Fields part 2 Flashcards
1
Q
What is capacitance
A
Charge stored by a capacitor per unit potential difference
2
Q
Capacitance formula
A
C = Q / V
3
Q
What is a capacitor
A
Electrical component that stores charge
4
Q
What are capacitors made up of
A
2 conducting parallel plates with a gap between them, may be seperated by inulsating material called a dielectric
5
Q
How do capacitors cause uniform electric fields
A
When capacitor is connected to a source of power, opposite charge builds up on the two parallel plates, causing an electric field to be formed
6
Q
What is the permittivity of a dielectric
A
Ability to store an electric field in the material
7
Q
What is the relative permittivity of a dielectric
A
Also known as dielectric constant, used to calculate capacitance of capacitor, ration of permittivity of dielectric to permittivity of free space
8
Q
Dielectric constant formula
A
Er = E / E0
9
Q
Capacitance formula using area of plates
A
C = AE0Er / d
10
Q
C = AE0Er / d what is A
A
Area of plates
11
Q
C = AE0Er / d what is d
A
Distance between plates
12
Q
C = AE0Er / d what is E0
A
Permittivity of free space
13
Q
C = AE0Er / d what is Er
A
Relative permittivity
14
Q
How to find electrical energy stored by capacitor on a Q-V graph
A
Area under graph
15
Q
Link between charge and potential difference on capacitors
A
Potential difference is directly proportional to charge
16
Q
Q-V graph line
A
Straight line, through origin
17
Q
Electrical energy stored, charge and potential difference formula
A
E = (1/2)QV
18
Q
Electrical energy stored, charge and capacitance formula
A
E = Q^2 / 2C
19
Q
What does a capacitor need to be connected to in order to charge
A
Power supply and resistor
20
Q
What is the gradient on a Q-t graph
A
Current
21
Q
What is the shape if an I-t graph for an chargin capacitor
A
Decreasing exponential
22
Q
What is the shape if an V-t graph for an charging capacitor
A
Increasing exponential
23
Q
What is the shape if an Q-t graph for an charging capacitor
A
Increasing exponential
24
Q
How to find charge on an I-t graph
A
Area under graph
25
What is used to measure the capacitance
Charge stored, pd between plates
26
What is the capacitance
Charge stored per volt
27
What are the units for capacitance
Farads - usually measured in microfarads
28
What are polar molecules
Molecules that are slightly positively charged on one side and slightly negatively on the other
29
Where are polar molecules found in capacitors
The insulator
30
How do polar molecules move in electric fields
Rotate until they rest in line with the field - arrows of field point to +ve
31
What is it called when all polar molecules rest symmetrically
Polarised
32
Relationship between permittivity and capacitance
Directly proportional
33
Relationship and explanation between area and capacitance
Directly proportional - electrons spread out, less repulsion, so more electrons fit on
34
What is the dielectric
Insulating material that polarises in the presense of an electric field
35
Charge breakdown of polarised dielectric - what is it equivalent to
One side positive, one side negative, middle is neutral - equal to 2 plates (1 -ve and 1 +ve)
36
What happens after the dielectrics are polarised
A second electric field is created in the opposite direction to the first - this weakens the first field, so less potential difference is required to charge the capacitor, causing capacitance to increase
37
What is permittivity
How easy it is for a dielectric to polarise
38
How does permittivity influence the electric field
Higher permittivity weakens the electric field (polarised = second field)
39
How to calculate the relative permittivity
Permittivity of material / permittivity of free space
40
How is a potential difference created across a capacitor when connected to a power supply
Current starts to flow, negative charge builds up on plate connected to negative terminal, due to this electrons are repelled from other plate, so electrons move to positive terminal and an equal and opposite charge is formed on each plate, hence creating a potenital difference
41
How the charge of a capacitor increasing effects the current
Charge increases so pd increases but electron flow decreases due to electrostatic force increasing, so current decreases and eventually reaches 0
42
How to discharge a capacitor through a resistor
Must connect it to a closed circuit with just a resistor
43
What is the shape if an I-t graph for a discharging capacitor
Decreasing exponential
44
What is the shape if an V-t graph for a discharging capacitor
Decreasing exponential
45
What is the shape if an Q-t graph for a discharging capacitor
Decreasing exponential
46
Why do the current, charge and pd all fall exponentially when a capacitor is decreasing
Current flows in opposite direction, will take the same amount of time for all the values to half
47
Capacitor charging formula for current
I = I_0 x e^(-t / RC)
48
Capacitor charging formula for potential difference
V = V_0 (1 - e^(-t / RC))
49
Capacitor charging formula for charge
Q = Q_0 (1 - e^(-t / RC))
50
Capacitor discharging formula for current
I = I_0 x e^(-t / RC)
51
Capacitor discharging formula for potential difference
V = V_0 x e^(-t / RC)
52
Capacitor discharging formula for charge
V = V_0 x e^(-t / RC)
53
What is RC
Time constant
54
What is the time constant equal to
Time to discharge a capacitor to 37% of its initial value (charge, current or voltage) or to charge to 63% of its initial value (charge or voltage)
55
How to find time constant using lnQ
Gradient of lnQ-t graph is -1 / RC so RC is -1 / gradient
56
Time to half when discharging formula
T_1/2 = 0.69RC
57
When does a capacitor stop charging
When the pd across the plates is equal to the pd of the battery
58
What are the 3 factors affecting capacitance
Permittivity, area, distance
59
Potential difference, electric field and distance between plates formula
Electic field = voltage / distance
60
Explanation for distance and electric field strength relationship
E = V/D - E stays the same, D increases so V increases to, C = Q/V Q stays the same, V increases, so C increases to
61
Relationship between distance and capacitance
Indirectly proportional
62
Relative permittivity symbol formula
ɛ_r = ɛ/ɛ_0
63
Charge / time =
Current
64
Formula for charge stored against time when charging a capacitor
Q = Q_0 (1-e^-t/RC)
65
How much charge is stored in a capacitor at the time constant whilst charging
0.63
66
Relationship between voltage across capacitor and charge stored
Proportional
67
What has to be passed through a wire for a magnetic field to be induced
A current
68
What is the magnetic flux density
Measure of strength of the field, measured in Tesla, B
69
What is 1 Tesla defined as
Force of 1N on 1m of wire carrying 1A of current perpendicular to a magnetic field
70
What happens if a current-carrying wire is placed in a magnetic field
A force is exerted on the wire
71
What is the force exerted on a current-carrying wire parallel to a the magnetic field
0N
72
Why is the force exerted on a current-carrying wire placed parallel to a magnetic field 0
No components of the field are perpendicular to the current'
73
Force, length, current and flux density formula when field is perpendicular to current
F = BIL
74
F = BIL what is B
Magnetic flux denisty of field
75
F = BIL what is I
Current in wire
76
F = BIL what is L
Length of wire
77
What does Fleming's left hand rule find
Direction of the force exerted on the wire
78
Fleming's left hand rule - what does the thumb represent
Direction of motion/force
79
Fleming's left hand rule - what does the first finger represent
Direction of the field
80
Fleming's left hand rule - what does the second finger represent
Direction of conventional current (opposite of direction of electron flow)
81
What is the direction of a magnetic field on a magentic
North to South
82
Why is a force exerted on a current-carrying wire
A force acts on charged particles moving in a magnetic field, a current-carrying wire contrain negatively charged particles (electrons)
83
F, B, Q, v formula
F = BQv
84
F = BQv what is B
Flux density
85
F = BQv what is Q
Charge of particle
86
F = BQv what is v
Velocity of particle moving perpendicular to a field
87
What does BQv equal
Force exerted on a particle
88
How to use Fleming's left hand rule to find the direction of the force exerted on a particle
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
89
What is the relationship between the direction of the force exerted and the direction of travel
Perpendicular
90
Why do charged particles follow a circular path when in a magnetic field
Force induced by magnetic field acts as a centripetal force as perpendicular to motion of travel
91
Formula for radius of a charged particles circular path
r = mv / BQ
92
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
93
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
94
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
95
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
96
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
97
What does the magnetic flux symbol look like
Circle with a line through it
98
What is the magnetic flux
Value which describes the magnetic field or magnetic field lines passing through a given area
99
How to calculate the magnetic flux
Product of magnetic flux denisty and given area when the field is perpendicular to the area so = BA
100
Magntetic flux linkage symbol
N(circle with line through) so (N)(magnetic flux)
101
What is the magnetic flux linkage
Magnetic flux times by number of turns (N) of a coil
102
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
103
What is the magnetic flux for a component of a field parallel to the coil of wire
0 Wb
104
Magnetic flux formula when not magnetic field not perpendicular to coil of wire
BA cos(theta)
105
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
106
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
107
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
108
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
109
2 laws governing the effects of electromagnetic induction
Faraday's law and Lenz's law
110
What is Faraday's law
Magnitude of induced emf is equal to the rate of change of flux linkage
111
What is Lenz's law
Direction of induced current is such as to oppose motion causing it
112
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
113
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)
114
Lenz's law demonstration explanation - magnet passes through wire
No change in flux so no emf induced
115
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)
116
Faraday's law equation
E = N (change in magnetic flux) / (change in time)
117
E = N (change in magnetic flux) / (change in time) what is E
Magnitude of induced emf
118
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
119
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)
120
Magnitude of emf induced by a straight conductor of length l, moving in an electric field of flux density B
E = Blv
121
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
122
Formula for magnetic flux linkage in a rotating coil
N(magnetic flux) = BANcos(wt)
123
N(magnetic flux) = BANcos(wt) what is wt
Angular speed x time
124
N(magnetic flux) = BANcos(wt) using sine function
E = BANw sin(wt)
125
E = BANw sin(wt) significance of using the sine function
Induced emf is alternating meaning it will change direction with time
126
What type of current can be displayed on an oscilloscope
Any
127
What does an oscillioscope do
Shows variation of voltage with time
128
What can you turn off on an oscillioscope
Time-base
129
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
130
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
131
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
132
How can you make taking measurements easier on an oscilloscope
Adjust scale of both axes of grid
133
How to change scale of Y-axis on oscilloscope
Select number of volts per divison using a Y-gain control dial
134
How to change scale of X-axis on oscilloscope
Adjust the time base
135
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
136
What can you measure on an oscilloscope
Peak voltage (V_0), peak-to-peak voltage, root mean square (rms) voltage, time period (T)
137
How to measure peak voltage on an oscilloscope
Distance from equilibrium to highest or lowest point
138
How to measure peak-to-peak voltage on an oscilloscope
Distance from minimum point to maximum point
139
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
140
How to measure time period on an oscilloscope
Distance between 2 adjacents points in phase
141
What is the voltage to the energy to UK homes
230V
142
What sort of electricity is supplied to homes in the UK
Alternating
143
If an alternating electric supply is delivered to UK homes, what value is 230V
rms of voltage (root mean square)
144
What sort of current do transformers use
Alternating
145
Basic structure of transformers
Primary coil attached to input voltage, secondary coil is connected to output voltage, has an iron core
146
How is a voltage induced in a transformer
Primary coil provides a changing magnetic field, passes through iron core and interacts with secondary coil
147
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
148
Faraday's law effect on ratio of transformers formula
Ns / Np = Vs / Vp
149
Ns / Np = Vs / Vp what is N
Number of turns
150
What are the different types of transformers
Step up, step down
151
What do step up transformers do
Increase input voltage by having more turns on secondary coil than primary
152
What do step down transformers do
Decrease input voltage by having less turns on the secondary coil than primary
153
Transformer efficiency formula
(Is Vs) / (Ip Vp) so power output / power input
154
What is the main form of energy loss in a transformer
Production of eddy currents
155
How are eddy currents formed
Transformers - induced by alternative magnetic field in primary coil and form a loop
156
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
157
How can eddy currents be reduced
Using a laminated iron core or using a core made from a high resistivity metal
158
What is a laminated iron core
Core made using layers of iron between layers of an isulator
159
How does using a laminated iron core reduce eddy currents
Eddy currents can't pass through the insulator and so amplitude is reduced
160
Besides eddy currents, how can energy be lost in transformers
Resistance in coils causes heating, if core isn't easily magnetised
161
How to reduce energy lost due to resistance in coils
Use a thick wire which will have a low resistance
162
How to reduce energy lost due to core not being easily magnetised
Magnetically soft iron core can be used allows easy magnetisation and demagnetisation
163
Power lost due to resistance formula
I^2R
164
How to reduce energy loss when transferring electrical power
Reducing current to a minimum value
165
What sort of transformer should be used to transmit electricity over a long distance
Step up, increase voltage, decrease current
