Mains Electricity and Power

GCSE Physics · Electricity

Mains electricity

In the UK, mains electricity is supplied at about 230 V and a frequency of 50 Hz. It is alternating current (a.c.) — the current constantly changes direction. Cells and batteries provide direct current (d.c.) — current flows in one direction only.

The three-pin plug

A UK plug has three wires, each with a job and a colour code:

Wire	Colour	Job
Live	Brown	Carries the alternating p.d. from the supply (~230 V) — dangerous
Neutral	Blue	Completes the circuit (near 0 V)
Earth	Green & yellow	Safety wire — carries current away if a fault makes the case live

The earth wire and a fuse protect the user: if a fault connects the live wire to a metal case, a large current flows to earth, the fuse melts (blows) and disconnects the appliance, preventing electric shock.
The live wire is dangerous even when a device is switched off, because it is still at ~230 V relative to earth.

Power in appliances

Power is the energy transferred per second. Useful equations:

power = potential difference × current   (P = V × I)
power = current² × resistance            (P = I² × R)
energy transferred = power × time        (E = P × t)
energy transferred = charge × p.d.       (E = Q × V)

The power rating of an appliance (in watts) tells you how much energy it transfers per second.

National grid

The National Grid distributes electricity across the country. Transformers are used to change the voltage:

Step-up transformers raise the voltage (to very high values) for transmission — high voltage means lower current, which reduces energy wasted as heat in the cables (more efficient).
Step-down transformers lower the voltage again to a safe level for homes.

Worked example

A 230 V hairdryer draws a current of 5 A. What is its power?

P = V × I = 230 × 5 = 1150 W (1.15 kW)

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Common mistakes

Mixing up wire colours — live = brown, neutral = blue, earth = green/yellow.
Saying a.c. flows one way — a.c. alternates; d.c. is one direction.
Forgetting the grid uses high voltage/low current to reduce heat losses.

Exam tips

Learn the plug wire colours and jobs, and how the earth wire + fuse keep you safe.
Know both power equations (P = VI and P = I²R).
Explain the National Grid using step-up (transmission) and step-down (homes) transformers.

Key facts to remember

UK mains = ~230 V, 50 Hz, a.c.; cells/batteries = d.c.
Plug: live (brown, ~230 V), neutral (blue), earth (green/yellow, safety); fuse + earth protect against faults.
P = V × I (also P = I²R); the National Grid uses step-up transformers for efficient high-voltage transmission and step-down for homes.

Mains Electricity and Power