Electrical power and mains electricity
Electrical power equations
Power is the rate at which energy is transferred:
P = I × V (power = current × potential difference)
Combining with V = IR gives two more useful forms:
P = I² × R and P = V² ÷ R
- Units: W (watt) = J/s.
- A 60 W lamp transfers 60 J of energy per second.
Energy transferred
E = P × t (joules = watts × seconds)
For domestic billing, energy is measured in kilowatt-hours (kWh): 1 kWh = energy used by a 1 kW appliance for 1 hour = 3.6 × 10⁶ J.
✦Worked example
A kettle uses a current of 8 A on the 230 V mains supply. It boils water in 4 minutes (240 s). Calculate (a) the power and (b) the energy transferred.
(a) P = IV = 8 × 230 = 1840 W (1.84 kW). (b) E = Pt = 1840 × 240 = 441 600 J (or 0.123 kWh).
UK mains electricity
The UK mains is alternating current (a.c.):
- Voltage: 230 V rms.
- Frequency: 50 Hz — direction reverses 50 times per second.
- Compare with direct current (d.c.) from a battery — flows one way only.
Three-pin plug wiring
A UK plug has three colour-coded wires:
| Wire | Colour | Purpose |
|---|---|---|
| Live | brown | Carries the alternating potential difference (~230 V); the dangerous wire. |
| Neutral | blue | Completes the circuit, near 0 V relative to earth. |
| Earth | green-and-yellow stripes | Safety only — connects the metal case to the ground. |
The fuse is in the live wire just inside the plug; the cable is held by a strain-relief grip.
Earth wire and fuse — how they protect you
If a fault makes the live wire touch the metal casing of an appliance:
- A large current flows from the live wire through the earth wire (low-resistance path) to ground.
- This large current melts the fuse (or trips the circuit breaker), breaking the live circuit.
- The casing is no longer at a dangerous voltage; you cannot get a shock.
A fuse must be rated just above the appliance’s normal current. Common values: 3 A, 5 A, 13 A. A fuse rated too high won’t blow in time; one rated too low blows during normal use.
Double-insulated appliances (with the square-in-square symbol) have a plastic case that cannot become live, so they don’t need an earth wire — only live and neutral.
Edexcel exam tip
When asked "explain how a fuse and earth wire protect a user", always use the sequence: fault makes case live → current flows through earth → large current melts fuse → live circuit broken → case safe. Marks are awarded for the chain, not just naming the parts.
AI-generated · claude-opus-4-7 · v3-edexcel-combined-science-leaves