Practical Electricity: Complete O Level Physics Cheatsheet
O Level Physics, Chapter 18 · Read time: ~8 minutes
These notes come from TGC Academy's O Level Physics tuition in Singapore.
Key Formulas
Power and energy
Electrical power is the rate at which electrical energy is converted into other forms: P = VI. Combining with V = IR gives two more versions, P = I²R and P = V²/R. Pick whichever uses the quantities the question gives you.
Electrical energy converted is E = Pt = VIt. With power in watts and time in seconds, the energy is in joules.
Heating appliances (kettles, irons, water heaters) use a high-resistance heating element that converts electrical energy into thermal energy. Their power ratings are usually the highest in a home.
Trap: Before using P = I²R, check the current really flows through that resistance. For appliances on the mains, the voltage is fixed at 230 V, so P = V²/R shows that a *smaller* resistance gives a *larger* power, the opposite of what many students expect.
Paying for electricity: the kilowatt-hour
Electricity bills use the kilowatt-hour (kWh), the energy converted by a 1 kW appliance running for 1 hour. 1 kWh = 1000 W × 3600 s = 3.6 × 10⁶ J.
To find energy in kWh, use power in kW and time in hours: E = P × t. Then multiply by the price per kWh to get the cost.
This is why a 2 kW heater left on for 3 hours uses 6 kWh, the same as a 100 W lamp left on for 60 hours.
Trick: Convert before you multiply: W to kW (÷ 1000) and minutes to hours (÷ 60). Writing the units next to every number in your working catches most mistakes in this calculation.
Electrical safety: fuses, earthing and insulation
A fuse is a thin wire that melts and breaks the circuit when the current is too large, preventing overheating and fire. It's connected in the live wire. Choose a fuse rating slightly above the normal operating current, for example a 13 A fuse for an appliance drawing 8.7 A, from common ratings of 3 A, 5 A and 13 A.
Earthing: the metal casing of an appliance is connected to the earth wire. If the live wire touches the casing, a large current flows to earth, which blows the fuse and cuts off the supply, so the user isn't electrocuted.
Switches are also placed in the live wire, so the appliance is not live when switched off. Double-insulated appliances have a plastic casing and need no earth wire. Main hazards to watch for: damaged insulation, overheated cables from overloaded sockets, and using appliances with wet hands.
Trap: A fuse rated too high won't blow when it should, and one rated too low blows during normal use. Exam answers must justify the choice: 'the normal current is 8.7 A, so a 13 A fuse is the smallest rating above it'.
Worked Example
An electric heater is rated 1.5 kW, 230 V. It is used for 4 hours a day for 30 days. Electricity costs $0.30 per kWh.
(a) Calculate the energy used by the heater over 30 days, in kWh. [2]
E = P × t
= 1.5 kW × (4 × 30) h
= 180 kWh
(b) Calculate the cost of using the heater over the 30 days. [1]
Cost = 180 × $0.30
= $54
(c) Calculate the normal operating current of the heater, and suggest a suitable fuse from 3 A, 5 A and 13 A. [3]
I = P ÷ V = 1500 ÷ 230 = 6.5 A
A 13 A fuse is suitable.
It is the smallest available rating above the normal current of 6.5 A, so it won't blow in normal use but will blow if a fault causes a much larger current.
Why this question is a good test of the topic: it mixes units on purpose (kW in one place, W needed in another). Part (c) also tests the reasoning behind the fuse choice, not just the number.
Frequently Asked Questions
What is a kilowatt-hour?
It's the energy converted by a 1 kW appliance running for 1 hour. 1 kWh = 3.6 × 10⁶ J, and it's the unit used on electricity bills.
How do you calculate the cost of electricity?
Find the energy used in kWh (power in kW × time in hours), then multiply by the price per kWh.
Why is the fuse connected to the live wire?
So that when the fuse blows, the appliance is disconnected from the high voltage of the live wire and is no longer dangerous to touch.
How does earthing protect the user?
If the live wire touches the metal casing, a large current flows through the earth wire, blowing the fuse and cutting off the supply before anyone touches the live casing.
How do you choose the right fuse?
Calculate the normal operating current with I = P/V, then choose the smallest available fuse rating that is above that current.
Still mixing up power, energy and kWh?
Small group O Level Physics classes at TGC Academy's Bishan, Bukit Timah and Potong Pasir centres, helmed by Andrew Seah, MOE Award-Winning Teacher and Marshall Cavendish textbook author, with a team of tutors.