Mains Electricity and Power

GCSE Physics · Electricity

Mains supply

UK mains is 230 V, alternating current (a.c.), 50 Hz. Cells/batteries give direct current (d.c.).

The three-pin plug

Wire	Colour	Job
Live	Brown	Carries 230 V
Neutral	Blue	Completes the circuit (~0 V)
Earth	Green/Yellow	Safety — carries current away in a fault

The fuse (in the live wire) melts if the current is too high, breaking the circuit.

Power equations

P = V × I              (power = pd × current)
P = I² × R
E = P × t              (energy = power × time)
E = V × I × t

Worked example

A 230 V kettle drawing 10 A:

P = V × I = 230 × 10 = 2300 W = 2.3 kW

Run for 120 s: E = P × t = 2300 × 120 = 276 000 J.

Exam tip

Touching the live wire is dangerous because there's a large potential difference between it (230 V) and you (at earth, 0 V), so a current flows through you.