Define mutual inductance

(i) Define mutual inductance.
(ii) A pair of adjacent coils has a mutual inductance of 1.5 H. If the current in one coil changes from 0 to 20 A in 0.5 s, what is the change of flux linkage with the other coil ?

(i) Mutual Inductance : It is numerically equal to the magnetic flux linked with one coil, when a unit current flows through the neighbouring coil.
(ii)Magnetic flux
$As{ I }{ 1 }=0$
$\therefore { \phi }
{ 1 }=0$
${\phi_2}=M{I_2}=1.5\times 20=30$
Change in flux $d\phi = {\phi _2} - {\phi _1}$