Mains Electricity

IGCSE Edexcel Physics
2.2–2.6 Electrical safety, heating effect of current, power, energy and AC/DC
Key Concepts: Mains electricity in the UK is AC at ~230 V, 50 Hz. DC comes from cells and batteries. Safety devices — fuses, circuit breakers, earthing and insulation — protect users and devices. A current through a resistor transfers energy as heat: $P = I \times V$; $E = I \times V \times t$.

Section A — AC and DC

1. State the difference between alternating current (AC) and direct current (DC). Give one source of each. [4]
2. State the approximate voltage and frequency of UK mains electricity. [2]

Section B — Electrical Safety

3. Explain how each of the following safety features protects a user or device: [6]

a) Insulation

b) Earthing

c) Fuse

4. Explain what double insulation means and why a doubly-insulated appliance does not need an earth wire. [2]
5. Describe how a circuit breaker works and state one advantage it has over a fuse. [3]
6. A student plugs a faulty hairdryer into the mains. The metal casing becomes live. Explain how the earth wire prevents the user from receiving an electric shock. [3]

Section C — Heating Effect and Power

7. Explain why a current flowing through a resistor causes the resistor to get hot. [2]
8. Write the equation for electrical power and use it to calculate the power of a kettle that draws 10 A from a 230 V supply. [3]
9. Write the equation for energy transferred by an electrical device. Calculate the energy transferred by the kettle in question 8 if it runs for 3 minutes. [3]
10. A laptop is rated at 45 W and is plugged into the 230 V mains. Calculate the current it draws and suggest an appropriate fuse rating (3 A, 5 A or 13 A). [3]

Total marks: 31

Mark Scheme

1. AC: current direction reverses repeatedly [1]; e.g. mains electricity [1]; DC: current flows in one direction only [1]; e.g. cell/battery [1] [4]
2. ~230 V [1]; 50 Hz [1] [2]
3. a) Insulation: covers live conductors with non-conducting material to prevent contact with live parts [2]; b) Earthing: connects metal casing to ground — if casing becomes live, current flows safely to earth and blows the fuse [2]; c) Fuse: contains a thin wire that melts if current exceeds a safe value, breaking the circuit [2] [6]
4. Double insulation means the device has two separate layers of insulating material around live parts [1]; there is no conducting casing that could become live, so no earth connection is needed [1] [2]
5. A circuit breaker uses an electromagnet (or bimetallic strip) that trips and breaks the circuit when current exceeds a safe level [2]; advantage: it can be reset (switched back on) rather than having to be replaced like a fuse [1] [3]
6. The earth wire connects the metal casing to earth (0 V) [1]; if the casing becomes live, a large current flows through the earth wire [1]; this blows the fuse / trips the circuit breaker, cutting off the supply before the user is harmed [1] [3]
7. Charge carriers (electrons) collide with ions in the resistor lattice [1]; these collisions transfer kinetic energy to the ions, increasing their vibration and raising the temperature [1] [2]
8. $P = I \times V$ [1]; $P = 10 \times 230 = 2300\,\text{W}$ (2.3 kW) [2] [3]
9. $E = I \times V \times t$ [1]; $t = 3 \times 60 = 180\,\text{s}$; $E = 10 \times 230 \times 180 = 414\,000\,\text{J}$ (414 kJ) [2] [3]
10. $I = P/V = 45/230 \approx 0.20\,\text{A}$ [2]; a 3 A fuse is appropriate — it is the lowest rating above the operating current [1] [3]