CBSE Class 12 · 2015 · Delhi · Electromagnetic InductionLong Answer

(a) The mutual inductance $M$ of two coils is the flux linked with one coil per unit current in the other; its S.I. unit is the henry (H).
(b) Let inner solenoid $S_1$ ($n_1$ turns/length) carry current $I_1$, producing field $B_1={\mu }_0{n}_1{I}_1$ over cross-section $A$. The flux linking the outer solenoid $S_2$ (turns $n_2\ell$) is

so that

(c) If current $I_2$ in coil $c_2$ links flux $\varphi =M{I}_2$ with coil $c_1$, then the emf induced in $c_1$ is