A long solenoid of radius $\mathrm{R}$, and having $\mathrm{N}$ turns per unit length carries a time dependent current $I \; (t)=I_{0} \cos (\omega t)$. The magnitude of induced electric field at a distance $R / 2$ readily from the axis of the solenoid is
- $\frac{\mathrm{R}}{2} \mu_{0} \mathrm{NI}_{0} \omega \sin (\omega t)$
- $\frac{\mathrm{R}}{2} \mu_{0} N \mathrm{I}_{0} \omega \cos (\omega t)$
- $\frac{\mathrm{R}}{2} \mu_{0} N \mathrm{I}_{0} \omega \sin (\omega t)$
- $\frac{\mathrm{R}}{2} \mu_{0} N I_{0} \omega \cos (\omega t)$