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A Level Physics Paper 1 > Electricity > Flashcards

Electricity Flashcards

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1
Q

Charge, current, time equation

A

Q=It

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2
Q

Current

A

Rate of change of charge

Measured in Amperes (A)

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3
Q

What is one coulomb

A

The amount of charge that passes in 1 second through a current of 1 Ampere

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4
Q

Electron flow vs conventional current

A

E: what actually happens, electrons flowing from negative terminal of cell to positive terminal

C: what is shown in diagrams, current going positive to negative

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5
Q

Which part of a battery symbol is positive and which is negative

A

Positive is the long terminal

Negative the short terminal

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6
Q

What causes electrons to flow

A

A potential difference

Flow from a higher potential to a lower potential

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7
Q

What happens to current if a kink is made in a wire

A

Causes resistance (difficulty in the flow of electrons) so decreases the current

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8
Q

What is Ohm’s law

A

The current flowing through a metallic conductor is directly proportional to the potential difference applied across it
At a constant temperature

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9
Q

Conditions for Ohm’s law

A

Metallic conductor

Constant temperature

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10
Q

Equation for Ohm’s law

A

V=IR

V∝I

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11
Q

Charge of one electron

A

-1.6x10^-19C

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12
Q

How many electrons needed to produce a charge of 1C

A

6.25x10^18

1/(1.6x10^-19)

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13
Q

Potential difference

A

The potential difference between two points is the amount of energy transformed when one coulomb of charge is moved between the points

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14
Q

What is resistance

A

A ratio of voltage to current

Measured in Ohm’s

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15
Q

1 Volt =

A

1 Joule per coulomb

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16
Q

Equation for work done, charge and voltage

A

W=QV

W=E

E=QV

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17
Q

Power

A

The rate at which work is done
The rate at which energy is transferred from one type to another
Measured in joules per second or watts

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18
Q

Equation for power

A

P=W/t or P=E/t
P=VI
P=I^2R
P=V^2/R

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19
Q

How is P=VI obtained

A

Q=It (1)
W=QV (2)
W=Pt (3)

2 and 3;
QV=Pt
P=QV/t

1;
Q/t=I

1 and 2 and 3;
P=IV

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20
Q

How is P=I^2R obtained

A

P=IV
V=IR

P=IIR
P=I^”R

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21
Q

How is P=V^2R obtained

A

P=IV
V=IR

I=V/R
P=(V/R)V
P=V^2/R

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22
Q

Energy transformed by a component

A

E=VIt

Since E=Pt and P=VI

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23
Q

Current voltage graph for a diode

A

X=V
Y=I

Threshold voltage of around 0.6V
Very little current can flow in reverse
For very large voltages, current will be forced through diode in wrong way and will eventually break it

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24
Q

Gradient of a current voltage graph

A

NOT RESISTANCE

Ratio of current to voltage

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25
Q

How do you get negative results

A

Flip battery

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26
Q

Current voltage graph for a fixed resistor

A

X=V
Y=I

Straight line through origin

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27
Q

Current voltage graph for filament bulb

A

X=V
Y=I

Initially a straight line through 0 where it acts as an ohmic conductor with constant resistance
Then at high positive and negative voltages it starts to bend towards X axis

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28
Q

What is a short circuit

A

Zero/very small resistance
Infinite/excessive current flows
May damage the component and break the circuit

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29
Q

What is an ammeter

A

Piece of equipment that measures the current in a circuit
Its resistance should be zero in order for it not to contribute to the resistance of the circuit or remove any voltage/no potential difference, lower the current and hence give an inaccurate reading of current

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30
Q

What is a voltmeter

A

Piece of equipment that measures the potential difference between two points in a circuit/across a component
It acts as an observer so connected in parallel
Resistance is infinite so current is zero

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31
Q

What is a rheostat

A

A piece of equipment that can be used as a variable resistor (using 2 terminals) or a potential divider (using all 3 terminals)

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32
Q

Pros vs Cons for a variable resistor circuit

A

Simple to set up
Used in sensor circuits
Series circuit so current can be higher than in a potential divider circuit

Can’t ever get the voltage to be zero

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33
Q

Pros vs Cons for potential divider circuit

A

Allows full range of supply voltage to be used
Current/p.d can be reduced to zero
Used for volume control and dimmer switches

More complicated to set up than a variable resistor
Current has to be split due to the parallel branches so maximum current lower

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34
Q

Ohmic component

A

Follows Ohm’s law
Where the current flowing through a metallic conductor is directly proportional to the potential difference applied across it at a constant temperature
So the resistance is constant

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35
Q

Non-Ohmic component

A

Does not follow Ohm’s law
So current and potential difference are not directly proportional
And there is varying resistance

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36
Q

What is a series circuit

A

Only one path current can take that connects the positive terminal to the negative terminal
No junctions or branches

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37
Q

What is the current like in a series circuit

A

Constant at every point

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38
Q

What is the potential difference like in a series circuit

A

May vary through different components if the resistance is different
Bu the total voltage adds up to the supply voltage

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39
Q

What is a parallel circuit

A

Multiple paths that current can take from the positive terminal to negative terminal
Contains junctions and branches

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40
Q

Current in a parallel circuit

A

Current in each branch may be different but the total current is the same as the supply current

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41
Q

Potential difference in a parallel circuit

A

p.d across each branch is the same and equals the supply voltage

42
Q

Kirchoff’s first law

A

At any junction in a circuit the sum of current flowing into the junction is equal to the sum of current flowing away from it

43
Q

Kirchoff’s second law

A

In any complete loop of a circuit the sum of potential difference is equal to the source potential difference

44
Q

How do you work out the total resistance in a parallel circuit

A

Rt=(1/R1 + 1/R2 + 1/R3 + …)^-1

45
Q

What determines brightness

A

Power

Not current or voltage

46
Q

Effect of connecting two cells/power supplies in series

A

Potential difference is equal to the sum of the individual potential differences

47
Q

Effect of connecting two cells/power supplies in parallel

A

Potential difference will be the same as the potential difference of each source
But the current flowing through each will be half so they will last twice as long

48
Q

NTC

A

Negative temperature coefficient thermistor

As temperature increases the resistance of the thermistor decreases

Assume all NTC unless told otherwise

49
Q

PTC

A

Positive temperature coefficient thermistor

As temperature increases resistance of the thermistor increases

50
Q

LDR

A

Light dependent resistor

As light intensity increases the resistance of an LDR decreases

51
Q

Uses of potential dividers

A

Sensor circuits, often with NTCs and LDRS

52
Q

If the temperature increases explain how the voltage changes in an NTC in series

A

As temperature increases the thermistor resistance decreases
So now receives a smaller proportion of the total resistance
Hence receives a smaller proportion of the total voltage
Since V=IxR, and I is constant

53
Q

If the temperature increases explain how the voltage changes in another resistor in series with an NTC

A

As temperature increases the thermistors resistance decreases
So now receives a smaller proportion of the total resistance
So the other resistor receives a larger proportion of the total resistance
Hence receives a larger proportion of the total voltage
So voltage increases
Since V=IxR, and I is constant

54
Q

If the temperature decreases, explain how the voltage changes in an NTC in series

A

As temperature decreases the thermistor resistance increases
So receives a larger proportion of the total resistance
Hence receives a larger proportion of the total voltage
Since V=IxR, and I is constant

55
Q

If the temperature decreases explain how the voltage changes in another resistor in series with an NTC

A

As temperature decreases the thermistors resistance increases
So now receives a larger proportion of the total resistance
So the other resistor receives a smaller proportion of the total resistance
Hence receives a smaller proportion of the total voltage
So voltage decreases
Since V=IxR, and I is constant

56
Q

If the temperature increases explain how the current changes in another resistor in series with an NTC

A

As temperature increases the thermistor resistance decreases
So total resistance decreases
From Ohm’s law, if the total resistance decreases the total current increases
Since I=V/R and V is constant
So the current increases

57
Q

If the temperature decreases explain how the current changes in another resistor in series with an NTC

A

As temperature decreases the thermistor resistance increases
So total resistance increases
From Ohm’s law, if the total resistance increases the total current decreases
Since I=V/R and V is constant
So the current decreases

58
Q

If the temperature increases explain how the current changes in an NTC in series

A

As temperature increases the thermistor resistance decreases
So total resistance decreases
From Ohm’s law, if the total resistance decreases the total current increases
Since I=V/R and V is constant
So the current increases

59
Q

If the temperature decreases explain how the current changes in an NTC in series

A

As temperature decreases the thermistor resistance increases
So total resistance increases
From Ohm’s law, if the total resistance increases the total current decreases
Since I=V/R and V is constant
So the current decreases

60
Q

If the light intensity increases explain how the voltage changes in an LDR in series

A

As intensity increases the resistance of the LDR decreases
So receives a smaller proportion of the total resistance
Hence a smaller proportion of the total voltage
So voltage decreases

61
Q

If the light intensity increases explain how the voltage changes in another resistor in series with an LDR

A

As intensity increases the resistance of the LDR decreases
So receives a smaller proportion of the total resistance
Hence a smaller proportion of the total voltage
So the other resistor receives a larger proportion of the total resistance and hence total voltage
So voltage increases

62
Q

If the light intensity decreases, explain how the voltage changes in an LDR in series

A

As intensity decreases the resistance of the LDR increases
So receives a larger proportion of the total resistance
Hence a larger proportion of the total voltage
So voltage increases

63
Q

If the light intensity decreases explain how the voltage changes in another resistor in series with an LDR

A

As intensity decreases the resistance of the LDR increases
So receives a larger proportion of the total resistance
Hence a larger proportion of the total voltage
So the other resistor receives a smaller proportion of the total resistance and hence total voltage
So voltage decreases

64
Q

If the light intensity increases explain how the current changes in another resistor in series with an LDR

A

As the light intensity increases the LDR’s resistance decreases
So the total resistance decreases
From Ohm’s law if the total resistance decreases then the total current increases
Since I=V/R and V is constant
So the current increases

65
Q

If the light intensity decreases explain how the current changes in another resistor in series with an LDR

A

As the light intensity decreases the LDR’s resistance increases
So the total resistance increases
From Ohm’s law if the total resistance increases then the total current decreases
Since I=V/R and V is constant
So the current decreases

66
Q

If the light intensity increases explain how the current changes in an LDR in series

A

As the light intensity increases the LDR’s resistance decreases
So the total resistance decreases
From Ohm’s law if the total resistance decreases then the total current increases
Since I=V/R and V is constant
So the current increases

67
Q

If the light intensity decreases explain how the current changes in an LDR in series

A

As the light intensity decreases the LDR’s resistance increases
So the total resistance increases
From Ohm’s law if the total resistance increases then the total current decreases
Since I=V/R and V is constant
So the current decreases

68
Q

What is 1 Volt

A

The energy transformed per coulomb of charge between 2 points

69
Q

Explain why the resistance of an NTC thermistor decreases when its temperature increases

A

As temperature increases the weakly bound electrons going enough energy to become free
They can carry a current
Lattice ion vibrations still increase but the freeing of electrons is more significant

70
Q

What is resistivity

A

A measure of how much a particular material resists current flow
Measured in Ohm metres
And is a property of the material

The resistance of a 1m length wire with a cross sectional area of 1m^2

71
Q

Explain the resistivity for a material that conducts electricity

A

Low resistivity

72
Q

How does thickness effect resistance

A

Larger cross sectional area
Bigger lattice ion structure
Increases number of paths for electrons to travel in
Resistance decreases

73
Q

How does thickness effect resistivity

A

It doesn’t
Resistivity is constant and is a property of the material
It will instead affect resistance

74
Q

How can you stop a circuit overheating

A

Use a switch and turn off the circuit when not in use/between readings

75
Q

What is a superconductor

A

A material that conducts electricity with zero resistance below a critical temperature
Hence can carry a current without losing energy

76
Q

What is a superconductor

A

A material that conducts electricity with zero resistance below a critical temperature
Hence can carry a current without dissipating energy as heat

77
Q

Uses of superconductors

A

Maglev trains
Speed up connections between computer chips
Superconducting coils make possible the very powerful electromagnets in MRI machines

78
Q

Uses of superconductors

A

Power cables that transmit electricity without any power losses
Really strong electromagnets like those in maglev trains or MRI
Electronic circuits that work very fast with minimal energy losses since there is no resistance to slow the current down

79
Q

EMF

A

Electromotive force
The amount of energy per unit charge produced inside a cell
Or
The voltage across terminals of a cell when no current is flowing through it

80
Q

TPD

A

Terminal Potential Difference

The voltage measured across terminals of a cell when current is flowing through it

81
Q

What is internal resistance

A

The resistance of the cell itself

So it cannot be separated from it

82
Q

Why is TPD less than EMF

A

Some voltage from the cell is used up by the cells own internal resistance
This voltage is know as the lost volts

83
Q

TPD, EMF and lost volts equation

A

V=E-U

TPD = EMF - lost volts

84
Q

EMF, current and resistance equation

A

E=I(Rxr)

R=Resistance of circuit
r=Internal resistance

85
Q

EMF, current and resistance equation

A

E=I(R+r)

R=Resistance of circuit
r=Internal resistance

86
Q

Equation for the energy wasted due to total internal resistance

A

EMFI=I^2R+I^2r

EMFI=Energy per second supplied by the source
I^2R=Energy per second supplied to external components
I^2r=Energy per second used up by internal resistance/Dissipated as heat in the battery

87
Q

EMFI=I^2R+I^2r derivation

A

P=IV and EMF=IR+Ir

So to get power, you multiply everything by I

88
Q

Explain the ideal internal resistance for a car battery

A

Cars need to supply a large current to have a large power
But internal resistance limits current
So having a low internal resistance means it can provide a higher current
Also means less energy wasted on internal resistance
So batteries last longer between charges
A higher TPD would be possible since less p.d lost across internal resistance

89
Q

Explain the graph of a diode

A

Below the threshold voltage of 0.6V the resistance is infinite so current is zero
As the voltage increases beyond the threshold value, the weakly bound electrons in the semiconductor gain energy
So become free and able to conduct electricity
Lattice ion vibrations will still increase (working to increase resistance) but this has a lesser effect than the freeing of electrons

90
Q

Explain the graph of a filament bulb

A

Initially acts as an ohmic conductor
But as the current increases so does the temperature of the filament
Causing lattice ion vibrations to increase
Increasing the number of electron collisions per second with the lattice ion structure per second
The current will continue to increase but at a lesser rate

91
Q

Explain the graph of an ohmic conductor

A

As voltage increases by fixed amounts so does current
Resistance at each point calculated by dividing the voltage by the current
This value of resistance stays constant for the graph

92
Q

What is a semiconductor

A

A group of materials that aren’t as good as conducting electricity as metals due to having less electrons
If energy is supplied to them (e.g. increasing the temperature) more charge carries/electrons can be released from their weakly bound electron structure
And the resistivity of the material decreases
Hence make good sensors for detecting changes in their environment
Such ass thermistors, LDR’s and diodes

93
Q

Energy, emf and charge equation

A

emf=E/Q

Since emf is just a voltage

94
Q

4 different emf equations using the same variables

A

E=I(R+r)
E=V+U
V=E-u
V=E-Ir

95
Q

What does resistivity depend on

A

The structure of the material

Environmental factors such as temperature and light intensity

96
Q

What is a potential divider

A

A circuit with a voltage source and more than one resistor in series

97
Q

Useful equation for potential divider circuits

A

I=V/Rtotal

I=Vout/Rout

V/Rtotal = Vout/Rout

98
Q

Define the volt

A

1 joule per coulomb

99
Q

Advantage of using a potential divider to control current vs advantage for a variable resistor

A

Potential divider can provide sensitive control from zero to max

Variable resistor can provide a larger current but can’t get near 0

100
Q

Explain internal resistance

A

Comes from electrons colliding with atoms and losing their energy
In a battery chemical energy is used to make electrons move
As they move they collide with atoms in the battery
Meaning it must have resistance

A Level Physics Paper 1 (8 decks)

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