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A-level Physics > Electricity > Flashcards

Electricity Flashcards

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1
Q

How do you calculate resistivity (formula)?

A

ρ = RA / l

A = cross-sectional area of wire

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2
Q

Formula for Charge, Time and Current?

A

I = ΔQ / Δt

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3
Q

Define potential difference.

A

The difference between the potential electrical energy of the electrons before and after travelling through a component (such as a bulb) per unit of charge is the potential difference.

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4
Q

Formula for Potential difference, Current, Power.

A

P = IV

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5
Q

How does current change in a series circuit?

A

It stays the same.

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6
Q

How does current change in a parallel circuit?

A

Current is split across the different pathways. Where the pathways meet the current is summed.

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7
Q

Why can metals create a current in a circuit?

A

Most electrons are attached to atoms BUT some are not.
These non-attached electrons are free to flow throughout the metal.
A potential difference will attract electrons to the positive terminal of the metal and create a current.

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8
Q

Why can’t insulators carry a current in a circuit?

A

All electrons are attched to atoms and so are not free to move.
This means applying a potential difference will not create a current.

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9
Q

How does potential difference change across a series circuit (3 light bulbs)?

A

The potential difference will decrease between each light bulb.

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10
Q

How does potential difference change across a parallel circuit (3 light bulbs)?

A

The voltage will be the same across the light bulbs.

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11
Q

Define resistance.

A

A material’s opposition to the flow of electric current: measured in ohms.

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12
Q

What is ohms law?

A

Ohms law states that I is proportional to V which is inversely proportional to resistance.

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13
Q

When does Ohms Law hold true?

A

Under constant physical conditions (such as temperature).

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14
Q

Why does increasing potential difference make a bulb shine brighter?

A

Higher pd means the energy the electrons transfer to the bulb is greater and therefore the bulb shines brighter.

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15
Q

State the relationship between current and potential difference in a circuit.

A

Pd and current are proportional to each other as long as temperature is constant.
As pd increases, so does current and vice versa.

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16
Q

What is an NTC thermistor?

A

Negative temperature coefficient.

As temperature increases, resistance decreases.

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17
Q

What can cause the temperature to change in a circuit?

A

As current increases, temperature also increases.

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18
Q

Describe the potential difference-current graph for a filament lamp.

A

A proportional relationship between V and I for lower values of I (straight).
For higher value of I the graph begins to lose proportionality and pd will begin to increase ‘exponentially’ (curve towards pd axis).
This is due to increasing temperatures causing increased resistance.

19
Q

Describe the potential difference-current graph for a resistor at a constant temperature.

A

A graph where pd and current are always proportional to one another (straight line).

20
Q

Describe the potential difference-current graph for a diode.

A

Pd increases as current stays at 0.
When a high enough pd is reached current will also increase.
Graph is only in positive direction.

21
Q

Why does a diode have a forward bias?

A

Because the resistance in the opposing direction is very high and a certain pd is required to conduct.

22
Q

What is the forward bias of a silicon diode?

A

0.7 Volts

23
Q

What is a metallic conductor?

A

Resitance increases SLOWLY with temperature (PTC)
This is due to positive ions vibrating more with increasing temperature/energy. So electrons can’t pass through easily when pd is applied.

24
Q

What is a semiconductor?

A

Resistance decreases QUICKLY with temperature (NTC).

The number of electrons increases far quicker than the impedence caused by vibrating ions.

25
Q

What are thermistors used for?

A

Generally used in sensor circuits to set off alarms or to turn devices on or off at certain temperatures.
Used for fans in cooling units of electrical circuits.

26
Q

What is resistivity?

A

A measure of the resisting power of a specified material to the flow of an electric current.

27
Q

What factors affect resistance?

A

Length of a wire.
Cross-sectional area of wire.
The material (resistivity)

28
Q

What is resistivity measured in?

A

Ωm

29
Q

What things tend to have low resistivity?

A

Metals and other good conductors.

30
Q

How do superconductors work?

A

Superconductive materials can be cooled below a transition temperature below which the resistivity is low enough to be negligible. Therefore, no energy is lost as heat energy.

31
Q

What is a use of superconductors?

A

Power transmission in overhead cables creates heating in the cables due to resistance and ∴ a loss of energy.
Superconductive cables don’t have this effect as they reduce resistance and therefore the heating effect.

32
Q

Why does increased current create increased temperature?

A

There are more electrons flowing throughout the circuit.

This means more collisions between electrons and positive ions and more energy lost as heat energy.

33
Q

How do you calculate total resistance across a series circuit?

A

Rₜ = R₁ + R₂ + R₃…

34
Q

How do you calculate total resistance across a parallel circuit?

A

1/Rₜ = 1/R₁ + 1/R₂ + 1/R₃…

35
Q

What is the EMF of a cell?

A

The quantity of chemical energy transferred to electrical energy per coulomb of charge.

36
Q

What is internal resistance?

A

The opposition to the flow of electric current in the source/cell.

37
Q

How do you calculate the EMF of a cell?

A 
ɛ = I(R + r)
EMF = terminal pd + lost pd
38
Q

How does total EMF change with more cells (series)?

A

Total EMF = sum of individual EMFs

39
Q

How does total EMF change with more identical cells (parallel)?

A

Stays the same

40
Q

How is lost pd calculated with multiple identical cells ‘n’ (parallel)?

A

Lost pd = Ir/n

41
Q

What is a potential divider?

A

A circuit that uses resistors to split a source, emf and ∴ supply a variable pd.

42
Q

Why might two lamps have different power ratings but same light intensity output?

A

Lamps energy is wasted as heat energy.

Higher power = more wasted energy.

43
Q

Why does increasing current decrease voltage from a cell?

A

Terminal pd = ε - lost pd (from internal resistance)

Greater current, greater proportion of emf across internal resistance ∴ more lost pd.

44
Q

Why is the voltage lower than ε?

A

Current flows, some energy is transferred into heat energy due to the internal resistance of the cell ∴ some pd is lost.

A-level Physics (14 decks)

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