A current of $2 \mathrm{A}$ is increasing at a rate of $4 \mathrm{A} {\mathrm{s}}^{-1}$ through a coil of inductance $2 \mathrm{H}$. The energy stored in the inductor per unit time is 

A solenoid $30 \mathrm{cm}$ long is made by winding $2000$ loops of wire on an iron rod whose cross-section is $1.5 {\mathrm{cm}}^2$. If the relative permeability of the iron is $6000$ . What is the self-inductance of the solenoid? (${\mu }_{\circ }=4\pi \times {10}^{-7} \mathrm{F} {\mathrm{m}}^{-1}$)