IGCSE Physics / Mains electricity
Electrical power and choosing a fuse
Edexcel International GCSE Physics (4PH1), chapter 6: Mains electricity, specification point 2.4. Free lesson.
Read the revision notes · IGCSE Physics past papers
Electrical power is the energy transferred each second, and it can be calculated using P = I × V. This lets you find an appliance’s normal current and choose a fuse that protects its wiring.
What power and the equation mean
Power is the energy transferred per second. One watt is one joule per second, so 1 W = 1 J/s. The equation P = I × V is on the equation list, but you need to know what each symbol represents. P is power, measured in watts (W). I is current, measured in amps (A). V is voltage, measured in volts (V). Appliance ratings may be in kilowatts. Convert to watts before substituting: 1 kW = 1000 W.
How to calculate current
To find current, put the power and voltage into P = I × V, then rearrange to make I the subject. Check that power is in watts first, because using a value in kilowatts gives the wrong answer.
How to choose the right fuse
A fuse should be rated just above the normal current of the appliance. If its rating is too low, it melts during normal use and the appliance stops working. If it is too high, a fault may not melt it soon enough, so the cable can overheat and create a fire risk. Plug fuses usually come in ratings of 3 A, 5 A and 13 A. A kettle draws about 10 A. A 3 A or 5 A fuse melts when it is switched on, so choose 13 A. A small lamp draws about 0.3 A. A 13 A fuse would hardly ever melt. If the lamp develops a fault, its thin cable could overheat before the fuse melts. A fuse that is too small is a…
Use the calculation to select a fuse
Once you have found the appliance’s normal current, choose the smallest available fuse rating that is above it. This protects the appliance during normal use while allowing the fuse to melt if the current becomes too high.