An air-cored solenoid with length $20 \, cm$,area of cross-section $20 \, cm^2$,and number of turns $400$ carries a current of $2 \, A$. The current is suddenly switched off within $10^{-3} \, s$. The average back $emf$ induced across the ends of the open switch in the circuit is (ignore the variation in magnetic field near the ends of the solenoid) ........ $V$.

In a coil of self-inductance $0.5 \, H$,the current varies at a constant rate from $0 \, A$ to $10 \, A$ in $2 \, s$. The $e.m.f.$ generated in the coil is ... $V$.

The current through an inductor is given by $i = (10 + 20t) \, A$,where $t$ is in $s$. The induced $emf$ in the inductor is $4 \, V$. The total flux linked with the inductor at $t = 2 \, s$ is ...... $Wb$.

$A$ circuit having a self-inductance of $1 \text{ H}$ carries a current of $1 \text{ A}$. To prevent sparking when the circuit is broken,a capacitor which can withstand $500 \text{ V}$ is connected across the switch. What is the minimum value of the capacitance of the capacitor (in $\mu \text{ F}$)?

An emf of $2.8 \ mV$ is induced in a rectangular loop of area $150 \ cm^2$ when the current in the loop changes from $3 \ A$ to $8 \ A$ in a time of $0.2 \ s$. Then the self-inductance of the loop is (in $\mu H$)

An air-cored solenoid with length $30 \ cm$,area of cross-section $25 \ cm^2$ and number of turns $500$ carries a current of $2.5 \ A$. The current is suddenly switched off for a brief time of $10^{-3} \ s$. How much is the (nearly) average back e.m.f. induced across the ends of the open switch in the circuit (in $V$)? (Ignore the variation of magnetic field near the ends of solenoid)