State Joule's law of heating and show that H = i²Rt. Find the heat produced when a current of 2 A flows through a coil of resistance 50 Ω for 5 minutes.

1. 1 · State the law. The heat produced in a conductor is proportional to the square of the current, its resistance and the time: H = i²Rt.
2. 2 · Start from work done. Work done in moving charge Q across p.d. V is W = VQ, and Q = it, so W = Vit.
3. 3 · Use Ohm's law. V = iR, so W = i²Rt. This work appears as heat: H = i²Rt.
4. 4 · Write what is given. i = 2 A, R = 50 Ω, t = 5 min = 300 s.
5. 5 · Substitute and answer. H = (2)² × 50 × 300 J = 60,000 J = 6 × 10⁴ J.

Scoring tip: Square the current before you multiply, and change minutes to seconds. Give the answer in joules.