Electricity

📌 Key points

1. Like charges repel and unlike charges attract. Rubbing an insulator transfers electrons: the object that gains electrons becomes negative.
2. Current is the rate of flow of charge: I = Qt. Measure it with an ammeter in series. Conventional current flows from + to −; electrons flow the other way.
3. Electromotive force (e.m.f.) is the energy supplied by a source per unit charge; potential difference (p.d.) is the energy transferred per unit charge across a component. Measure them with a voltmeter in parallel.
4. Resistance R = VI, in ohms (Ω). A longer wire has more resistance; a thicker wire has less (R is proportional to length and inversely proportional to cross-sectional area).
5. A fixed resistor at constant temperature has a straight-line I–V graph through the origin. A filament lamp’s graph curves because its resistance increases as it gets hotter.
6. Series: the current is the same everywhere, the p.d.s add up to the e.m.f., and total R = R₁ + R₂.
7. Parallel: each branch has the same p.d., the branch currents add up to the total, and the combined resistance is less than either resistor: 1R = 1R₁ + 1R₂.
8. Power P = IV and energy E = IVt. The kilowatt-hour (kWh) is the energy used by a 1 kW appliance in 1 hour.
9. A light-dependent resistor’s resistance decreases as light intensity increases; a thermistor’s resistance decreases as temperature increases.
10. Safety: a fuse or circuit breaker in the live wire cuts off the current if it becomes too large; the earth wire connected to a metal case stops it from becoming live.

📖 Key terms

Current

The rate of flow of charge; measured in amperes (A).

Electromotive force

The energy supplied by a source in driving unit charge around a complete circuit.

Potential difference

The work done (energy transferred) per unit charge passing through a component.

Resistance

The p.d. across a component ÷ the current through it; measured in ohms (Ω).

Fuse

A thin wire that melts and breaks the circuit if the current is too high.

Thermistor

A resistor whose resistance decreases as temperature increases.

Light-dependent resistor

A resistor whose resistance decreases as light intensity increases.

Kilowatt-hour

The energy transferred by a 1 kW appliance in 1 hour.