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Fields part 2 Flashcards

1
Q

What is capacitance

A

Charge stored by a capacitor per unit potential difference

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2
Q

Capacitance formula

A

C = Q / V

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3
Q

What is a capacitor

A

Electrical component that stores charge

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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

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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

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6
Q

What is the permittivity of a dielectric

A

Ability to store an electric field in the material

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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

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8
Q

Dielectric constant formula

A

Er = E / E0

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9
Q

Capacitance formula using area of plates

A

C = AE0Er / d

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10
Q

C = AE0Er / d what is A

A

Area of plates

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11
Q

C = AE0Er / d what is d

A

Distance between plates

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12
Q

C = AE0Er / d what is E0

A

Permittivity of free space

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13
Q

C = AE0Er / d what is Er

A

Relative permittivity

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14
Q

How to find electrical energy stored by capacitor on a Q-V graph

A

Area under graph

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15
Q

Link between charge and potential difference on capacitors

A

Potential difference is directly proportional to charge

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16
Q

Q-V graph line

A

Straight line, through origin

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17
Q

Electrical energy stored, charge and potential difference formula

A

E = (1/2)QV

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18
Q

Electrical energy stored, charge and capacitance formula

A

E = Q^2 / 2C

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19
Q

What does a capacitor need to be connected to in order to charge

A

Power supply and resistor

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20
Q

What is the gradient on a Q-t graph

A

Current

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21
Q

What is the shape if an I-t graph for an chargin capacitor

A

Decreasing exponential

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22
Q

What is the shape if an V-t graph for an charging capacitor

A

Increasing exponential

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23
Q

What is the shape if an Q-t graph for an charging capacitor

A

Increasing exponential

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24
Q

How to find charge on an I-t graph

A

Area under graph

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25
Q

What is used to measure the capacitance

A

Charge stored, pd between plates

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26
Q

What is the capacitance

A

Charge stored per volt

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27
Q

What are the units for capacitance

A

Farads - usually measured in microfarads

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28
Q

What are polar molecules

A

Molecules that are slightly positively charged on one side and slightly negatively on the other

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29
Q

Where are polar molecules found in capacitors

A

The insulator

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30
Q

How do polar molecules move in electric fields

A

Rotate until they rest in line with the field - arrows of field point to +ve

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31
Q

What is it called when all polar molecules rest symmetrically

A

Polarised

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32
Q

Relationship between permittivity and capacitance

A

Directly proportional

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33
Q

Relationship and explanation between area and capacitance

A

Directly proportional - electrons spread out, less repulsion, so more electrons fit on

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34
Q

What is the dielectric

A

Insulating material that polarises in the presense of an electric field

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35
Q

Charge breakdown of polarised dielectric - what is it equivalent to

A

One side positive, one side negative, middle is neutral - equal to 2 plates (1 -ve and 1 +ve)

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36
Q

What happens after the dielectrics are polarised

A

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

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37
Q

What is permittivity

A

How easy it is for a dielectric to polarise

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38
Q

How does permittivity influence the electric field

A

Higher permittivity weakens the electric field (polarised = second field)

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39
Q

How to calculate the relative permittivity

A

Permittivity of material / permittivity of free space

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40
Q

How is a potential difference created across a capacitor when connected to a power supply

A

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

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41
Q

How the charge of a capacitor increasing effects the current

A

Charge increases so pd increases but electron flow decreases due to electrostatic force increasing, so current decreases and eventually reaches 0

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42
Q

How to discharge a capacitor through a resistor

A

Must connect it to a closed circuit with just a resistor

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43
Q

What is the shape if an I-t graph for a discharging capacitor

A

Decreasing exponential

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44
Q

What is the shape if an V-t graph for a discharging capacitor

A

Decreasing exponential

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45
Q

What is the shape if an Q-t graph for a discharging capacitor

A

Decreasing exponential

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46
Q

Why do the current, charge and pd all fall exponentially when a capacitor is decreasing

A

Current flows in opposite direction, will take the same amount of time for all the values to half

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47
Q

Capacitor charging formula for current

A

I = I_0 x e^(-t / RC)

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48
Q

Capacitor charging formula for potential difference

A

V = V_0 (1 - e^(-t / RC))

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49
Q

Capacitor charging formula for charge

A

Q = Q_0 (1 - e^(-t / RC))

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50
Q

Capacitor discharging formula for current

A

I = I_0 x e^(-t / RC)

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51
Q

Capacitor discharging formula for potential difference

A

V = V_0 x e^(-t / RC)

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52
Q

Capacitor discharging formula for charge

A

V = V_0 x e^(-t / RC)

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53
Q

What is RC

A

Time constant

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54
Q

What is the time constant equal to

A

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)

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55
Q

How to find time constant using lnQ

A

Gradient of lnQ-t graph is -1 / RC so RC is -1 / gradient

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56
Q

Time to half when discharging formula

A

T_1/2 = 0.69RC

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57
Q

When does a capacitor stop charging

A

When the pd across the plates is equal to the pd of the battery

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58
Q

What are the 3 factors affecting capacitance

A

Permittivity, area, distance

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59
Q

Potential difference, electric field and distance between plates formula

A

Electic field = voltage / distance

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60
Q

Explanation for distance and electric field strength relationship

A

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

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61
Q

Relationship between distance and capacitance

A

Indirectly proportional

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62
Q

Relative permittivity symbol formula

A

ɛ_r = ɛ/ɛ_0

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63
Q

Charge / time =

A

Current

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64
Q

Formula for charge stored against time when charging a capacitor

A

Q = Q_0 (1-e^-t/RC)

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65
Q

How much charge is stored in a capacitor at the time constant whilst charging

A

0.63

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66
Q

Relationship between voltage across capacitor and charge stored

A

Proportional

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67
Q

What has to be passed through a wire for a magnetic field to be induced

68
Q

What is the magnetic flux density

A

Measure of strength of the field, measured in Tesla, B

69
Q

What is 1 Tesla defined as

A

Force of 1N on 1m of wire carrying 1A of current perpendicular to a magnetic field

70
Q

What happens if a current-carrying wire is placed in a magnetic field

A

A force is exerted on the wire

71
Q

What is the force exerted on a current-carrying wire parallel to a the magnetic field

72
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’

73
Q

Force, length, current and flux density formula when field is perpendicular to current

74
Q

F = BIL what is B

A

Magnetic flux denisty of field

75
Q

F = BIL what is I

A

Current in wire

76
Q

F = BIL what is L

A

Length of wire

77
Q

What does Fleming’s left hand rule find

A

Direction of the force exerted on the wire

78
Q

Fleming’s left hand rule - what does the thumb represent

A

Direction of motion/force

79
Q

Fleming’s left hand rule - what does the first finger represent

A

Direction of the field

80
Q

Fleming’s left hand rule - what does the second finger represent

A

Direction of conventional current (opposite of direction of electron flow)

81
Q

What is the direction of a magnetic field on a magentic

A

North to South

82
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)

83
Q

F, B, Q, v formula

84
Q

F = BQv what is B

A

Flux density

85
Q

F = BQv what is Q

A

Charge of particle

86
Q

F = BQv what is v

A

Velocity of particle moving perpendicular to a field

87
Q

What does BQv equal

A

Force exerted on a particle

88
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

89
Q

What is the relationship between the direction of the force exerted and the direction of travel

A

Perpendicular

90
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

91
Q

Formula for radius of a charged particles circular path

A

r = mv / BQ

92
Q

Application of circular deflection of charged particles in a magnetic field

A

A type of particle accelarator called a cyclotron, has many uses including producing ion beams for radiotherapy and radioactive tracers

93
Q

Structure of a cyclotron

A

2 semi-circular electrodes called ‘dees’, uniform magnetic field acting perpendicular to the plane of electrodes, high frequency alternating voltage applied between electrodes

94
Q

Why is there an alternating electric field between electrodes in a cyclotron

A

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
Q

What happens once the particles reach the edge of the electrode in a cyclotron

A

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
Q

Roles of the electric vs magnetic field in cyclotrons

A

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
Q

What does the magnetic flux symbol look like

A

Circle with a line through it

98
Q

What is the magnetic flux

A

Value which describes the magnetic field or magnetic field lines passing through a given area

99
Q

How to calculate the magnetic flux

A

Product of magnetic flux denisty and given area when the field is perpendicular to the area so = BA

100
Q

Magntetic flux linkage symbol

A

N(circle with line through) so (N)(magnetic flux)

101
Q

What is the magnetic flux linkage

A

Magnetic flux times by number of turns (N) of a coil

102
Q

How to find magnetic flux or magnetic flux linkage when magnetic field is not perpendicular to coil of wire

A

Use trigonometry to resolve the magnetic field vector into components which are parallel and perpendicular to the coil

103
Q

What is the magnetic flux for a component of a field parallel to the coil of wire

104
Q

Magnetic flux formula when not magnetic field not perpendicular to coil of wire

A

BA cos(theta)

105
Q

BA cos(theta) what is theta if this is the formula for magnetic flux when M field not perpendicular to coil of wire

A

Angle between field and normal to the plane of the coil

106
Q

What happens to the electrons in a conducting rod when it moves relative to a magnetic field

A

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
Q

What is electromagnetic induction

A

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
Q

1 other scenario for electromagnetic induction not involving a conducting rod

A

Moving a bar magnet relative to a coil of wire - if the coil forms a complete circuit then a current is also induced

109
Q

2 laws governing the effects of electromagnetic induction

A

Faraday’s law and Lenz’s law

110
Q

What is Faraday’s law

A

Magnitude of induced emf is equal to the rate of change of flux linkage

111
Q

What is Lenz’s law

A

Direction of induced current is such as to oppose motion causing it

112
Q

Demonstration of Lenz’s law premise

A

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
Q

Lenz’s law demonstration explanation - magnet approaching wire

A

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
Q

Lenz’s law demonstration explanation - magnet passes through wire

A

No change in flux so no emf induced

115
Q

Lenz’s law demonstration explanation - magnet moving away from wire

A

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
Q

Faraday’s law equation

A

E = N (change in magnetic flux) / (change in time)

117
Q

E = N (change in magnetic flux) / (change in time) what is E

A

Magnitude of induced emf

118
Q

E = N (change in magnetic flux) / (change in time) what is N (change in magnetic flux) / (change in time)

A

Rate of change of flux linkage

119
Q

How does Lenz’s law effect Faraday’s equation

A

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
Q

Magnitude of emf induced by a straight conductor of length l, moving in an electric field of flux density B

121
Q

How to calculate the emf induced when a coil rotates at a constant frequency in a magnetic field

A

Derivate of formula for magnetic flux linkage with respect to time as induced emf is equal to rate of change of flux linkage

122
Q

Formula for magnetic flux linkage in a rotating coil

A

N(magnetic flux) = BANcos(wt)

123
Q

N(magnetic flux) = BANcos(wt) what is wt

A

Angular speed x time

124
Q

N(magnetic flux) = BANcos(wt) using sine function

A

E = BANw sin(wt)

125
Q

E = BANw sin(wt) significance of using the sine function

A

Induced emf is alternating meaning it will change direction with time

126
Q

What type of current can be displayed on an oscilloscope

127
Q

What does an oscillioscope do

A

Shows variation of voltage with time

128
Q

What can you turn off on an oscillioscope

129
Q

Significance of being able to turn off the time-base on an oscilliscope

A

Causes the trace to show all the possible voltages at any time in one area which is usefule for taking measurements

130
Q

Oscilliscope readings for a direct current with/without time base

A

With - straight line parallel to axis at height of output voltage, without - dot at height of output voltage

131
Q

Oscilliscope readings for an alternating current with/without time base

A

With - sinusoidal waeform showing the variation of output voltage with time, without - straight vertical line

132
Q

How can you make taking measurements easier on an oscilloscope

A

Adjust scale of both axes of grid

133
Q

How to change scale of Y-axis on oscilloscope

A

Select number of volts per divison using a Y-gain control dial

134
Q

How to change scale of X-axis on oscilloscope

A

Adjust the time base

135
Q

How to take measurements from an oscilloscope

A

Count number of divisions (adjusting axes to make easier) and multiple them by either volts per division or time base

136
Q

What can you measure on an oscilloscope

A

Peak voltage (V_0), peak-to-peak voltage, root mean square (rms) voltage, time period (T)

137
Q

How to measure peak voltage on an oscilloscope

A

Distance from equilibrium to highest or lowest point

138
Q

How to measure peak-to-peak voltage on an oscilloscope

A

Distance from minimum point to maximum point

139
Q

How to measure root mean square voltage on an oscilloscope

A

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
Q

How to measure time period on an oscilloscope

A

Distance between 2 adjacents points in phase

141
Q

What is the voltage to the energy to UK homes

142
Q

What sort of electricity is supplied to homes in the UK

A

Alternating

143
Q

If an alternating electric supply is delivered to UK homes, what value is 230V

A

rms of voltage (root mean square)

144
Q

What sort of current do transformers use

A

Alternating

145
Q

Basic structure of transformers

A

Primary coil attached to input voltage, secondary coil is connected to output voltage, has an iron core

146
Q

How is a voltage induced in a transformer

A

Primary coil provides a changing magnetic field, passes through iron core and interacts with secondary coil

147
Q

What does Faraday’s law show about transformers and ratio’s

A

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
Q

Faraday’s law effect on ratio of transformers formula

A

Ns / Np = Vs / Vp

149
Q

Ns / Np = Vs / Vp what is N

A

Number of turns

150
Q

What are the different types of transformers

A

Step up, step down

151
Q

What do step up transformers do

A

Increase input voltage by having more turns on secondary coil than primary

152
Q

What do step down transformers do

A

Decrease input voltage by having less turns on the secondary coil than primary

153
Q

Transformer efficiency formula

A

(Is Vs) / (Ip Vp) so power output / power input

154
Q

What is the main form of energy loss in a transformer

A

Production of eddy currents

155
Q

How are eddy currents formed

A

Transformers - induced by alternative magnetic field in primary coil and form a loop

156
Q

Lenz’s law, how do eddy currents cause a loss of energy in transformers

A

Oppose the field that produced them, reduces fields flux density, generate heat which causes energy to be lost

157
Q

How can eddy currents be reduced

A

Using a laminated iron core or using a core made from a high resistivity metal

158
Q

What is a laminated iron core

A

Core made using layers of iron between layers of an isulator

159
Q

How does using a laminated iron core reduce eddy currents

A

Eddy currents can’t pass through the insulator and so amplitude is reduced

160
Q

Besides eddy currents, how can energy be lost in transformers

A

Resistance in coils causes heating, if core isn’t easily magnetised

161
Q

How to reduce energy lost due to resistance in coils

A

Use a thick wire which will have a low resistance

162
Q

How to reduce energy lost due to core not being easily magnetised

A

Magnetically soft iron core can be used allows easy magnetisation and demagnetisation

163
Q

Power lost due to resistance formula

164
Q

How to reduce energy loss when transferring electrical power

A

Reducing current to a minimum value

165
Q

What sort of transformer should be used to transmit electricity over a long distance

A

Step up, increase voltage, decrease current

Physics (8 decks)

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