Magnetic fields Flashcards
(105 cards)
1
Q
What has to be passed through a wire for a magnetic field to be induced
A
A current
2
Q
What is the magnetic flux density
A
Measure of strength of the field, measured in Tesla, B
3
Q
What is 1 Tesla defined as
A
Force of 1N on 1m of wire carrying 1A of current perpendicular to a magnetic field
4
Q
What happens if a current-carrying wire is placed in a magnetic field
A
A force is exerted on the wire
5
Q
What is the force exerted on a current-carrying wire parallel to a the magnetic field
A
0N
6
Q
Why is the force exerted on a current-carrying wire placed parallel to a magnetic field 0
A
No components of the field are perpendicular to the current’
7
Q
Force, length, current and flux density formula when field is perpendicular to current
A
F = BIL
8
Q
F = BIL what is B
A
Magnetic flux denisty of field
9
Q
F = BIL what is I
A
Current in wire
10
Q
F = BIL what is L
A
Length of wire
11
Q
What does Fleming’s left hand rule find
A
Direction of the force exerted on the wire
12
Q
Fleming’s left hand rule - what does the thumb represent
A
Direction of motion/force
13
Q
Fleming’s left hand rule - what does the first finger represent
A
Direction of the field
14
Q
Fleming’s left hand rule - what does the second finger represent
A
Direction of conventional current (opposite of direction of electron flow)
15
Q
What is the direction of a magnetic field on a magentic
A
North to South
16
Q
Why is a force exerted on a current-carrying wire
A
A force acts on charged particles moving in a magnetic field, a current-carrying wire contrain negatively charged particles (electrons)
17
Q
F, B, Q, v formula
A
F = BQv
18
Q
F = BQv what is B
A
Flux density
19
Q
F = BQv what is Q
A
Charge of particle
20
Q
F = BQv what is v
A
Velocity of particle moving perpendicular to a field
21
Q
What does BQv equal
A
Force exerted on a particle
22
Q
How to use Fleming’s left hand rule to find the direction of the force exerted on a particle
A
Second finger is direction of travel, if negative then reverse the direction of the second finger because it represents convential current which flows positive to negatice
23
Q
What is the relationship between the direction of the force exerted and the direction of travel
A
Perpendicular
24
Q
Why do charged particles follow a circular path when in a magnetic field
A
Force induced by magnetic field acts as a centripetal force as perpendicular to motion of travel
25
Formula for radius of a charged particles circular path
r = mv / BQ
26
r = mv / BQ what is r
Radius of particles paths in a magnetic field
27
r = mv / BQ what is m
Particles mass
28
r = mv / BQ what is v
Particles velocity
29
r = mv / BQ what is b
Magnetic flux density
30
r = mv / BQ what is q
Charge
31
Application of circular deflection of charged particles in a magnetic field
A type of particle accelarator called a cyclotron, has many uses including producing ion beams for radiotherapy and radioactive tracers
32
Structure of a cyclotron
2 semi-circular electrodes called 'dees', uniform magnetic field acting perpendicular to the plane of electrodes, high frequency alternating voltage applied between electrodes
33
Why is there an alternating electric field between electrodes in a cyclotron
Charged particles move from centre of one electrode and are deflected in a circular path by the magnetic field (force exerted perpendicular to direction of travel), particles speed will not increase due to the magnetic field so there is an alternating field between the electrodes
34
What happens once the particles reach the edge of the electrode in a cyclotron
Particles begin to move across the gap between the electrodes where they are accelerated by the electric field so radius of circular path will increase as they move through second electrode, when they reach the gap again the alternating electric field changes direction allowing the particles to be accelerated again, process repeats until the required speed is reach by the particles and the exit the cyclotron
35
Roles of the electric vs magnetic field in cyclotrons
Alternating electric field increases speed of particles between the dees, uniform magnetic field forces particles into a circular path (with increasing radii due to increasing speed) inside the dees
36
What does the magnetic flux symbol look like
Circle with a line through it
37
What is the magnetic flux
Value which describes the magnetic field or magnetic field lines passing through a given area
38
How to calculate the magnetic flux
Product of magnetic flux denisty and given area when the field is perpendicular to the area so = BA
39
Magntetic flux linkage symbol
N(circle with line through) so (N)(magnetic flux)
40
What is the magnetic flux linkage
Magnetic flux times by number of turns (N) of a coil
41
Magnetic flux linkage formula when field is perpendicular to area
N(magnetic flux) = BAN
42
How to find magnetic flux or magnetic flux linkage when magnetic field is not perpendicular to coil of wire
Use trigonometry to resolve the magnetic field vector into components which are parallel and perpendicular to the coil
43
What is the magnetic flux for a component of a field parallel to the coil of wire
0 Wb
44
Magnetic flux formula when not magnetic field not perpendicular to coil of wire
BA cos(theta)
45
BA cos(theta) what is theta if this is the formula for magnetic flux when M field not perpendicular to coil of wire
Angle between field and normal to the plane of the coil
46
What happens to the electrons in a conducting rod when it moves relative to a magnetic field
Experience a force (due to being charged) and will build up on one side of the rod - hence causing an emf to be induced in the rod - this is known as electromagnetic induction
47
What is electromagnetic induction
When a conducting rod moves relative to magnetic field so electrons experience a force causing them to build up on one side of the rod, inducing an emf
48
1 other scenario for electromagnetic induction not involving a conducting rod
Moving a bar magnet relative to a coil of wire - if the coil forms a complete circuit then a current is also induced
49
2 laws governing the effects of electromagnetic induction
Faraday's law and Lenz's law
50
What is Faraday's law
Magnitude of induced emf is equal to the rate of change of flux linkage
51
What is Lenz's law
Direction of induced current is such as to oppose motion causing it
52
Demonstration of Lenz's law premise
Measure speed of magnet falling through a coil of wire and its speed when falling from same height without going through a coil, the magnet will take longer to reach the ground when it moves through the coil
53
Lenz's law demonstration explanation - magnet approaching wire
Change in flux through coil so emf and current induced as magnet approaches coil, direction of induced current opposed motion of magnet so same pole as which is approaching the coil is induced at the top of the coil to repel magnet, slows down magnet (repulsion)
54
Lenz's law demonstration explanation - magnet passes through wire
No change in flux so no emf induced
55
Lenz's law demonstration explanation - magnet moving away from wire
As magnet leaves coil, change in flux so current is induced to oppose motion of magnet, so opposite pole is induced at the bottom of the coil causing it slow down (attraction)
56
Faraday's law equation
E = N (change in magnetic flux) / (change in time)
57
E = N (change in magnetic flux) / (change in time) what is E
Magnitude of induced emf
58
E = N (change in magnetic flux) / (change in time) what is N (change in magnetic flux) / (change in time)
Rate of change of flux linkage
59
How does Lenz's law effect Faraday's equation
Lenz's law states that direction of induced current will act to oppose change in flux that created it so becomes negative E = - N (change in magnetic flux) / (change in time)
60
Magnitude of emf induced by a straight conductor of length l, moving in an electric field of flux density B
E = Blv
61
How to calculate the emf induced when a coil rotates at a constant frequency in a magnetic field
Derivate of formula for magnetic flux linkage with respect to time as induced emf is equal to rate of change of flux linkage
62
Formula for magnetic flux linkage in a rotating coil
N(magnetic flux) = BANcos(wt)
63
N(magnetic flux) = BANcos(wt) what is wt
Angular speed x time
64
N(magnetic flux) = BANcos(wt) using sine function
E = BANw sin(wt)
65
E = BANw sin(wt) significance of using the sine function
Induced emf is alternating meaning it will change direction with time
66
What type of current can be displayed on an oscilloscope
Any
67
What does an oscillioscope do
Shows variation of voltage with time
68
What can you turn off on an oscillioscope
Time-base
69
Significance of being able to turn off the time-base on an oscilliscope
Causes the trace to show all the possible voltages at any time in one area which is usefule for taking measurements
70
Oscilliscope readings for a direct current with/without time base
With - straight line parallel to axis at height of output voltage, without - dot at height of output voltage
71
Oscilliscope readings for an alternating current with/without time base
With - sinusoidal waeform showing the variation of output voltage with time, without - straight vertical line
72
How can you make taking measurements easier on an oscilloscope
Adjust scale of both axes of grid
73
How to change scale of Y-axis on oscilloscope
Select number of volts per divison using a Y-gain control dial
74
How to change scale of X-axis on oscilloscope
Adjust the time base
75
How to take measurements from an oscilloscope
Count number of divisions (adjusting axes to make easier) and multiple them by either volts per division or time base
76
What can you measure on an oscilloscope
Peak voltage (V_0), peak-to-peak voltage, root mean square (rms) voltage, time period (T)
77
How to measure peak voltage on an oscilloscope
Distance from equilibrium to highest or lowest point
78
How to measure peak-to-peak voltage on an oscilloscope
Distance from minimum point to maximum point
79
How to measure root mean square voltage on an oscilloscope
Average of all squares of possible voltages - average value of voltage output by supply (in either direction) I_rms = I_0 / root 2 or V_rms = V_0 / root 2 where I_0 and V_0 are peak values of current and voltage
80
How to measure time period on an oscilloscope
Distance between 2 adjacents points in phase
81
What is the voltage to the energy to UK homes
230V
82
What sort of electricity is supplied to homes in the UK
Alternating
83
If an alternating electric supply is delivered to UK homes, what value is 230V
rms of voltage (root mean square)
84
What sort of current do transformers use
Alternating
85
Basic structure of transformers
Primary coil attached to input voltage, secondary coil is connected to output voltage, has an iron core
86
How is a voltage induced in a transformer
Primary coil provides a changing magnetic field, passes through iron core and interacts with secondary coil
87
What does Faraday's law show about transformers and ratio's
Ratio of voltage in primary coil to secondary coil is the same as ratio of number of turns on primary coil to secondary coil
88
Faraday's law effect on ratio of transformers formula
Ns / Np = Vs / Vp
89
Ns / Np = Vs / Vp what is N
Number of turns
90
What are the different types of transformers
Step up, step down
91
What do step up transformers do
Increase input voltage by having more turns on secondary coil than primary
92
What do step down transformers do
Decrease input voltage by having less turns on the secondary coil than primary
93
Transformer efficiency formula
(Is Vs) / (Ip Vp) so power output / power input
94
What is the main form of energy loss in a transformer
Production of eddy currents
95
How are eddy currents formed
Transformers - induced by alternative magnetic field in primary coil and form a loop
96
Lenz's law, how do eddy currents cause a loss of energy in transformers
Oppose the field that produced them, reduces fields flux density, generate heat which causes energy to be lost
97
How can eddy currents be reduced
Using a laminated iron core or using a core made from a high resistivity metal
98
What is a laminated iron core
Core made using layers of iron between layers of an isulator
99
How does using a laminated iron core reduce eddy currents
Eddy currents can't pass through the insulator and so amplitude is reduced
100
Besides eddy currents, how can energy be lost in transformers
Resistance in coils causes heating, if core isn't easily magnetised
101
How to reduce energy lost due to resistance in coils
Use a thick wire which will have a low resistance
102
How to reduce energy lost due to core not being easily magnetised
Magnetically soft iron core can be used allows easy magnetisation and demagnetisation
103
Power lost due to resistance formula
I^2R
104
How to reduce energy loss when transferring electrical power
Reducing current to a minimum value
105
What sort of transformer should be used to transmit electricity over a long distance
Step up, increase voltage, decrease current
