Question

In figure a $120-turn$ coil of radius $1.8$ cm and resistance $5.3\Omega$ is placed outside a solenoid. The current in the solenoid is $1.5A$ and it reduces to zero at a steady rate in $25ms$. What current (in mA) appears in the coil ? The number of turns per unit length of the solenoid is $220$ turns/cm and its diameter $D=3.2cm$.

Answer 1

Answer: 30

The induced emf,$\left|e\right|=A\left(\frac{\Delta B}{\Delta t}\right)<br>={\left(\pi R\right)}^2{\mu }_{0}n\left(\begin{array}{c}\Delta i\\ \Delta t\end{array}\right)$
$=\pi {\left(1.8\times 10^{-2}\right)}^2\times \left(4\pi \times 10^{-7}\right)\times 220\times 10^2$
$\times \left(\frac{1.5}{25\times 10^{-3}}\right)=68.6mV$
Current,$i=\frac{e}{R}=30mA$