State Faraday’s law for electromagnetic induction.
A solenoid $S$ is wrapped round a soft-iron core, as shown in Fig. 9.1. A coil $C$ with 120 turns of wire is wound onto one end of the core. The cross-sectional area of coil $C$ is $1.5\,\text{cm}^2$. A Hall probe is placed near the other end of the core. When a steady current flows in solenoid $S$, the flux density in the core is $0.19\,\text{T}$. The voltmeter connected to the Hall probe reads $0.20\,\text{V}$. The current in solenoid $S$ is then reversed over $0.13\,\text{s}$ at a constant rate.
Calculate the reading on the voltmeter connected to coil $C$ while the current is changing.
Complete Fig. 9.2 by filling in the voltmeter readings for the times before, during and after the current direction is reversed.