Electricity

9. Electricity

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Lesson summary

9. Electricity — Cambridge AS & A Level Physics 9702 — CAMBRIDGE AS & A LEVEL PHYSICS 9702 — COMPLETE NOTES notes: Use current, charge, potential difference, power, resistance, Ohm's law, I–V characteristics, resistivity and sensor resistors to analyse electrical circuits.Electri…

Electric current
Conventional current
Quantisation of charge
Charge carriers
Drift speed
Carrier sign

Part 1 of 9Electric current

1Electric current1 point

= the flow of charge carriers, measured in amperes (A).

Charge carriers can be positive or negative.

2Conventional current1 point

= the flow of positive charge from the positive terminal to the negative terminal.

It is opposite to the electron flow in a metal, because it was defined before electrons were discovered.

3Ammeter

— it measures current and is always connected in series.

4Quantisation of charge

— charge occurs in multiples of the elementary charge e = 1.60 × 10⁻¹⁹ C (electron −e, proton +e).

Part 2 of 9Charge and current

Equation — current is the charge passing per unit time, I = Q/t, so Q = It, with charge in coulombs.
Sign — conventional current is the flow of positive charge, so current is given as a positive value.

Part 3 of 9Current in a conductor

Charge carriers — in a metal they are free electrons, and drift speed is their average speed along the wire.
Drift speed — it is very slow, about a millimetre to a centimetre per second, yet current appears instantaneously because the number density of carriers is huge.
Equation — I = Anvq.
- A is the cross-sectional area (m²) and n the number density of carriers (m⁻³).
- v is the average drift speed (m s⁻¹) and q the charge of each carrier (C).
Carrier sign — the same equation is used for positive or negative carriers, which have the same effect, so the positive-charge model works.

Part 4 of 9Potential difference and power

Potential difference (p.d.) = the energy transferred per unit charge as it moves from one point to another, V = W/Q.
- A cell makes one end of a circuit positive and the other negative, setting up a p.d.
Unit — the volt, 1 V = 1 J C⁻¹, so a 3 V bulb receives 3 J from each coulomb.
Voltmeter — it measures p.d. and is always connected in parallel across the component.
Series circuit — the supply p.d. is shared between the components.
Parallel circuit — each branch has the supply p.d.
Brightness — with the same current, a 240 V lamp transfers 20 times as much energy per coulomb as a 12 V lamp.
Power = the rate of doing work, P = IV.
What the symbols mean: P = power (W) · I = current (A) · V = potential difference (V)
- Substituting V = IR gives P = I²R, and substituting I = V/R gives P = V²/R.
- For a fixed resistance, doubling the current or p.d. makes the power four times greater.

Part 5 of 9Resistance and Ohm's law

1Resistance2 points

= the opposition to current, R = V/I, measured in ohms (1 Ω = 1 V A⁻¹).

For a given p.d., a higher resistance gives a smaller current.
All components, including wires, have some resistance.

2Ohm's law

= for a conductor at constant temperature, the current is proportional to the p.d. across it.

3Test for ohmic behaviour

— the I–V graph is a straight line through the origin, and the resistance is 1/gradient.

4Measuring resistance3 points

— use a low supply voltage (1 to 2 V) to avoid heating.

Connect the component with an ammeter in series and a voltmeter in parallel.
Use a variable resistor to change the current and record pairs of V and I.
Plot I against V and take R = 1/gradient.

Part 6 of 9I–V characteristics

Component	Graph	Reason
Metal conductor at constant temperature (resistor)	Straight line through the origin	Resistance is constant
Filament lamp	S-shaped curve that flattens at high V	The filament heats, so resistance increases and current rises more slowly
Semiconductor diode	Almost zero current in reverse bias; sharp rise above about 0.7 V in forward bias	It conducts in one direction only

Straight lineconstant resistance; a curve means resistance is changing.
Resistance at a pointit is V/I at that point, not the gradient of a curve.
Forward biascurrent in the direction of the arrowhead symbol.
Reverse biasthe diode does not conduct, so current is zero.

Part 7 of 9Resistance and temperature

Heating mechanismfree electrons collide with vibrating metal ions and transfer kinetic energy to them, so the wire heats.
Temperature effecta higher temperature gives larger vibrations and more collisions, so more resistance and less current.
Filament lampthis is why its resistance rises as it gets hotter.

Part 8 of 9Resistivity

Resistance of a wire — R = ρL/A, where ρ is the resistivity (Ω m), L the length and A the cross-sectional area.
- A longer wire has more resistance and a thicker wire has less.
- For a circular wire A = πd²/4, so doubling the diameter quarters the resistance.
Resistivity — a property of the material that depends only on temperature, and a higher value means more resistance.

Material	Resistivity / Ω m
Copper	1.7 × 10⁻⁸
Gold	2.4 × 10⁻⁸
Aluminium	2.6 × 10⁻⁸
Germanium	0.6
Silicon	2.3 × 10³
Glass	10¹²
Sulfur	10¹⁵

Copper — it is used in wires because of its low resistivity.
Insulators — their resistivity is so high that virtually no current flows.

Part 9 of 9Sensor resistors

1Light-dependent resistor (LDR)3 points

— a non-ohmic sensor whose resistance falls as light intensity rises.

In the dark it has millions of ohms and in bright light only tens of ohms.
It switches on street lights and garden lights at dusk.
At constant light intensity its I–V graph is a straight line.

2Thermistor (NTC)2 points

— a non-ohmic sensor whose resistance falls as temperature rises and rises as it cools.

It is used in ovens, fire alarms and digital thermometers.
In series with a fixed resistor, cooling raises its resistance, lowers the current and increases its share of the p.d.

Last updated: 13 October 2026