Magnetic Fields Flashcards
1
Q
What is a magnetic field
A
A region in which magnetic forces act
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
3
Q
Two things a magnetic field and electric field affect
A
A conductor carrying an electric current
A moving charged particle
4
Q
What 2 things can magnetic fields be
A
Attractive
Repulsive
5
Q
Direction of field lines
A
North to South
The direction of the force on a North pole
6
Q
Compass
A
Always points along field lines
7
Q
Can you have a single monopole
A
No
Always NS or SN
8
Q
Where is the magnetic field strongest
A
Where field lines are closest together
At the poles for a bar magnet
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
10
Q
Arctic
A
Location:
Pole:
Real pole:
11
Q
Antarctic
A
Location:
Pole:
Real Pole:
12
Q
Circle with a cross
A
Field entering the page
13
Q
Circle with a dot
A
Field coming out of the page
14
Q
Technical names for magnetic field lines
A
Lines of magnetic flux
15
Q
Magnetic flux density
A
Strength of the magnetic field
B
16
Q
SI unit for strength of a magnetic field
A
Magnetic flux density
T (Tesla)
17
Q
Expected values for tesla/magnetic field strength
A
Micro or milli
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
19
Q
How many tesla to lift a frog
A
10T
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
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
22
Q
Force when the field is parallel to the current
A
0
23
Q
Maximum force in terms of F=BIL
A
When perpendicular
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 -)
25
Explain Flemings left hand rule
Like a gun = FBI
F=Force=Thumb
B=Magnetic field strength=First finger
I=Current=Middle finger
26
What do you do when the force on a current carrying wire isn't perpendicular to the field
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
27
How would you draw the graph of force on a wire against the angle between the current and field in degrees
Sine graph
F∝sinθ
Passes through zero since force 0 when field parallel to the current
Max at 90 when perpendicular
28
Explain the set up for measuring the magnetic force on a current carrying conductor
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
29
Method for measuring the magnetic force on a current carrying wire
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
30
Give an application of the left hand rule
electric motor (DC)
31
Why add more loops in a DC motor
Makes rotation smoother
32
Purpose of commutator in a DC motor
Prevents wires tangling together
| Works with carbon brushes to switch polarity of coil
33
What would happen if there were no commutators
Wires would twist and tangle
| Polarity of coil would be constant so would not complete full rotations
34
Purpose of the brushes in a DC motor
Provide a connection between battery and commutators and allow polarity of the coil to switch so it can continue rotating
35
Explain the changes in energy in a DC motor
Electrical energy
to
Mechanical energy
36
Torque
The turning moment/resultant moment of a couple
Of a motor
Since T=Fd, and F=BIL, T=BILN
37
How do you make a motor have an increased torque
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
38
Equation for force on a moving charge
F=BQv
F=Force
B=Flux density
Q=Charge in coulombs
v=Velocity
39
How is F=BQv obtained
F=BIL
I=Q/t
F=BQL/t
l/t=v (distance by time)
F=BQv
40
Lorentz equation
F=BQv
41
Condition for lorentz equation
Force perpendicular to the velocity
Aka circular motion
Velocity tangent to particle
42
In Fleming left hand rule, the 3rd finger acts opposite to
The way electrons move
| Since it is conventional it shows
43
Positive charge...
| Negative charge...
Current aligns with velocity
Current opposes velocity
44
Why is the work done in moving a charge in a magnetic field zero
W=FS
Where F and S are parallel
But here they are perpendicular
No displacement in a circle
45
Why can't the magnetic force be used to speed up or slow down a moving charged particle
No magnetic force on the direction of the particle motion
| Does no work on a charged particle
46
How do you derive the expression for the radius of a charged moving particle
Fcentri=Fmagnetic
mv²/r=BQv
mv/r=BQ
So r=mv/BQ
47
Radius of orbit of a charged particle in a magnetic field
r=mv/BQ
R=v/BC (specific charge)
48
Mass spectrometers
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
49
4 stages of mass spectrometer
Ionisation
Acceleration
Velocity selection
Mass separation
50
Explain the first stage of mass spectrometry
Ionisation
| Gives atoms or molecules a charge
51
Explain step 2 of mass spectrometry
Acceleration
| Particles accelerated through a high potential difference (W=QV)
52
Explain step 3 of mass spectrometry
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
53
Explain step 4 of mass spectrometry
Particles with the same charge but different masses are separated by curving in a magnetic field through circular arms different radii
r=mv/BQ
54
Explain the process of mass spectrometry
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
55
In a particle accelerator
Electric fields....
Magnetic fields
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)
56
Example of a particle accelerator
Large hadron collider
| Cyclotron
57
Circumference of LHC
27km
58
Explain how a particle accelerator works
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
59
Explain how the cyclotron works
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
60
Why do you apply A.C potential difference between Dees
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
Why don't you need to change the frequency of A.C. applied across the Dees of a cyclotron
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
How can you chose what velocity to select
Since v=E/Q
| You can calibration the velocity selector by adjusting the magnetic and electric field to get the desired velocity
63
When is the resultant force on a particle in the velocity selector zero
When the electric field strength is equal to the magnetic flux density
64
What does the magnitude of flux passing through an area
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
What is flux/magnetic flux
Field lines passing through an area
| The product of the area and the perpendicular component of magnetic flux density to the area
66
SI unit for flux
Wb
Weber
1 Wb = 1 Tm²
67
What is magnetic flux linkage
The magnetic flux which passes through a coil of wire consisting of N equal turns/loops
NΦ
Measured in Wb (Weber turns)
68
Caution with the equation NΦ=BANcos(°)
Do not just cancel the N!!!!!
69
Express flux linkage in terms of magnetic flux density, area of the coil and the angle between the field lines and the normal
NΦ=BANcos(°)
70
SI unit for flux linkage
Usually expressed as Wb turns
71
Motor vs generator
Motor: electrical to mechanical
Generator: mechanical to electrical
72
What happens when a magnetic is pushed into a wire
An EMF is induced in the opposite direction
| Creating a magnetic field which repels the magnet
73
What happens when a magnet is being pulled out of a coil of wire
An EMF is induced in the opposite direction
| Creating a magnetic field that attracts the magnetic
74
What is produced when you move a magnetic in and out of a wire
An alternating current
75
What happens if you push a magnet into a coil faster
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
What happens if a magnet is stationary in a coil of wire
Magnet not moving
No change in flux
So EMF induced is zero
No longer mechanical energy
77
When is EMF negative
Moving magnet into field
| If flux linkage in increasing
78
When is EMF positive
When magnet is being moved out of the field
| If the change in flux linkage is decreasing
79
Induced EMF of a circuit
The electrical work done on a unit charge flowing once around the circuit
Measured in volts
80
Faraday's law
The magnitude of the emf is proportional to the rate of change of flux linkage
81
Lenz's law
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
Lenz's law equation
```
EMF= -NBA/t
EMF= -Φ/t
```
Change in flux linkage/time taken to move magnet
AKA the rate of change of flux linkage
83
Why must Lenz's Law exist
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
Explain a falling magnet in a coil of wire
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
Which rule for generators
Flemings right hand rule
86
Flemings right hand rule
The direction of an induced EMF
F
B
I
Thumb is the motion/thrust
B is the magnetic field
I is the current
87
Left hand rule vs right hand rule
Right hand rule = geneRIGHTing current (generators)
Left hand rule = producing force of movement (motors)
88
When is the EMF induced in a rotating coil in a magnetic field zero
Coil perpendicular to the field
Maximum flux linkage
Rate of change of flux linkage is zero
So induced EMF is zero
89
When is the EMF induced in a coil of wire in a magnetic field maximum
```
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
Explain the graph for flux linkage and EMF induced
Differentiate flux linkage to get EMF induced
| Min flux linkage is maximum EMF since steepest gradient
91
When and why is flux linkage negative
Now passing through the back of the coil
92
Equation that describes a flux linkage graph
NΦ=BANcos(°)
In degrees (in radians it woukd be wt)
The angle between the field lines and the normal of the coil
93
Differentiate NΦ=BANcos(°)
EMF=BANwsin(wt)
Must have calculator in radians
Maximum means EMF=BANw since sin(wt)=0 is maximum
94
How many times is the EMF maximum in one cycle
2
95
CRO
Cathode rays oscillators
| Instrument uses to look at AC waveforms
96
How is a CRO connected
In parallel
| Like a voltmeter
97
What can a CRO tell you
```
Peak to peak voltage
Frequency
Shape of the wave
Phase of two separate waves
Time taken between two pulses
```
98
CRO can only display wave forms that are...
Repeated regularly
99
Explain a voltage time graph for CRO
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
Why is peak to peak voltage better than voltage
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
Motor vs generator
M: current used to generate movement/G: movement used to generate a current
Generator needs no battery
102
How can you make a generator DC
Use slit rings
103
Galvanometer
Measures tiny currents
104
What type of current is produced in a generator
AC
Alternating current
Current flips every 180° with the flipping of the coil
105
Purpose of carbon brushes in a generator
Provides a connection between the coil and the generator/external circuit
106
Purpose of slip rings in a generator
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
How can you increase the magnitude of the current produced in a generator
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
In a plug socket, what are all three points
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
Europe rate of change of current
50Hz in Europe
60Hz in USA
110
AC vs DC for uses
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
Why do you have to use rms value for voltage in AC
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
Peak voltage
Highest value of voltage in the cycle
113
Peak to peak voltage
The difference from the maximum positive to maximum negative voltage
114
For a sinusoidal waveform the root mean square voltage or current is
The value of DC voltage or current that would provide the same average power pee cycle as the AC supply
115
Why is the waveform sinusoidal for voltage and time
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
How do you display an AC input as a single vertical line on a CRO
Turn off the time base
117
DC graph with time base switched on
Horizontal line through peak voltage
118
DC graph with time base turned off
Dot at peak voltage
119
Two ways you can measure AC voltages
Peak to peak voltage, easily done on a CRO
| Measure the root mean squared value or the effective value
120
Why do we use the rms value
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
rms value
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
Why is power always positive
A negative voltage means a negative current
| So negative times negative is positive
123
Average power for sinusoidal AC waveform
1/2 V0I0
124
How do you find the equivalent DC power value
rms
Formula booklet
I/Vrms=I0/V0/root2
125
rms=peak=peak to peak
```
NO
NONE
EQUAL
TO
EACHOTHER
```
126
What are transformers
Increase or decrease the voltage being applied to equipment from the mains voltage
127
How do transformers work
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
What happens if you use a transformer with a direct current
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
Ns>Np
Increases voltage
| Step up
130
Np>Ns
Decrease voltage
| Step down
131
What does the output voltage of a transformer depend on
The input voltage
| The ratio of number of turns on the input and output coils
132
Why does more turns mean it is a step up in transformers
```
EMF=-change in flux/change in time
More cold means greater change in flux
Means greater EMF
Less current
Increased voltage
```
133
Path of transformers from generators to home
```
Source
Step up
Voltage increases
Journey
Step down
Voltage decreases
Location
```
134
Equation for input and output voltage and coils
Ns/Np=Vs/Vp
135
Equation to use for a 100% efficient transformer
VpIp=VsIs
136
Equation to use for a non fully efficient transformer
efficiency=IsVs/IpVp
137
Advantage of using transformers
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
4 ways transformers are not efficient and state their counter action
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
Explain resistive heat losses/copper losses/I^2R losses
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
Explain the eddy currents in transformers
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
Explain the purpose of laminating transformers cores
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
What is eddy current breaking
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
Explain hysteresis losses
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
Explain flux losses
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
Step up
Increase voltage
| Decrease current
146
Step down
Decrease voltage
| Increase current
147
If a turns ratio of 1:1000 is used before the power cable by what factor would the power loss in the cable change
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
Right hand thumb rule
Finger curl in direction of current
| Thumb points towards the north pole
149
Is an electromagnet a permanent magnet
No, just behaves like one when current flows
