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A-level Physics 2021-22 > 5. Electricity > Flashcards

5. Electricity Flashcards

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1
Q

What is electric current?

A

Net flow of charged particles

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2
Q

What is the metal wire made up of?

A

A lattice of positive ions, surrounded by ‘free electrons’

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3
Q

How do ions move in a metal wire?

A

They vibrate about fixed positions

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4
Q

How do electrons move in a wire?

A

They are free to move from one ion to another

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5
Q

What happens to electrons when a battery is connected to a wire?

A

Free electrons are repelled by the negative terminal and attracted to the positive

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6
Q

What is drift velocity?

A

When a wire is connected to a battery and electrons move randomly, but in the same direction

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7
Q

At what point is there a current in a wire?

A

When electrons move in the same direction (electrons carry charge, current = flow of charge)

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8
Q

How does electric current flow through liquids?

A
  • electrolyte (e.g. salt solution)

* when power supply connected, +ve ions move to -ve terminal and vice versa

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9
Q

In the equation ▲Q / ▲T, what is the quantity being calculated?

A

Rate of flow of charge

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10
Q

How to calculate rate of flow of charge?

A

▲Q / ▲T

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11
Q

How is the coulomb defined?

A

The quantity of electric charge that passes a point in a circuit in 1s when a current of 1A is present

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12
Q

What direction does the current go in?

A

In the direction of positive charge

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13
Q

How is the ohm defined in words?

A

A conductor has resistance of 1Ω if a current of 1A flows across when a p.d. of 1V is applied

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14
Q

How is the ohm defined in an equation?

A

1Ω = 1V / 1A

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15
Q

What is Ohm’s Law?

A

Current through a wire is proportional to the p.d. across it (if temperature is constant)

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16
Q

What are Ohmic conductors?

A

Materials that obey Ohm’s law

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17
Q

What are the charge carriers in metals?

A

Conduction electrons

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18
Q

What are the charge carriers in a salt solution?

A

Ions

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19
Q

What is the convention for the direction of a current in a circuit?

A

Positive to negative

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20
Q

What can materials be classified as in electrical terms?

A

Conductors, insulators or semiconductors

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21
Q

How is potential difference defined?

A

As the work done (or energy transfer) per unit charge

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22
Q

What is resistance caused by?

A

The repeated collisions between the charge carriers in the material with each other and with the fixed ions of the material

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23
Q

How is the resistance of any component defined?

A

The p.d. across the component / the current through it

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24
Q

How is an Ohmic conductor represented on an I-V graph?

A

A straight line through the origin

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25
Q

How will an I-V graph show that a component is obeying Ohm’s Law?

A

Straight line through the origin

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26
Q

Why does a straight line through the origin on an I-V show that Ohm’s Law is being obeyed?

A

Gradient is constant so resistance is constant - for both directions of current flow

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27
Q

What is the I-V graph for a filament lamp?

A

A curve

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28
Q

What does the curved graph for a filament lamp mean?

A

Resistance increases as current increases

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29
Q

For a filament lamp, why does resistance increase as current increases?

A

The current has a heating effect

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30
Q

What is the I-V graph for a semiconductor diode?

A

Reverse bias for a negative p.d. and forward biased p.d. of 0.6V needed before diode conducts in forward direction

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31
Q

For which component’s graph showing I-V characteristics can the gradient be used for resistance for all values?

A

For ohmic conductors - for filament lamps and diodes, read values off the graph for R at a certain point

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32
Q

What is resistance proportional to, in terms of length and area?

A

Resistance of a uniform conductor is ∞ to length and 1/∞ to its cross sectional area

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33
Q

Equation for resistance? (using resistivity)

A

R = ρl / A

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34
Q

Equation for resistivity?

A

ρ = RA / l

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35
Q

Unit for resistivity?

A

Ωm

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36
Q

Units for length, area and resistivity for the equation?

A

length = m

cross-sectional area = m²

resistivity = Ωm

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37
Q

How can resistivity be defined?

A

As numerically equal to the resistance of a sample of the material of unit length and unit cross-sectional area at a particular temperature

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38
Q

Typical value of resistivity of copper at 20°C?

A

Good conductor - 1.7x10^-8 Ωm

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39
Q

Typical value of resistivity of nichrome at 20°C?

A

Conductor - 1.1x10^-6 Ωm

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40
Q

Typical value of resistivity of silicon at 20°C?

A

Semiconductor - 2.3x10^3 Ωm

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41
Q

Typical value of resistivity of glass at 20°C?

A

Insulator - 1.0x10^12 Ωm

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42
Q

A wire of uniform cross-section has a resistance of RΩ. What would be the resistance of a similar wire, made of the same material, but twice as long and twice the diameter?

A

1/2 RΩ

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43
Q

A wire of uniform cross-section has a resistance of RΩ. It is drawn to three times the length, but the volume remains constant. What will be its resistance?

A

9 RΩ

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44
Q

In general, how are resistivity and temperature of metals linked?

A

As temperature ↑, resistivity ↑

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45
Q

What happens, in metals at high temperatures, to resistance as temperature increases?

A

Resistance increases linearly with temperature

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46
Q

What does increased temperature mean for particles within a wire?

A

Ions vibrate faster, with greater amplitude, so it’s more difficult for the electrons to pass through the lattice (resistance ↑)

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47
Q

When can a metal become a superconductor?

A

Below its critical temperature

48
Q

What does it mean if a metal becomes a superconductor?

A

It loses all of its resistance

49
Q

Why do superconductor wires not become hot?

A

As electrons can flow through them without any transfer of energy

50
Q

What are the particles like in insulators at room temperature?

A

There are few free electrons available for conduction

51
Q

What are the particles like in insulators at high temperatures?

A

Some electrons have enough energy to ‘escape’ from their atoms and the insulator is able to conduct

52
Q

What happens to resistance in insulators as temperature increases?

A

Resistance decreases

53
Q

What is one of the best known semiconductor materials?

A

Silicon

54
Q

How are semiconductors at low temperatures?

A

Poor conductors

55
Q

What happens in semiconductors as temperature rises?

A

Electrons break free from their atoms and so it becomes a better conductor

56
Q

What happens when silicon reaches 150°C?

A

Breakdown occurs and it becomes permanently damaged

57
Q

When might a semiconductor material become permanently damaged?

A

When it reaches a certain temperature (e.g. when silicon reaches 150°C)

58
Q

What is the relationship between temperature and resistance for semiconducting materials?

A

As temperature ↑, resistance ↓ (for many)

59
Q

For materials where temperature ↑ as resistance ↓, what is a term that can be used to describe them?

A

They have a negative temperature coefficient of resistance (NTC)

60
Q

What is an NTC?

A

A negative temperature coefficient of resistance

61
Q

What are semiconductor materials used to make?

A

Thermistors

62
Q

Is there a p.d. when a material is a superconductor?

A

No - as a current flows but resistance = 0

63
Q

When will a superconductor lose its superconductivity?

A

When it raises above its critical temperature

64
Q

What is electrical power?

A

The rate at which electrical energy is converted to other forms of energy

65
Q

What is the unit of power?

A

The watt

66
Q

What is 1J per second equivalent to?

A

1 W

67
Q

Equation for power?

A

P = W/t (P=ItV/t)

68
Q

Equation for energy in terms of charge?

A

W=QV (W=ItV)

69
Q

What happens in terms of energy transfers when current flows through a resistor?

A

Electrical energy is transferred to heat

70
Q

What equation is given when P=IV and V=IR are combined?

A

P=I²R

71
Q

In which direction is the conventional current?

A

The flow of positive charge

72
Q

In liquids, gases and semiconductors, which particles move in the direction of the conventional current?

A

Positive ions

73
Q

In liquids, gases and semiconductors, which way do charge carriers flow?

A

In the opposite direction to conventional charge

74
Q

In metals, what is the only type of charge that flows?

A

Negative - as the only charge carrier is the electrons

75
Q

How does the current behave in a series circuit?

A

It is the same at all points

76
Q

How does the current behave in a parallel circuit?

A

Current leaving and returning to the supply is the sum of the currents in the separate branches

77
Q

What is Kirchoff’s first law?

A

The sum of the currents flowing into any junction in a circuit is equal to the sum of currents flowing out of that junction i.e. charge is conserved

78
Q

What is Kirchoff’s second law?

A

Around any closed loop in a circuit, the sum of the emfs is equal to the sum of the p.d.s

79
Q

Explanation of Kirchoff’s second law?

A
  • we know that a coulomb gains electrical energy as it moves through each emf and loses electrical energy as it moves through each p.d.
  • after one loop of the circuit, the energy it has gained must be equal to the energy it has dissipated
80
Q

What does emf stand for?

A

Electromotive force

81
Q

If two identical lamps are connected in series, what is their brightness?

A

They are equally bright, but not as bright as if they were connected to the battery on its own

e.g. total p.d. across both = 6V, and shared between 2 = 3V each

82
Q

In a series circuit, what is the total p.d. equal to?

A

total p.d. across all components = sum of p.d.s across the separate components

83
Q

When two identical lamps are connected in parallel, what happens to their brightness?

A

Each lamp is as bright as if it were connected to the battery on its own

84
Q

What happens to p.d. in a parallel circuit?

A

It is the same across each branch

85
Q

What model of energy transfer is used to explain why p.d. is equal across all branches in parallel?

A
  • each coulomb transports all 6J of energy to the lamp
  • each lamp receives same energy as if it were connected to battery on its own
  • twice as many coulombs pass per second through the battery - so battery runs out more quickly
86
Q

How is electromotive force defined?

A

The energy (chemical, mechanical, thermal etc,) converted into electrical energy when unit charge (ie 1C) passes through it

87
Q

What is emf and what is p.d. in any closed loop?

A

emf - supply of electrical energy

p.d. - conversion of electrical energy to other forms

88
Q

For cells in series, how is total emf of their combination calculated?

A

By adding their individual emfs

89
Q

For identical cells in parallel, how is total emf of their combination calculated?

A

Total emf is the same size as each of the cells individually

90
Q

Why, for identical cells in parallel, is the total emf the same size as each of the cells individually?

A

Each charge only passes through one cell so gains energy from the single cell

91
Q

Why, for a combination of cells in series, is the total emf calculated by adding the individual emfs?

A

Each charge passes through each cell so gains energy from all three

92
Q

When resistors are in series, what is the current going through them and p.d. across them?

A

Current through each resistor is the same, and total p.d. across resistors is the sum of the p.ds across the separate resistors ie. V=V1+V2+V3

93
Q

How is the combined resistance in a series circuit calculated?

A

R=R1+R2+R3

94
Q

When resistors are in parallel, what is the current going through them and p.d. across them?

A

Current in main circuit is the sum of the currents in each parallel branch, and p.d. across each resistor is the same

95
Q

How is the combined resistance in a parallel circuit calculated?

A

1/R= 1/R1 + 1/R2 + 1/R3

96
Q

In a simple potential divider (two resistors in series), what happens to p.d. when resistors are identical?

A

Resistors share voltage equally

97
Q

In a simple potential divider (two resistors in series), one resistor has a value of 200Ω and the other 100Ω, and the emf is 6V. What is the p.d. through each resistor?

A

200Ω = 4V

100Ω = 2V

98
Q

Equation to calculate output and input voltage in a potential divider?

A

Vₒᵤₜ = Vᵢₙ (R₁ / R₁+R₂)

99
Q

What components can be used as sensors in electronic circuits?

A

Thermistors and light dependant resistors

100
Q

As a thermistor cools, what happens to its resistance?

A

Resistance rises

101
Q

As an LDR receives less light, what happens to its resistance?

A

Resistance rises

102
Q

In a circuit diagram, what is a loop around a cell and a resistor showing?

A

They are one component; the resistor is the cells internal resistance

103
Q

What is emf equal to?

A

Terminal p.d. + lost volts

104
Q

What two equations are used to form the equation emf = I (R +r)?

A

emf = IR and emf = Ir

105
Q

What happens if a driver starts a car with the head-lamps on?

A

The current through the battery is so large that the ‘lost volts’ are high - even though the battery’s internal resistance is low. Terminal p.d. drops and headlights dim.

106
Q

What is emf equal to when there is no current in a cell?

A

The terminal p.d.

107
Q

What is terminal p.d. ?

A

The p.d. across the terminals of the cell

108
Q

What happens to p.d. as soon as current is drawn from the cell?

A

P.d. drops

109
Q

Why does p.d. drop as soon as current is drawn from the cell?

A

The cell has internal resistance

110
Q

What is an open circuit?

A

A circuit that is off, with no current

111
Q

What is the p.d. defined as?

A

The amount of electrical energy changed to other forms of energy per coulomb of charge flowing between them

112
Q

How is p.d. calculated with energy and charge?

A

V = W / Q

113
Q

How can p.d. be defined using an equation?

A

p.d. = energy used by the component / charge passing

114
Q

What does W equal in terms of I, t and V?

A

W = ItV

115
Q

What is one volt?

A

The p.d. between 2 points in a circuit in which 1J of energy in converted to other forms when 1C of charge passes between them

116
Q

How is the unit of 1V defined?

A

1J / 1C

117
Q

Why is a loop drawn around E and r (battery and resistor)?

A

To show that they are one component

A-level Physics 2021-22 (11 decks)

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