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A LEVEL - Physics > Section 6 - Electricity > Flashcards

Section 6 - Electricity Flashcards

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1
Q

What is current?

A

The rate of flow of charge in a circuit.

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2
Q

What are the conditions for current to pass through a circuit?

A
  • Circuit must be complete

* Source of p.d.

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3
Q

What is the unit for current?

A

Ampere (A)

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4
Q

What are charge carriers?

A

Charged particles that move around a circuit, allowing current to flow.

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5
Q

What are the charge carriers in metals?

A

Conduction electrons.

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6
Q

What are the charge carriers in a salt solution?

A

Ions.

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7
Q

What is the unit for charge?

A

Coulomb (C)

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8
Q

Which way does conventional current flow?

A

From + to - terminals.

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9
Q

Which way do electrons flow in a circuit?

A

From - to + terminals.

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10
Q

What must be remembered about the direction of current in a circuit?

A
  • Although electrons flow from - to + terminals, this is not used.
  • Conventional current is in fact considered, which flows from + to - terminals.
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11
Q

What is the symbol for current?

A

I

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12
Q

What is the symbol for charge?

A

Q

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13
Q

What is an ampere defined by?

A

The magnetic force between two parallel wires when they carry the same current.

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14
Q

What is a coulomb defined as?

A

The amount of charge that passes in 1 second if the current is 1 ampere.

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15
Q

What is the equation relating charge, current and time?

A

Q = I x t

Where:
Q - Charge (C)
I - Current (A)
t - Time (s)

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16
Q

What device is used to measure current and how must it be connected?

A
  • Ammeter

* Connected in series

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17
Q

Why can’t current flow in an insulator?

A
  • Each electron is attached to an atom and can’t move away from the atom
  • Therefore, electrons can’t flow
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18
Q

Why can current easily flow in a metallic conductor?

A
  • Most electrons are attached to an atom, but some are delocalised
  • These delocalised electrons act as charge carriers
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19
Q

What is a semiconductor and why does it behave like this?

A
  • Number of charge carriers increases with temperature -> Resistance decreases
  • This is because electrons break free and act as charge carriers
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20
Q

What is a pure semiconducting material called and why?

A

Intrinsic semiconductor (intrinsic means natural) -> Conduction is due to electrons that break free from atoms of the material

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21
Q

What are the uses of semiconductors?

A

Sensors for detecting changes in the environment (e.g. thermistors and diodes).

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22
Q

Give two examples of semiconductors.

A

Thermistors and diodes.

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23
Q

What must be done in order to make electric charge flow through a conductor?

A

Do work on it.

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24
Q

What is potential difference?

A

The work done (or energy transferred) per unit charge.

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25
Q

What is the unit for potential difference?

A

Volt (V)

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26
Q

What is the equation relating potential difference, work done and charge?

A

V = W / Q

Where:
V - Potential difference (V)
W - Work done (J)
Q - Charge (C)

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27
Q

What is the symbol for potential difference?

A

V

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28
Q

What device is used to measure potential difference and how must it be connected?

A
  • Voltmeter

* In parallel

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29
Q

Describe the energy changes to an electron in a circuit.

A
  • A battery transfers chemical energy to the electrons
  • Each electron has to do work to pass through a component and transfers energy to it
  • The battery resupplies electrons with energy
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30
Q

When a electron passes through a component, the work done by the electron is equal to…

A

The loss of energy.

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31
Q

Describe what potential difference is in terms of a component.

A
  • Each electrons has to do work to pass through a component (this is equal to the energy it loses)
  • The potential difference across a component is the work done per unit charge
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32
Q

What is a volt?

A
  • The potential difference across a component is 1 volt when you convert 1 joule of energy moving 1 coulomb of charge through the component.
  • 1V = 1J/C
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33
Q

Describe and explain the energy changes in an electrical heater.

A
  • Heater has high resistance
  • Charge carriers collide repeatedly with atoms in the device and transfer Ek to them
  • Atoms vibrate faster
  • Resistor heats up -> Transferred as thermal energy
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34
Q

Describe and explain the energy changes in an electric motor.

A
  • WD on motor = Energy transferred to the load and surroundings -> Constant speed
  • Electrons need to forced through the wires of the spinning motor coil against the opposing force exerted by the magnetic field
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35
Q

Describe and explain the energy changes in a loudspeaker.

A
  • WD is transferred as sound energy

* Electrons need to forced through the wires of the speaker coil against the force on them due to the loudspeaker magnet

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36
Q

For a particular potential difference across a component, what determines the current?

A

The resistance of the component.

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37
Q

What is resistance?

A

A measure of how difficult it is for current to pass through the component.

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38
Q

What is the unit for resistance?

A

Ohms (Ω)

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39
Q

What is the symbol for resistance?

A

R

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40
Q

What equation relates potential difference, current and resistance?

A

V = I x R

Where:
V - Potential difference (V)
I - Current (A)
R - Resistance (Ω)

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41
Q

What is an ohm?

A

A component has a resistance of 1Ω if a potential difference of 1V makes a current of 1A flow through it.

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42
Q

What an the resistance of a voltmeter and ammeter be assumed to be?

A

Voltmeter - Infinite

Ammeter - Zero

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43
Q

What is Ohm’s law?

A
  • Provided that the physical conditions (e.g. temperature) remain constant, the current through an ohmic conductor us directly proportional to the potential difference across it.
  • V = I x R
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44
Q

What causes resistance?

A

Repeated collisions between the charge carriers in the material with each other and the fixed ions in the material.

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45
Q

What is an ohmic conductor?

A
  • A material or component that obeys Ohm’s law.

* Has constant resistance, regardless of the current.

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46
Q

Describe the I/V graph for an ohmic conductor.

A
  • Goes through the origin

* Straight line

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47
Q

Name two physical factors that could affect Ohm’s law.

A
  • Light level

* Temperature

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48
Q

What conditions must be met for Ohm’s law?

A
  • Ohmic conductor

* Constant physical conditions

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49
Q

Why must light levels and temperature be kept constant in experiments on ohmic conductors?

A

They could affect the resistance of the conductor.

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50
Q

Are most components ohmic conductors?

A

No, most are non-ohmic components and have their own I-V graph.

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51
Q

What does the term “I/V characteristic” refer to?

A
  • A graph of I (y-axis) against V (x-axis)

* Shows how current through a component changes as the potential difference is increased.

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52
Q

On an I/V graph, what is on the x-axis?

A

V

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53
Q

On a V/I graph, what is on the x-axis?

A

I

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54
Q

Describe the circuit that can be used to determine the I/V graph for a component.

A
  • Ammeter
  • Component
  • Voltmeter (across component)
  • Variable resistor
  • Battery
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55
Q

How can you find the resistance at a certain point on a V/I or I/V graph?

A
  • Look at the value for V and for I

* R = V/I

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56
Q

On an I/V graph for an ohmic conductor, what does a steep gradient signify?

A

Low resistance

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57
Q

On an V/I graph for an ohmic conductor, what does a steep gradient signify?

A

High resistance

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58
Q

Remember to revise + practise drawing out all of the I/V graphs.

A

Pg 76-77 of revision guide.

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59
Q

Describe the I/V graph for a filament lamp.

A
  • Curve
  • Starts steep but gets shallower with voltage
  • Rotated around origin(See pg 76 for graphs)
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60
Q

Why isn’t a filament bulb an ohmic conductor?

A

The wire heats up with current and potential difference, causing the resistance to increase.

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61
Q

Explain how and why the resistance of a filament lamp changes as potential difference is increased.

A
  • As potential difference increases, so does current.
  • Increasing current increases the temperature (due to electron-ion collisions)
  • Positive ions in metal vibrate more -> More difficult for charge carriers to pass -> Resistance increased
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62
Q

Explain how and why the resistance of a filament lamp changes as current is increased.

A
  • Increasing current increases the temperature (due to electron-ion collisions)
  • Positive ions in metal vibrate more -> More difficult for charge carriers to pass -> Resistance increased
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63
Q

Compare the resistance of semiconductors and metals..

A
  • Metals are better conductors (i.e. lower resistance)

* Resistance of metals increases with temperature, while resistance of semiconductors decreases with temperature

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64
Q

At low temperatures, why are metals better conductors than semiconductors?

A

There are more charge carriers available.

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65
Q

What is a thermistor?

A

A resistor with a resistance that changes with temperature.

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66
Q

What type of thermistor do you need to know about?

A

Negative Temperature Coefficient (NTC) - Resistance decreases as temperature increases

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67
Q

How does the resistance of a thermistor change with temperature?

A

As temperature increases, resistance decreases.

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68
Q

Describe the graph of resistance (y-axis) against temperature (x-axis) for a thermistor.

A
  • Downwards curve

* Gradient becomes less steep with temperature(See diagram pg 77)

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69
Q

Describe the I/V graph for a thermistor.

A
  • Upwards curve
  • Curved upwards away from x-axis
  • Rotated around origin

(See diagram pg 77)

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70
Q

Explain the I/V graph for a thermistor.

A
  • As p.d. increases, current increases
  • This causes temperature to increase
  • More electrons have enough energy to escape from their atoms
  • More charge carriers -> Resistance decreases
  • More current can flow, so graph curves upwards
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71
Q

What is a diode?

A

A component that allows current to flow in one direction only.

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72
Q

What is the forward bias of a diode?

A

The direction in which the current is allowed to flow.

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73
Q

What does LED stand for?

A

Light emitting diode.

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74
Q

Describe how the resistance of a diode changes with potential difference.

A
  • With a negative voltage (in reverse bias) the resistance is very high
  • Up to a threshold voltage (usually about 0.6V) the resistance remains high
  • After this voltage, the resistance falls rapidly
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75
Q

What is the threshold voltage of most diodes?

A

About 0.6V

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76
Q

What is an LED?

A

A diode that emits light when current flows through it.

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77
Q

Describe the I/V graph for a diode.

A
  • At negative voltage -> Very small negative current
  • At low positive voltage -> Small positive current
  • Above threshold voltage -> Current increases linearly

(See diagram pg 77)

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78
Q

Name a use of a diode.

A

Protection of d.c. circuits.

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79
Q

What is a cell?

A

A source of electrical energy.

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80
Q

What is a LDR?

A
  • Light-depedent resistor

* Resistance decreases as temperature is increased.

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81
Q

What is a positive temperature coefficient?

A

When resistance increases with increasing temperature.

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82
Q

What is a negative temperature coefficient?

A

When resistance decreases with increasing temperature.

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83
Q

Give an example of a material/component with a positive temperature coefficient.

A

Metals

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84
Q

Give an example of a material/component with a negative temperature coefficient.

A

Thermistor

85
Q

What is the circuit symbol for a thermistor?

A

Rectangle with diagonal line across it, with a short line at the end.

86
Q

What is the circuit symbol for a diode?

A

Circle with a triangle and line in it.

87
Q

What is the circuit symbol for an LED?

A

Circle with a triangle and line in it. Two lines pointing away from it.

88
Q

What does the symbol for a diode in a circuit tell us?

A

Current can flow in the direction that the triangle points.

89
Q

What is the circuit symbol for an ammeter?

A

A circle with an “A” in it.

90
Q

What is the circuit symbol for a voltmeter?

A

A circle with a “V” in it.

91
Q

What is the circuit symbol for a cell?

A

A long line and a short line.

92
Q

What is the circuit symbol for a light bulb?

A

A circle with a cross in it.

93
Q

What is the circuit symbol for a resistor?

A

A rectangle.

94
Q

What is the circuit symbol for a variable resistor?

A

A rectangle with a diagonal arrow.

95
Q

What is the circuit symbol for a LDR?

A

A rectangle with two arrows pointing towards it.

96
Q

What is the circuit symbol for a heater?

A

A rectangle divided into 4 squares.

97
Q

What is the circuit symbol for an electric motor?

A

A circle with an “M” with a line under it that points down and up at either end.

98
Q

Remember to revise circuit symbols.

A

Pg 209 of textbook.

99
Q

What is the resistivity of a material?

A

The resistance of a 1m length of the material of 1m2 cross-sectional area.

100
Q

What 3 factors affect the resistance of a piece of wire?

A

1) Length
2) Area (Cross-sectional)
3) Resistivity

101
Q

How does length of a wire affect its resistance?

A

The longer the wire, the higher the resistance.

102
Q

How does the cross-sectional area of a wire affect its resistance?

A

The wider the wire, the lower the resistance.

103
Q

What factors affect the resistivity of a material?

A

Environmental factors (e.g. temperature and light intensity)

104
Q

What is the unit for resistivity?

A

Ohm-metre (Ωm)

105
Q

What is the symbol for resistivity?

A

p (Greek letter “rho”)

106
Q

What is the equation for resistivity?

A 
p = RA/l
Where:
p - Resistivity (Ωm)
R - Resistance (Ω)
A - Area (m2)
l - Length (m)
107
Q

Is the resistivity of a material a set quantity?

A
  • No, it depends on temperature.

* Resistivity is usually quoted at a set temperature (e.g. 25*)

108
Q

What is the size of a typical value for the resistivity of a conductor?

A

Very small - e.g. 1.72 x 10^-8

109
Q

Describe how you can calculate the resistivity of a piece of wire.

A

Calculate area:
1) Measure the diameter at at least 3 points along the wire using a micrometer -> Find average -> Divide by two to get radius
2) Area = πr2
Calculate R/l:
1) Set up a circuit with an ammeter, wire and voltmeter.
2) Attach wire along a ruler -> Attach one end where the ruler reads 0cm
3) Move the crocodile clip at the other end to adjust the length of the wire
4) Record the length of the wire and the resistance (R = V/I)
5) Repeat this to find an average resistance for that length
6) Vary the length from 0.10 to 1.00m
7) Plot a graph of resistance (y) against length (x) + draw a line of best fit
Find the resistivity:
1) The gradient is R/l, so it can be subbed in to the equation p = RA/l by multiplying by the area.
2) Take note to maintain the temperature of the wire constant at all times (since resistivity depends on temperature).

(See diagram pg 78)

110
Q

When calculating the resistivity of a piece of wire, what is it important to keep constant and how?

A
  • Temperature -> Resistivity depends on it

* Only have small currents flow through the wire

111
Q

Remember to revise the experiment to find the resistivity of a piece of wire.

A

Pg 78 of revision guide.

112
Q

What is the problem with most materials having some resistivity?

A

When current is passed through them, they heat up and energy is wasted as thermal energy.

113
Q

What is the easiest way to lower the resistivity of most materials?

A

Cool them down.

114
Q

What is a superconductor?

A

A wire or device made of a material that has 0 resistivity below a critical temperature, which depends on the material.

115
Q

Describe how the resistance of a superconductor changes with temperature.

A
  • Below critical temperature -> Zero resistance

* Above critical temperature -> Resistance increases

116
Q

What is the temperature below which a superconductor has no resistance called?

A

Threshold temperature / Critical temperature

117
Q

Describe how the resistance of these components varies with temperature:
• Metal
• Semiconductor
• Superconductor

A
  • Metal -> Resistance increases as temperature increases
  • Semiconductor -> Resistance decreases as temperature increases
  • Superconductor -> Non-zero resistance above critical temperature, zero resistance below it
118
Q

What are the implications of a superconductor and why?

A
  • With no resistance, there is no heating effect -> No energy lost
  • You could start a current using a magnet and it would flow forever
119
Q

What is the problem that makes the use of superconductors difficult?

A
  • Most have a transition temperature below 10 kelvin (263*C)

* It is hard and expensive to cool things that much

120
Q

What is a high-temperature superconductor?

A

A superconductor with a critical temperature above 77K (the boiling point of nitrogen).

121
Q

What is 77K equal to?

A

The boiling point of nitrogen.

122
Q

Describe the graph of resistivity (y) against temperature (x) for a superconductor.

A
  • Line along the x-axis up to critical temperature
  • Vertical spike at the critical temperature
  • Straight line with upwards gradient after this
123
Q

What is the next development in terms of superconductors?

A

Room-temperature superconductors

124
Q

Give some current uses of superconductors.

A

High-power electromagnets used in:
• MRI scanners
• Particle accelerators

125
Q

Give some potential uses of superconductors.

A
  • Power cables that transmit energy without energy loss
  • Strong electromagnets without a constant power source
  • Electronic circuits that work really fast since there’s no resistance
126
Q

What is power?

A

The rate of transfer of energy.

127
Q

What is the symbol for power?

A

P

128
Q

What is the unit for power?

A

Watt (W)

129
Q

What is a watt?

A

1 joule transferred per second.

130
Q

What is the formula for power?

A

P = E/t

Where:
P - Power (W)
E - Energy transferred (J)
t - Time (s)

131
Q

What is the formula for power in terms of current and voltage?

A

P = I x V

132
Q

What is the formula for power in terms of resistance and voltage?

A

P = V²/R

133
Q

What is the formula for power in terms of current and resistance?

A

P = I² x R

134
Q

How can the different equations for power by derived?

A

Combining:
• P = I x V
• V = I x R

135
Q

What are the 4 equations for power?

A
  • P = E/t
  • P = I x V
  • P = V² /R
  • P = I² x R
136
Q

What is the equation for energy transferred in terms of power and time?

A

E = P x t

NOTE: Any of the equations for P can be subbed in here (e.g. E = V x I x t).

137
Q

What causes resistance?

A

Electrons colliding with atoms in the material and losing energy.

138
Q

What is internal resistance and what causes it?

A
  • The resistance of a battery itself.

* Caused by electrons colliding with the atoms in the battery.

139
Q

What causes cells and batteries to warm up when they are used?

A

Their internal resistances.

140
Q

What is the symbol for internal resistance?

A

r

141
Q

What is the name for the resistance of a cell?

A

Internal resistance

142
Q

What is load resistance?

A

The total resistance of all the components in a circuit except the battery.

143
Q

What is the symbol for load resistance?

A

R

144
Q

What is another name for load resistance?

A

External resistance

145
Q

What are the two types of resistance in a circuit?

A
  • Internal resistance (r)

* Load resistance (R)

146
Q

What does e.m.f. stand for?

A

Electromotive force

147
Q

What is e.m.f.?

A
  • The amount of electrical energy the battery produces for each coulomb of charge
  • i.e. It is the battery’s effective output voltage when no current flows through it
148
Q

What is the symbol for e.m.f.?

A

ε

149
Q

What are the units for e.m.f.?

A

Volts (V)

150
Q

What is the equation that defines e.m.f.?

A

ε = E/Q

151
Q

What is terminal p.d.?

A
  • The potential difference across the load resistance (R).

* i.e. The energy transferred per coulomb of charge flowing through the load resistance.

152
Q

What is the symbol for terminal p.d.?

A

V

153
Q

Is the terminal p.d. the same as e.m.f.?

A
  • Usually no, unless there is no internal resistance or no current flowing
  • Usually energy is lost in overcoming the internal resistance
154
Q

What is the name for the energy wasted per coulomb in overcoming the internal resistance in a cell?

A

Lost volts

155
Q

What is the symbol for lost volts?

A

v

156
Q

What conservation of energy equation is used in e.m.f. calculations?

A

Energy per coulomb supplied by source (ε) = Energy per coulomb transferred in load resistance (V) + Energy per coulomb wasted in internal resistance (v)

ε = V + v

157
Q

Which e.m.f. equation are you given in the exam?

A

ε = I(R + r)

158
Q

In e.m.f. calculations, what are the different symbols?

A 
ε - emf of battery
V - Terminal pd 
v - Lost volts
R - Load resistance
r - Internal resistance 
I - Current
159
Q

Rearrange the “ε = I(R+r)” equation to give 3 more equations.

A
  • ε = I(R+r)
  • ε = V + v
  • V = ε - v
  • V = ε - Ir
160
Q

How can you work out the total e.m.f. of cells in series?

A
  • Add their individual e.m.f.s

* ε(total) = ε1 + ε2 + ε3 + …

161
Q

How can you work out the total e.m.f. of IDENTICAL cells in parallel?

A
  • Total e.m.f. is equal to each individual e.m.f.

* ε(total) = ε1 = ε2 = ε3 = …

162
Q

Three identical cells with an e.m.f. of 2.0V and an internal resistance of 0.20Ω are connected in parallel. A current of 0.90A is flowing through the circuit. Calculate the total p.d. across the cells.

(See diagram of 83 of revision guide)

A 
Calculate the lost volts for 1 cell:
• I = 0.90 / 3 = 0.30A
• v = Ir = 0.30 x 0.20 = 0.06V
Find terminal pd across 1 cell:
• V = ε - v
• V = 2 - 0.06 = 1.94V
163
Q

Describe an experiment to calculate the e.m.f. and internal resistance of a cell.

A

1) Connect the cell in series with an ammeter and variable resistor + connect a voltmeter across the cell
2) Vary the current using the variable resistor.
3) Record the voltage at each current.
4) Plot a graph of voltage (y) against current (x).
5) y-intercept = ε
Gradient = -r

164
Q

In the experiment to find the e.m.f. and internal resistance of a cell, how should the V and I values be plotted and why?

A 
• V = ε - Ir
Rearranges to:
• V = -rI + ε
• y = mx + c
Therefore:
• Plot V on the y and I on the x
• Gradient = -r
• y-intercept = ε
165
Q

What is an easy way to measure a cell’s e.m.f.?

A
  • Connect a high-resistance voltmeter across its terminals

* A small current flows through the voltmeter, so there are some lost volts, but this is negligible

166
Q

When connecting a high-resistance voltmeter across a cell to find its e.m.f., what is the error in the results?

A
  • Small amount of current flows through the voltmeter
  • So there are some lost volts
  • Measured value is very slightly less than the e.m.f.
  • But this is negligible
167
Q

Does charge get used up?

A

No

168
Q

What happens to charge and current at a junction?

A

They are conserved.

169
Q

What quantities are conserved at a junction?

A
  • Current

* Charge

170
Q

Is voltage conserved at a junction?

A

Not necessarily.

171
Q

What is Kirchhoff’s first law?

A

Total current entering junction = Total current leaving it

172
Q

Is energy in a circuit conserved?

A

Yes

173
Q

Energy transferred to a charge is…

A

e.m.f.

174
Q

Energy transferred from a charge is…

A

Potential difference.

175
Q

What is Kirchhoff’s second law?

A

Total e.m.f. around a series circuit = Sum of the p.d.s across each component

176
Q

Explain Kirchhoff’s second law.

A
  • e.m.f. is the energy transferred to a charge
  • p.d. is the energy transferred from a charge
  • Conservation of energy says that these two must be equal
  • Therefore: Total e.m.f. = Sum of p.d.s
177
Q

State both of Kirchhoff’s laws.

A

1) Total current entering a junction = Total current leaving it
2) Total e.m.f. around a series circuit = Sum of p.d.s across each component

178
Q

Describe current in a series circuit.

A

It is the same at all points.

179
Q

Describe e.m.f. in a series circuit.

A
  • e.m.f. is split between components

* ε = V1 + V2 + V3 + …

180
Q

Describe resistance in a series circuit.

A
  • Sum of the individual resistances gives total resistance

* R(total) = R1 + R2 + R3 + …

181
Q

Describe a series circuit in terms of:
• Current
• e.m.f.
• Resistance

A 
CURRENT:
• Same at all points
EMF:
• Shared between components
RESISTANCE:
• Total resistance is sum of the individual resistances
182
Q

Describe current in a parallel circuit.

A
  • Split between branches (at each junction)

* I(total) = I1 + I2 + I3 + …

183
Q

Describe e.m.f. in a parallel circuit.

A
  • Same p.d. on each branch
  • Within each branch, the sum of the p.d.s equals the e.m.f.
  • e.m.f. = Branch 1 = Branch 2 = Branch 3 = …
184
Q

Describe resistance in a parallel circuit.

A

1/R(total) = 1/R1 + 1/R2 + 1/R3 + …

185
Q

Describe a parallel circuit in terms of:
• Current
• e.m.f.
• Resistance

A

CURRENT:
• Split between branches (at each junction)
EMF:
• Same p.d. on each branch
• Within each branch, the sum of the p.d.s equals the e.m.f.
RESISTANCE:
• 1/R(total) = 1/R1 + 1/R2 + 1/R3 + …

186
Q

Remember to practice conservation of energy and charge questions.

A

See example pg 85 of revision guide.

187
Q

What is a potential divider?

A

A setup used to obtain an output voltage equal to a fraction of the source voltage.

188
Q

What is the simplest possible potential divider?

A

A voltage source with 2 resistors in series (with wires around one of the resistors).

189
Q

What range in potential difference can be provided by a potential divider?

A

From 0 to the source voltage.

190
Q

In what ratio is pd split in a potential divider?

A
  • In the ratio of the the resistances

* i.e. The higher the resistance, the higher the voltage across that resistor

191
Q

With a 2Ω and 3Ω resistor in series, what is the potential difference across each resistor?

A
  • 2Ω -> 2/5 of the source voltage

* 3Ω -> 3/5 of the source voltage

192
Q

Remember to practise drawing out a basic potential divider diagram.

A

Pg 86 of revision guide.

193
Q

For a source voltage, Vs, connected in series with resistors R1 and R2 (with output around R2), what equation gives the output voltage around R2?

A

V(out) = (R2 / (R1 + R2)) x Vs

i.e. Output voltage = (Output resistor voltage / Total load voltage) x Source voltage

194
Q

In potential divider diagrams, what is:
• Vs
• R1
• R2

A
  • Vs - Source voltage
  • R1 - Non-output resistor
  • R2 - Output resistor
195
Q

In a potential divider, if the source voltage is 9V and R1 is 100Ω, what must R2 be in order to get an output of 6V?

A

200Ω

196
Q

What is a simple potential divider used for?

A

Calibrating voltmeters, which have a high resistance.

197
Q

In a simple potential divider, what happens when you add a component with low resistance across R2 (the output resistor), what happens and why?

A
  • You now have two resistors in parallel, so the total resistance will be lower than R2.
  • This means that R2 will be lower than calculated.
198
Q

In a simple potential divider, what is the effect of replacing R1 (non-output resistor) with a variable resistor?

A

It allows the output voltage across R2 to be varied.

199
Q

In a simple potential divider with a variable resistor as R1, how does output voltage across R2 vary as the resistance R1 is increased?

A
  • When R1 = 0, V(out) = Vs

* As you increase R1, V(out) gets smaller

200
Q

How does the resistance of a light-dependent resistor vary?

A

Resistance decreases as light intensity increases.

201
Q

What is the circuit symbol for an LDR?

A
  • Circle with a rectangle inside it.
  • Wires reach to the rectangle, but not through it.
  • Arrows point to the circle.

(See diagram pg 86 of revision guide)

202
Q

How does the resistance of a NTC thermistor vary?

A

Resistance decreases as temperature increases.

203
Q

How can a potential divider output be varied according to temperature or light intensity?

A

Using a thermistor or LDR as one of the resistors in the potential divider.

204
Q

When an LDR is connected as R1 in a potential divider, with the output across another resistor, R2, how will the output vary as light increasing increases and why?

A
  • Light gives the LDR a lower resistance.
  • So it uses less of the pd and more can go to the output.
  • Output voltage increases as light intensity increases.

(See diagram pg 86 of revision guide)

205
Q

Give a use of LDRs and thermistors.

A

They can be included in circuits that control switches, e.g. to turn on a light or heating system.

206
Q

What is a potentiometer?

A

A type of potential divider that uses a single variable resistor instead of two fixed resistors to give a desired voltage output.

(See diagram of 87 of revision guide)

207
Q

How does a potentiometer divider work?

A
  • The slider can be moved along the variable resistor to change R1 and R2 as desired.
  • So V(out) can be varied from 0 to the source voltage
208
Q

When is a potentiometer useful?

A
  • When you need to change the voltage continuously

* e.g. The volume control of a stereo

209
Q

Remember to revise potential dividers and potentiometers.

A

Pgs 86-87 of revision guide.

A LEVEL - Physics (14 decks)

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