuit is changed?

A

Flux cutting

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29
Q

Flux cutting

A

when the flux density is kept constant and the area enclosed in the circuit is changed

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30
Q

What happens during flux cutting?

A

A conductor cuts across a magnetic field, cutting the lines of flux much as a lawnmower cuts grass

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31
Q

Equation for the area swept out per unit time by a conductor cutting across a magnetic field

A

A/t = lv

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32
Q

emf equation with flux cutting

A

emf = Blv

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33
Q

Equation for the flux cut through per unit time

A

Flux = flux linkage/t

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34
Q

What changes when we change the flux density in a circuit?

A

The flux linkage

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35
Q

Give 2 examples of when the flux linkage in a circuit would change

A

When a conductor experiences a changing magnetic field due to a changing current in a nearby wire
When a coil of wire is moved relative to a bar magnet

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36
Q

What is the emf induced in a conductor given by?

A

A statement known as Faraday’s law of electromagnetic induction

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37
Q

Transformers

A

Devices that transfer electric energy from one alternating current circuit to other circuits

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38
Q

What would cause the flux linkage in a circuit to change?

A

Transformers

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39
Q

Faraday’s law

A

The emf induced in a circuit is directly proportional to the rate of flux cutting or to the change of flux linkage through the circuit

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40
Q

Symbol for emf in electromagnetic induced theory

A

ϵ

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41
Q

Why do we use the symbol ϵ in electromagnetic induced theory?

A

To avoid confusion with the electric field E

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42
Q

Express Faraday’s law mathematically (explain)

A

emf = -rate of change of flux linkage

ϵ = dNφ
——
dt

ϵ = -BANcostheta/t

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43
Q

Flux linkage equation to learn

A

φ = BANcostheta

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44
Q

emf equation to learn (Faraday’s law)

A

ϵ = -BANcostheta/t

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45
Q

Explain what happens when flux linkage is changed in a circuit

A

emf induced = current induced = magnetic field

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46
Q

Describe the force in a circuit when the emf is zero

A

zero emf = force is zero

47
Q

What does Faraday’s law give us a means of doing?

A

Calculating the magnitude of the induced emf

48
Q

What does Lenz’s law add to Faraday’s law?

A

Direction of the emf

49
Q

Lenz’s law

A

The induced emf is always such as to opposite the change (e.g - magnetic flux linkage) which is producing it

50
Q

What is Lenz’s law responsible for in the emf equation?

A

The negative sign

51
Q

What does an induced emf give rise to?

A

A current

52
Q

Describe the magnetic field of a current given rise to by an induced emf

A

It acts so as to oppose the original change in magnetic flux which served to cause it

53
Q

Describe Lenz’s law in terms of a the N pole of a magnet approaching the end of a solenoid

A

Lenz’s law tells us that in this situation, the end induced in the solenoid causes a current to flow through the circuit, and the direction of this current is such that it will oppose the change causing it. The two north poles repel so the motion is opposed.

54
Q

Direction of a current that was caused by the emf induced in a solenoid

A

Such that it will oppose the change causing it

55
Q

What’s does an emf induced in a solenoid cause to happen?

A

Causes a current to flow

56
Q

In which direction does the current flow when removing a magnet from a solenoid?

A

The opposite direction to when putting it in

57
Q

If Lenz’s law weren’t true, what would happen when putting a magnet in a solenoid? Why is this wrong?

A

The magnet would be pulled further into the solenoid and you could make free energy
(Conservation of energy disobeyed)

58
Q

Describe the current when moving a magnet faster into a solenoid

A

Higher current

59
Q

Why is the motion of a magnet damped when placed in a solenoid? Whose law is this according to?

A

Lenz’s law - motion of the magnet will be damped since the induced current acts to oppose the change in magnetic flux linkage that causes it this reduced the KE of the magnet and hence damps the motion.

60
Q

How do we determine the direction of an induced current?

A

Using Fleming’s right hand rule

61
Q

What’s the difference between the left hand rule and the right hand rule?

A

Left hand rule = motor (current causing motion)
Right hand rule = generator (motion causing current)

62
Q

Difference between a motor and a generator

A

Motor = current causing motion
Generator = motion causing current

63
Q

Area swept out by a straight conductor in 1 second

A

lv

64
Q

How is an electric field induced inside a rod moving in an electric field?

A

Due to the motion of electrons along it

65
Q

What happens when a rod is moving with a steady velocity perpendicular to a magnetic field?

A

Due to the motion of the rod through the field, free electrons in the rod experience a force F and are pushed along the rod

66
Q

How is an electric field set up inside a rod moving along a magnetic field?

A

Due to the motion of the rod through the field, free electrons in the rod experience a force F and are pushed along the rod

67
Q

What happens when, due to the motion of a rod through a magnetic field, free electrons in the rod experience a force F and are pushed along the rod ?

A

An electric field is set up inside the rod

68
Q

What happens at equilibrium to the electrons in a rod that cause an electric field to be set up?

A

At equilibrium, force on an electron due to the electric field and the force on it due to the motion of the red through the magnetic field will be equal in magnitude and opposite in direction

69
Q

Equation for when a rod is moving perpendicular to a magnetic field and at equilibrium, the force on an electron due to the electric field and the force on it due to the motion of the rod through the magnetic field will be equal in magnitude and opposite in direction

A

Ee = BeV

70
Q

Equation for the emf induced in a straight conductor (+ explain)

A

Ee = BeV (at equilibrium, the force on an electron due to the electric field and the force on it due to the motion of the rod through the magnetic field will be equal in magnitude and opposite in direction)
E = Bv
E = ϵ/l

ϵ = Blv

71
Q

Equation for the emf induced in a straight conductor

A

ϵ = Blvcostheta

72
Q

Define the symbols in ϵ= Blv

A

B = flux density
lv = the area swept out by the rod each second

73
Q

What happens when a linear conductor is moving at right angles to a uniform magnetic field?

A

An emf is induced

74
Q

When is an emf induced in a linear conductor?

A

When it’s moving at right angles to a uniform magnetic field

75
Q

Why is ϵ = Blvcostheta usually ϵ = Blv?

A

Cos theta = 1 since it’s usually vertical

76
Q

When is an emf induced in a coil?

A

When its rotating at right angles to a magnetic field

77
Q

What is the emf induced in a rotating coil related to?

A

The position of the coil
Flux density
Coil area
Angular velocity

78
Q

Difference between the emf induced in a rotating coil and in a straight conductor?

A

The emf induced in a rotating coil is slightly more complex than that for a straight conductor

79
Q

What is emf proportional to in both a rotating coil and a straight conductor?

A

The rate of flux cutting

80
Q

What is proportional to the rate of flux cutting in both a rotating coil and a straight conductor?

A

emf

81
Q

What is the magnitude of the induced emf in a rotating coil given by?

A

Faraday’s law

82
Q

How will the magnitude of the induced emf in a rotating coil vary?

A

Sinusoidally with time as the coil rotates

83
Q

What does the sinusoidal varying position of a rotating coil produce?

A

A sinusoidal current and this a sinusoidal emf

84
Q

What does the emf induced in a rotating coil vary with?

A

Position

85
Q

When is the emf induced in a rotating coil zero?

A

At positions where the coil is moving parallel to the field

86
Q

When is the emf induced in a rotating coil maximum (be it positive or negative)?

A

As the coil cue perpendicular to the field

87
Q

Which parts of a rotating coil have a magnetic field?

A

The parts which are perpendicular

88
Q

List the factors which can effect the instantaneous emf in a rotating coil

A

The flux density
The area of the coil
The angular velocity of the rotation

89
Q

What does a larger flux density mean in terms of flux linkage?

A

Larger fluctuations linkage

90
Q

Theta in φ = BANcostheta

A

The angle between field B and the normal to the face of the coil

91
Q

Describe and explain the induced emf if there’s a larger flux density

A

A larger flux density means a larger flux linkage
In a given time, the change in flux linkage would be greater
Therefore, from Faraday’s law, the induced emf would be greater

92
Q

What does a larger area lead to in terms of flux linkage?

A

Larger flux linkage

93
Q

What does a larger flux density lead to in terms of flux linkage?

A

Larger flux linkage

94
Q

Describe and explain what a larger area of coil would lead to in term of emf

A

Larger area = larger flux linkage
In a given time, the change in flux linkage would be larger
From faraday’s law, the induced emf would therefore be greater

95
Q

What would a larger angular velocity lead to in terms of flux linage?

A

The same change in flux linkage would happen in a smaller time

96
Q

Explain what would be larger in a rotating coil if the angular velocity were larger

A

The induced emf would be larger and the frequency of the emf would be larger
A larger angular velocity means the same change in flux linkage would happen in a smaller time

97
Q

Describe and explain the emf in a rotating coil at theta = 90 degrees

A

Maximum positive emf induced (arrows the other way = maximum negative emf)
Coil is flat, rate of change of area (and flux linkage) is maximum

emf = BANcostheta/t = max

Minimum flux linkage

98
Q

Describe and explain the emf of a rotating coil at theta = 0 degrees

A

Coil is perpendicular to the field
Maximum flux linkage
Rate of change of area (and flux linkage) is minimum hence zero emf

99
Q

Cos 0

A

1

100
Q

Cos 90

A

0

101
Q

When does a rotating coil have zero emf?

A

When perpendicular to the field

102
Q

When does a rotating coil have maximum emf?

A

The vertical to the field

103
Q

flux linkage of a rotating coil at 90 to the field

A

Minimum

104
Q

Flux linkage of a coil perpendicular o the field

A

Maximum

105
Q

Why is the emf of a rotating coil at maximum when it’s at 90 degrees to the field?

A

The rate of change of area (and flux linkage) is maximum

106
Q

why is the emf of a rotating coil zero when perpendicular to the field?

A

The rate of change of area (and flux linkage) is minimum hence zero emf

107
Q

Area of a trapezium

A

a + b/2 xh

108
Q

How do we explain whether or not data is consistent with an equation without using a graph?

A

Calculate ratios

109
Q

cm2 to m2

A

x10^-4

110
Q

When does flux cutting not occur?

A

When something is perpendicular to the motion

111
Q

What do we need to do if we’re getting inaccurate results with a Hall probe?

A

The probe may not be in the centre of the solenoid so we need to make sure it’s properly aligned and also use a hall probe with good resolution

112
Q

What can happen once we have an emf due to flux cutting? Explain

A

The circuit is complete = current flows

113
Q

What do we need to remember to do when calculating a ratio?

A

Say “ok within uncertainties”

114
Q

Flux linkage and flux unit

A

Wb

A level Physics (29 decks)

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