Distinguish between induced emf and induced current. The flux through each turn of a 100-turn coil of resistance 5 Ω changes steadily by 2 mWb in 0.02 s. Find the induced emf, the current and the charge that flows.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Write Faraday's law. ε = N dΦ/dt in magnitude: an emf appears whenever the flux changes.
2 · Explain emf without current. In a coil with open ends the emf appears across the ends, but no current flows, as there is no closed path.
3 · Write the current. In a closed circuit of total resistance R, I = ε/R.
4 · Substitute. ε = 100 × 2 × 10⁻³ Wb ÷ 0.02 s = 10 V; I = 10 V ÷ 5 Ω = 2 A.
5 · Find the charge. The current is steady, so q = IΔt = 2 A × 0.02 s = 0.04 C.
6 · Close in one line. Therefore, an emf needs only a changing flux, but a current also needs a closed circuit.
Scoring tip: Write the unit in every line of working: Wb, s, V, Ω, A and C.
