State Joule's law of heating. Derive H = I²Rt and explain any two applications.

1. 1 · State the law. The heat produced in a conductor is directly proportional to the square of the current, the resistance and the time.
2. 2 · Derive the formula.
   • W = VQ, and this work becomes heat H.
   • Q = It, so H = VIt.
   • V = IR (Ohm's law), so H = I²Rt.
3. 3 · Explain the cause. Electrons collide with the atoms of the wire and pass on energy, so the wire heats up.
4. 4 · Application 1: heater. Nichrome coil: high resistivity, high melting point, does not oxidise easily.
5. 5 · Application 2: fuse. A tin-lead wire with a low melting point melts and breaks the circuit when the current is too high.
6. 6 · Real-life link. The iron box, water heater and toaster at home work on this effect.
7. 7 · Close. So heat depends most on current: doubling I gives four times the heat.

Scoring tip: Write the three proportionalities as (i), (ii) and (iii): examiners look for each one.