Electrical Power and Fuses — Electrical Power & Mains Electricity | IGCSE Physics

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 10% of your exam marks.

stable

Medium

Stable10%

P = IV and energy calculations, plus mains electricity safety, appear in most series.

Power is the rate at which energy is transferred, i.e. the amount of energy moved each second
Power is measured in watts (W); 1 watt is the same as 1 joule per second (1 W = 1 J/s)
For an electrical device, the power depends on two things:
- the voltage (potential difference) across the device
- the current flowing through the device

P = I × V

where:
- P = power transferred by the device (W)
- I = current through the device (A)
- V = voltage across the device (V)
Rearrangements:
- I = P / V (a high-power device at fixed voltage draws a large current)
- V = P / I

Example — a 150 W electric toaster runs from a 230 V mains supply. Calculate the current it draws.

A fuse is a safety component placed in the live wire of an appliance; it cuts the supply if the current grows dangerously large
A fuse looks like a small glass or ceramic cartridge with a fine metal wire running through it; if the current passes the rated value, the wire heats up and melts ("blows"), breaking the circuit
Once a fuse has blown it must be replaced, because it cannot reset itself
Without a fuse, an internal short circuit could deliver enough current to overheat the cable, set fire to nearby material, or electrify the appliance's metal casing

Common UK fuse ratings are 3 A, 5 A and 13 A
The right fuse for an appliance is the smallest one that is still above the appliance's normal operating current:
1. Use I = P / V to find the appliance's working current from its rated power
2. Pick the next standard fuse rating up from that figure
If the fuse is rated too low, it will blow during normal operation and the appliance will keep cutting out
If the fuse is rated too high, a fault current can pass right through it before the wire melts, leaving the appliance vulnerable to overheating

Example — a 1800 W electric kettle runs from a 230 V mains supply. Decide which standard fuse (3 A, 5 A or 13 A) is the right choice.

Normal current: I = P / V = 1800 / 230 = 7.83 A
Either a 3 A or a 5 A fuse would melt the instant the kettle is turned on, because the normal working current already exceeds their rating
A 13 A fuse is the next standard size above 7.83 A → choose the 13 A fuse