Electricity in the Home

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FreeScienceLessons · AQA PhysicsElectricity in the Home

Key facts

AC and DC

Direct current (DC) — current flows in one direction only. Source: batteries, cells.
Alternating current (AC) — current direction reverses regularly. UK mains: 230 V, 50 Hz.
Frequency = 50 Hz means current direction reverses 100 times per second (50 cycles).
Oscilloscope shows: DC = flat horizontal line. AC = sinusoidal wave.

Three-pin plug

Live wire (brown) — carries current at high voltage (230 V). Dangerous.
Neutral wire (blue) — completes the circuit at 0 V.
Earth wire (green/yellow) — safety wire. Connected to metal casing. If fault occurs, large current flows → fuse blows → circuit breaks.
Fuse — thin wire that melts if current is too high, breaking the circuit.
Circuit breaker — faster and reusable alternative to fuse.

Transformers and the National Grid

Transformers change voltage using electromagnetic induction. Only work with AC.
Step-up transformer — increases voltage, decreases current. Used at power station.
Step-down transformer — decreases voltage, increases current. Used at homes.
Transformer equation: Vp/Vs = np/ns (primary voltage/secondary voltage = primary turns/secondary turns).
Also: Vp × Ip = Vs × Is (power in = power out for ideal transformer).
High voltage transmission → low current → less energy wasted as heat (P = I²R).

Exam questions — 4 questions · 12 marks

1 markEarth wire purposeAQA 8463 P1 style

Why does an electric kettle with a metal casing need an earth wire?

A The earth wire provides a return path for the current in normal operation

B If the live wire touches the casing, the earth wire provides a low-resistance path for a large current, blowing the fuse and making the appliance safe

C The earth wire prevents static electricity building up in the kettle

D The earth wire increases the efficiency of the appliance

4 marksTransformer calculationAQA 8463 P1 style

A step-up transformer has 500 turns on the primary coil and 10,000 turns on the secondary coil. The primary voltage is 25 V. (a) Calculate the secondary voltage. (b) If the primary current is 8 A, calculate the secondary current (assume 100% efficiency). (4 marks)

Hint: (a) Vp/Vs = np/ns — cross multiply to find Vs. (b) Vp × Ip = Vs × Is — rearrange for Is.

+40 XP

1 markWhy high voltage transmissionAQA 8463 P1 style

The National Grid transmits electricity at very high voltages (e.g. 400,000 V). Why?

A High voltage stores more energy that can be released when needed

B High voltage allows electricity to travel faster between power stations and homes

C High voltage reduces current, which reduces power lost as heat in cables (since P = I²R)

D High voltage is safer for workers to handle at the power station

Electricity in the Home

AC and DC

Three-pin plug

Transformers and the National Grid

Module complete! 🎉