State Joule's law of heating. Give three applications of the heating effect of current and explain how a fuse protects a circuit.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · State the law. The heat produced in a resistor is H = I²Rt: directly proportional to the square of the current, the resistance and the time.
2 · Show where it comes from. Work done W = VQ = VIt. With V = IR, H = I²Rt.
3 · Give applications.
Electric iron, toaster, kettle, heater: nichrome coils
Electric bulb: tungsten filament in nitrogen and argon
Fuse: wire of low melting point
4 · Draw the fuse in a circuit. [Circuit: supply, fuse in series on the live wire, switch, appliance; fuse labelled]
5 · Explain the fuse. It is in series, so all the current passes through it. Too much current means large I²Rt heat, so the wire melts and breaks the circuit.
6 · Real life. An 880 W iron on 220 V draws 4 A, so it is protected by a 5 A fuse.
7 · Close in one line. Joule heating is useful in heaters and bulbs, and the fuse uses it for safety.
Scoring tip: Write H = I²Rt in the first line and say 'square of the current'. Many students lose a mark by writing H = IRt.
