A solenoid is connected in series with a battery and a switch, as illustrated in the figure below.
A small coil, connected to a sensitive ammeter, is situated near one end of the solenoid. As the current in the solenoid is switched on, there is a changing magnetic field inside the solenoid and a current is induced in the small coil. This induced current gives rise to a magnetic field in the small coil.
State Lenz’s law.
Use Lenz’s law to state and explain the direction of the magnetic field due to the induced current in the small coil.
The small coil has an area of cross-section 
and contains 75 turns of wire. A constant current in the solenoid produces a uniform magnetic flux of flux density 
throughout the small coil. The direction of the current in the solenoid is reversed in a time of 
. Calculate the average e.m.f. induced in the small coil.
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