Two coils are wound on the same iron rod so that the flux generated by one passes through the other also. The primary coil has $N_p$ loops and, when a current of 2.0 A flows through it, the flux in it is $2.5 \times 10^{-4} \mathrm{~Wb}$. Determine the mutual inductance of the two coils if the secondary coil has $N$, loops.
$$
\left|e_x\right|=N_3\left|\frac{\Delta \Phi_{\mu_2}}{\Delta t}\right| \text { and }\left|\psi_x\right|=M\left|\frac{\Delta i_p}{\Delta t}\right|
$$
give
$$
M=N_r\left|\frac{\Delta \Phi_{\mathrm{Mb}}}{\Delta t_p}\right|=N, \frac{\left(2.5 \times 10^{-4}-0\right) \mathrm{Wb}}{(2.0-0) \mathrm{A}}=\left(1.3 \times 10^{-4} N_s\right) \mathrm{H}
$$