Practical electricity

O Level / IGCSE Physics · Electricity and magnetism. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Electrical power is the energy transferred each second. It depends on both the current and the p.d. Electricity companies sell energy in kilowatt-hours: one kW h is the energy used by a 1 kW appliance in 1 hour.

A mains cable has a live wire, a neutral wire and an earth wire. The live wire is the dangerous one, so the switch, the fuse and the circuit breaker all go in the live wire. Then switching off, or a blown fuse, cuts the appliance off from the high voltage.

If a live wire touches an earthed metal case, a large current flows to earth through the low-resistance earth wire. This melts the fuse, so the case cannot give a shock.

Rules to remember

power = current × voltage (P = IV), in watts
energy = current × voltage × time (E = IVt), in joules with the time in seconds
energy in kW h = power in kW × time in hours; cost = energy in kW h × price of one kW h
1 kW h = 1000 W × 3600 s = 3.6 × 10⁶ J
Fuse rating: the next value above the normal working current (I = P/V)
Switches, fuses and circuit breakers go in the live wire. A metal case must be earthed; a plastic (double-insulated) case needs no earth wire
Lamps in parallel each get the full supply p.d., can be switched separately, and stay on if another lamp fails

Common mistake

Working out kilowatt-hours with the power in watts or the time in minutes. Change the power to kW (÷ 1000) and the time to hours (minutes ÷ 60) before you multiply.

Worked example

A heater takes a current of 4.0 A from a 230 V supply. Electricity costs Rs 45 per kW h. What does it cost to run the heater for 5.0 hours?

power = current × voltage = 4.0 × 230 = 920 W = 0.92 kW.
energy = power × time = 0.92 × 5.0 = 4.6 kW h.
cost = 4.6 × 45 = Rs 207.

Answer: Rs 207

Practice questions

Try each one, then open the answer.

1. A kettle draws a current of 10 A from a 230 V supply. What is its power?

A
23 W
B
230 W
C
2300 W
D
23 000 W

Show answer

Answer: C. P = IV = 10 × 230 = 2300 W (2.3 kW).

2. An appliance rated at 1150 W is used on a 230 V supply. Which fuse rating is most suitable?

A
1 A
B
3 A
C
5 A
D
13 A

Show answer

Answer: C. Normal current = P ÷ V = 1150 ÷ 230 = 5 A. The fuse should be just above the normal current; here 5 A fits (a 13 A fuse would allow a dangerous fault current).

3. A hairdryer has a plastic case, and its cable has only a live and a neutral wire. Why is it safe without an earth wire?

A
its case is an insulator, so it cannot become live
B
its fuse protects the user, so no earth is needed
C
it runs on a.c., which cannot give anyone a shock
D
its neutral wire does the job of an earth wire

Show answer

Answer: A. The hairdryer is double insulated: its outer case does not conduct, so even if a live wire came loose inside, touching the case could not give a shock. An earth wire is needed only for a metal case.

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