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A Level Physics Paper 2 > Magnetic Fields > Flashcards

Magnetic Fields Flashcards

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1
Q

What is a magnetic field

A

A region in which magnetic forces act

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2
Q

What can a magnetic field act on

A

Conductor carrying an electric current
A moving charged particle
A permenant magnet
A piece of magnetic material

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3
Q

Two things a magnetic field and electric field affect

A

A conductor carrying an electric current

A moving charged particle

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4
Q

What 2 things can magnetic fields be

A

Attractive

Repulsive

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5
Q

Direction of field lines

A

North to South

The direction of the force on a North pole

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6
Q

Compass

A

Always points along field lines

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7
Q

Can you have a single monopole

A

No

Always NS or SN

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8
Q

Where is the magnetic field strongest

A

Where field lines are closest together

At the poles for a bar magnet

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9
Q

Full name for a magnetic North pole and why

A

North seeking
Because if it was free to move in Earth’s magnetic field the force on it would move North
So if it is being attracted to what we call north there must be a south there

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10
Q

Arctic

A

Location:
Pole:
Real pole:

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11
Q

Antarctic

A

Location:
Pole:
Real Pole:

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12
Q

Circle with a cross

A

Field entering the page

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13
Q

Circle with a dot

A

Field coming out of the page

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14
Q

Technical names for magnetic field lines

A

Lines of magnetic flux

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15
Q

Magnetic flux density

A

Strength of the magnetic field

B

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16
Q

SI unit for strength of a magnetic field

A

Magnetic flux density

T (Tesla)

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17
Q

Expected values for tesla/magnetic field strength

A

Micro or milli

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18
Q

Define the tesla

A

The magnetic flux density that produces a force of one Newton per metre on a current of one ampere flowing in a direction perpendicular to the magnetic field

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19
Q

How many tesla to lift a frog

A

10T

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20
Q

Equation for field strength force and current

A

F=BIL

Force (N)
Magnetic flux density (T)
Current (A)
Length of current carrying wire inside magnetic field (m)

So B=F/IL [Nm-¹A-¹] is equivalent to T

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21
Q

Magnetic flux density definition

A

The magnetic flux density at a point is equal to the force per unit length on a conductor carrying a unit current in a direction perpendicular to the field

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22
Q

Force when the field is parallel to the current

A

0

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23
Q

Maximum force in terms of F=BIL

A

When perpendicular

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24
Q

How can you find the direction of a force on a current carrying wire

A

Fleming left hand rule

Where current is conventional (+ to -)

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25
Q

Explain Flemings left hand rule

A

Like a gun = FBI

F=Force=Thumb
B=Magnetic field strength=First finger
I=Current=Middle finger

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26
Q

What do you do when the force on a current carrying wire isn’t perpendicular to the field

A

Only interested in the component that is perpendicular so must use trig

F=BILsinθ

Where θ is the angle between the current direction and the magnetic field
In degrees

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27
Q

How would you draw the graph of force on a wire against the angle between the current and field in degrees

A

Sine graph
F∝sinθ

Passes through zero since force 0 when field parallel to the current
Max at 90 when perpendicular

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28
Q

Explain the set up for measuring the magnetic force on a current carrying conductor

A

Top pan balance measuring in newtons or massx9.81
Horse shoe magnet
Current carrying wire passing through the magnet
Attached to a circuit in series with a power source and a variable resistor and ammeter

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29
Q

Method for measuring the magnetic force on a current carrying wire

A

Set balance to zero
Measure the length of wire using vernier calipers
Vary the current by varying the resistance of the surface
Take current force readings each time resistance is changed
Repeat
Average
Plot a graph of force against current using means
Expect a straight line passing through the origin
With a gradient equal to Bl
Can find the magnetic fields strength using this and the length

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30
Q

Give an application of the left hand rule

A

electric motor (DC)

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31
Q

Why add more loops in a DC motor

A

Makes rotation smoother

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32
Q

Purpose of commutator in a DC motor

A

Prevents wires tangling together

Works with carbon brushes to switch polarity of coil

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33
Q

What would happen if there were no commutators

A

Wires would twist and tangle

Polarity of coil would be constant so would not complete full rotations

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34
Q

Purpose of the brushes in a DC motor

A

Provide a connection between battery and commutators and allow polarity of the coil to switch so it can continue rotating

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35
Q

Explain the changes in energy in a DC motor

A

Electrical energy
to
Mechanical energy

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36
Q

Torque

A

The turning moment/resultant moment of a couple
Of a motor

Since T=Fd, and F=BIL, T=BILN

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37
Q

How do you make a motor have an increased torque

A

T=NBIL

Increase the number of turns in the coil
Increase the magnetic field strength/magnetic flux density of the wire
Increase the current flowing through the wire by using a bigger power source
Increase the length of wire in the magnetic field

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38
Q

Equation for force on a moving charge

A

F=BQv

F=Force
B=Flux density
Q=Charge in coulombs
v=Velocity

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39
Q

How is F=BQv obtained

A

F=BIL
I=Q/t

F=BQL/t

l/t=v (distance by time)

F=BQv

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40
Q

Lorentz equation

A

F=BQv

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41
Q

Condition for lorentz equation

A

Force perpendicular to the velocity
Aka circular motion
Velocity tangent to particle

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42
Q

In Fleming left hand rule, the 3rd finger acts opposite to

A

The way electrons move

Since it is conventional it shows

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43
Q

Positive charge…

Negative charge…

A

Current aligns with velocity

Current opposes velocity

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44
Q

Why is the work done in moving a charge in a magnetic field zero

A

W=FS
Where F and S are parallel
But here they are perpendicular
No displacement in a circle

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45
Q

Why can’t the magnetic force be used to speed up or slow down a moving charged particle

A

No magnetic force on the direction of the particle motion

Does no work on a charged particle

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46
Q

How do you derive the expression for the radius of a charged moving particle

A

Fcentri=Fmagnetic

mv²/r=BQv

mv/r=BQ

So r=mv/BQ

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47
Q

Radius of orbit of a charged particle in a magnetic field

A

r=mv/BQ

R=v/BC (specific charge)

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48
Q

Mass spectrometers

A

Use magnetic and electric fields
To measure the masses of atoms
Determine their relative abundance in a chemical sample
By separating based off specific charge

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49
Q

4 stages of mass spectrometer

A

Ionisation
Acceleration
Velocity selection
Mass separation

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50
Q

Explain the first stage of mass spectrometry

A

Ionisation

Gives atoms or molecules a charge

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51
Q

Explain step 2 of mass spectrometry

A

Acceleration

Particles accelerated through a high potential difference (W=QV)

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52
Q

Explain step 3 of mass spectrometry

A

Velocity selection
Particles pass through a space where electric and magnetic fields act on particles to filter out all velocities but one

Fmagnetic=Felectric
EQ=BQv
v=E/B

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53
Q

Explain step 4 of mass spectrometry

A

Particles with the same charge but different masses are separated by curving in a magnetic field through circular arms different radii
r=mv/BQ

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54
Q

Explain the process of mass spectrometry

A

Electron gun fires electrons at a sample
Making ions charged (positive)
Negatively charged plates accelerate them with a high voltage
Ions run through an electric field at 90° to the magnetic field
Velocity selector ensures they all have the same velocity by filtering all others out
So they have the same velocity but different specific charges
Separated by specific charge in another magnetic field
Register radius of the path with a detector
Data recorded graphically

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55
Q

In a particle accelerator

Electric fields….
Magnetic fields

A

Accelerate the particles, electric force in the direction of travel (F=EQ)

Change the direction of the particle, magnetic force is perpendicular to travel (F=BQv)

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56
Q

Example of a particle accelerator

A

Large hadron collider

Cyclotron

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57
Q

Circumference of LHC

A

27km

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58
Q

Explain how a particle accelerator works

A

Accelerates charged particles to speeds close to light speed
Using electric fields
Strong magnetic fields provided by superconducting electromagnets are used to direct particles around the circular path they follow whilst being accelerated

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59
Q

Explain how the cyclotron works

A

Strong magnetic fields curve the path of charged particles
Whilst being accelerated by electric fields

Start on the centre and gradually spiral outwards until reaching desired speed
Then fired towards a target
Magnetic field directed vertically through the accelerator
Particles accelerated each time they cross the small gap between the Dees where there is a potential difference

Since they increase their velocity their radius increases

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60
Q

Why do you apply A.C potential difference between Dees

A

Since the particles cross the gap in alternating directions
The electric field must reverse direction every half cycle
So apply A.C. between the Dees with the same frequency as the particles circulate

61
Q

Why don’t you need to change the frequency of A.C. applied across the Dees of a cyclotron

A

As the particles are accelerates across the gap
Their velocity increases
So move to a higher radius
But the time stays the same because although travelling a larger distance, travelling at a larger speed
So the frequency doesn’t need to change

62
Q

How can you chose what velocity to select

A

Since v=E/Q

You can calibration the velocity selector by adjusting the magnetic and electric field to get the desired velocity

63
Q

When is the resultant force on a particle in the velocity selector zero

A

When the electric field strength is equal to the magnetic flux density

64
Q

What does the magnitude of flux passing through an area

A

Φ=BA
Φ=BAcos(°)
So the magnetic flux density, the area and the angle between the field lines and the line normal to the area

65
Q

What is flux/magnetic flux

A

Field lines passing through an area

The product of the area and the perpendicular component of magnetic flux density to the area

66
Q

SI unit for flux

A

Wb
Weber

1 Wb = 1 Tm²

67
Q

What is magnetic flux linkage

A

The magnetic flux which passes through a coil of wire consisting of N equal turns/loops

NΦ

Measured in Wb (Weber turns)

68
Q

Caution with the equation NΦ=BANcos(°)

A

Do not just cancel the N!!!!!

69
Q

Express flux linkage in terms of magnetic flux density, area of the coil and the angle between the field lines and the normal

A

NΦ=BANcos(°)

70
Q

SI unit for flux linkage

A

Usually expressed as Wb turns

71
Q

Motor vs generator

A

Motor: electrical to mechanical
Generator: mechanical to electrical

72
Q

What happens when a magnetic is pushed into a wire

A

An EMF is induced in the opposite direction

Creating a magnetic field which repels the magnet

73
Q

What happens when a magnet is being pulled out of a coil of wire

A

An EMF is induced in the opposite direction

Creating a magnetic field that attracts the magnetic

74
Q

What is produced when you move a magnetic in and out of a wire

A

An alternating current

75
Q

What happens if you push a magnet into a coil faster

A

Flux changes more quickly
Increased amount of the magnetic field of magnet in coil
EMF induced in the opposite direction is greater
So a larger current

76
Q

What happens if a magnet is stationary in a coil of wire

A

Magnet not moving
No change in flux
So EMF induced is zero
No longer mechanical energy

77
Q

When is EMF negative

A

Moving magnet into field

If flux linkage in increasing

78
Q

When is EMF positive

A

When magnet is being moved out of the field

If the change in flux linkage is decreasing

79
Q

Induced EMF of a circuit

A

The electrical work done on a unit charge flowing once around the circuit
Measured in volts

80
Q

Faraday’s law

A

The magnitude of the emf is proportional to the rate of change of flux linkage

81
Q

Lenz’s law

A

The direction of an induced emf acts to produce a current which opposes the change causing it
By producing a magnetic field

Current is conventional

82
Q

Lenz’s law equation

A 
EMF= -NBA/t
EMF= -Φ/t

Change in flux linkage/time taken to move magnet
AKA the rate of change of flux linkage

83
Q

Why must Lenz’s Law exist

A

If a magnet has a tiny velocity towards a stationary magnet then it induces a tiny EMF and so a current flows
This current produces a magnetic field in the coil which either attracts or repels the magnet
If the current was to cause an attraction then the magnet would speed up more and more
Creating energy

Lenz’s law exists so the current flows in a direction to create a magnetic field that acts to slow the magnet
So it’s kinetic energy is lost as electrical energy is produced

84
Q

Explain a falling magnet in a coil of wire

A

Falling magnet induces a current within the coil
Current induced so that it’s magnetic field works to oppose the motion of the falling magnet
Repulsion occurs so that the magnet is slowed down
It’s kinetic energy is converted into electrical energy

85
Q

Which rule for generators

A

Flemings right hand rule

86
Q

Flemings right hand rule

A

The direction of an induced EMF
F
B
I

Thumb is the motion/thrust
B is the magnetic field
I is the current

87
Q

Left hand rule vs right hand rule

A

Right hand rule = geneRIGHTing current (generators)

Left hand rule = producing force of movement (motors)

88
Q

When is the EMF induced in a rotating coil in a magnetic field zero

A

Coil perpendicular to the field
Maximum flux linkage
Rate of change of flux linkage is zero
So induced EMF is zero

89
Q

When is the EMF induced in a coil of wire in a magnetic field maximum

A 
Coil parallel to the field
No flux linkage
Rate of change of flux linkage is maximum
Because polarity of flux is changing
So induced EMF is maximum
90
Q

Explain the graph for flux linkage and EMF induced

A

Differentiate flux linkage to get EMF induced

Min flux linkage is maximum EMF since steepest gradient

91
Q

When and why is flux linkage negative

A

Now passing through the back of the coil

92
Q

Equation that describes a flux linkage graph

A

NΦ=BANcos(°)

In degrees (in radians it woukd be wt)

The angle between the field lines and the normal of the coil

93
Q

Differentiate NΦ=BANcos(°)

A

EMF=BANwsin(wt)

Must have calculator in radians

Maximum means EMF=BANw since sin(wt)=0 is maximum

94
Q

How many times is the EMF maximum in one cycle

A

2

95
Q

CRO

A

Cathode rays oscillators

Instrument uses to look at AC waveforms

96
Q

How is a CRO connected

A

In parallel

Like a voltmeter

97
Q

What can a CRO tell you

A 
Peak to peak voltage
Frequency 
Shape of the wave
Phase of two separate waves
Time taken between two pulses
98
Q

CRO can only display wave forms that are…

A

Repeated regularly

99
Q

Explain a voltage time graph for CRO

A

Voltage on the Y axis
Controlled by the y plates
Scale of y axis is known as the voltage sensitivity/y gain

Time on the X axis
Controlled by the x plates
Scale of x axis is known as the time base

By adjusting the dials you adjust the scales
Select the biggest divisions to get a visible trace

100
Q

Why is peak to peak voltage better than voltage

A

Does not change/is constant
It is the vertical length of the curve and not the distance from the x axis
So doesn’t change if the x axis is moved up or down
Whereas voltage will

101
Q

Motor vs generator

A

M: current used to generate movement/G: movement used to generate a current

Generator needs no battery

102
Q

How can you make a generator DC

A

Use slit rings

103
Q

Galvanometer

A

Measures tiny currents

104
Q

What type of current is produced in a generator

A

AC
Alternating current
Current flips every 180° with the flipping of the coil

105
Q

Purpose of carbon brushes in a generator

A

Provides a connection between the coil and the generator/external circuit

106
Q

Purpose of slip rings in a generator

A

Ensures the left side of the coil always connected to the left side of the circuit
And right always connected to right
So can produce and alternating current

107
Q

How can you increase the magnitude of the current produced in a generator

A

Use a stronger magnetic field
Increase the speed of rotation
Increase the number of coil
Increase the length of the coil in the field
Decrease the resistance to increase the current

108
Q

In a plug socket, what are all three points

A

Bottom two have AC current flowing through

Top one is a safety feature that can trip a fuse if there is an electrical fault and stop electricity

109
Q

Europe rate of change of current

A

50Hz in Europe

60Hz in USA

110
Q

AC vs DC for uses

A

Equally good at heating, lighting or running motors
DC essential for chemical processes like electrolysis
AC much more easily distributed than DC since it can be used with transformers
Infrastructure of AC much less expensive than DC

111
Q

Why do you have to use rms value for voltage in AC

A

Otherwise the mean would be 0
Removes negative values by taking absolute values
Without it would be a straight line and not a sinusoidal waveform

112
Q

Peak voltage

A

Highest value of voltage in the cycle

113
Q

Peak to peak voltage

A

The difference from the maximum positive to maximum negative voltage

114
Q

For a sinusoidal waveform the root mean square voltage or current is

A

The value of DC voltage or current that would provide the same average power pee cycle as the AC supply

115
Q

Why is the waveform sinusoidal for voltage and time

A

Ideally it would have a constant positive and constant negative voltage
So would form a square waveform
However there is a lag
So sinusoidal in nature
The corresponding current for the voltage is pi out of phase due to lagging

116
Q

How do you display an AC input as a single vertical line on a CRO

A

Turn off the time base

117
Q

DC graph with time base switched on

A

Horizontal line through peak voltage

118
Q

DC graph with time base turned off

A

Dot at peak voltage

119
Q

Two ways you can measure AC voltages

A

Peak to peak voltage, easily done on a CRO

Measure the root mean squared value or the effective value

120
Q

Why do we use the rms value

A

Allows you to do power and energy electrical calculations as if they were direct current
Also allows you to make comparisons with the DC

121
Q

rms value

A

The equivalent vue to a steady direct current which converts electrical energy into other forms of energy for a given resistance at the same rate as the AC

122
Q

Why is power always positive

A

A negative voltage means a negative current

So negative times negative is positive

123
Q

Average power for sinusoidal AC waveform

A

1/2 V0I0

124
Q

How do you find the equivalent DC power value

A

rms
Formula booklet
I/Vrms=I0/V0/root2

125
Q

rms=peak=peak to peak

A 
NO
NONE
EQUAL
TO
EACHOTHER
126
Q

What are transformers

A

Increase or decrease the voltage being applied to equipment from the mains voltage

127
Q

How do transformers work

A

AC flowing through the primary coil creates alternating magnetic field in core
Primary coil acts as an electromagnet
Core ensures change in magnetic field is linked to the secondary coil
Changing magnetic field in secondary coil induces an EMF or output voltage which can drive an output AC in secondary coil

128
Q

What happens if you use a transformer with a direct current

A

Need a change in magnetic field for a change in flux to induce and EMF and a current
Only produce EMF if there is a change in flux
For DC the change in flux is zero
Current doesn’t change
Magnetic field doesn’t change
Only get a tiny current when you turn it on and off, not throughout

129
Q

Ns>Np

A

Increases voltage

Step up

130
Q

Np>Ns

A

Decrease voltage

Step down

131
Q

What does the output voltage of a transformer depend on

A

The input voltage

The ratio of number of turns on the input and output coils

132
Q

Why does more turns mean it is a step up in transformers

A 
EMF=-change in flux/change in time
More cold means greater change in flux
Means greater EMF
Less current
Increased voltage
133
Q

Path of transformers from generators to home

A 
Source
Step up
Voltage increases
Journey
Step down
Voltage decreases
Location
134
Q

Equation for input and output voltage and coils

A

Ns/Np=Vs/Vp

135
Q

Equation to use for a 100% efficient transformer

A

VpIp=VsIs

136
Q

Equation to use for a non fully efficient transformer

A

efficiency=IsVs/IpVp

137
Q

Advantage of using transformers

A

In power cables transferring electricity over large distances, you want to lose as little energy as possible
Since P=I^2R the smaller the current the smaller the less energy loss during transfer
Hence why poser cables are high voltage

DO NOT USE P=V^2/R

138
Q

4 ways transformers are not efficient and state their counter action

A

Resistive heat loss/copper loss —- Coils made from good conducting material

Eddy currents —- Laminated core

Hysteresis losses —- Soft magnetic material for core

Flux losses —- Secondary coil wound around primary coil

139
Q

Explain resistive heat losses/copper losses/I^2R losses

A

As current is flowing in both p and s coils energy will be transformed into heat
Lost to surroundings (hence why they feel warm after use)

So coils are made from good conducting materials
Like copper
And have a large cross sectional area to reduce resistance

140
Q

Explain the eddy currents in transformers

A

Metal core is a conductor so as the magnetic field alternates inside the core, EMFs are induced in the core itself since it is a conductor so currents can flow
These are called eddy currents
Cause the core to heat up
Eventually escapes to surroundings and convert useful electrical energy into waste thermal energy

To minimise the core is laminated

141
Q

Explain the purpose of laminating transformers cores

A

Minimises the effect of eddy currents so less useful electrical energy is converted to waste thermal energy

Divide the core up into small strips separated by a thin layer of insulator
Little effect on the magnetic field passing through the core
But limits the size of the eddy currents
So reduces the amount of energy lost as heat via this
By giving the eddy currents smaller paths

142
Q

What is eddy current breaking

A

Provide a resistive force depending on the strength of the field
Currents induced in opposite directions
Different magnetic fields so oppose disk going into or out of the field
Stops disc moving

143
Q

Explain hysteresis losses

A

Field inside the core has to be reversed at the same rate as the AC being inputted
As the core is not a permanent magnet
Once it has been magnetised in one direction it resists being magnetised in the other
Reversing the direction of the field takes an input of energy from the primary coil

To minimise a soft magnetic material is used for the core
Easily magnetised
So minimises the amount of energy needed to reverse the field
Iron usually chosen
Or alloys of iron

144
Q

Explain flux losses

A

Not all of the flux created by the primary coil links with the secondary coil
Some of the field created is not channelled through the core to the secondary coil

To minimise this the secondary coil is wound around the primary coil
So as many field lines pass through as possible
Only works if the wires are insulated

145
Q

Step up

A

Increase voltage

Decrease current

146
Q

Step down

A

Decrease voltage

Increase current

147
Q

If a turns ratio of 1:1000 is used before the power cable by what factor would the power loss in the cable change

A

100000 less

If 100% efficient, Ns=1000Np
Vs=1000Vp

Power losses=I^2/R
Since R is constant, if I is 1000 less, I^2 is 1000000 less so power losses are 1000000 less

148
Q

Right hand thumb rule

A

Finger curl in direction of current

Thumb points towards the north pole

149
Q

Is an electromagnet a permanent magnet

A

No, just behaves like one when current flows

A Level Physics Paper 2 (7 decks)

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