The shape of wavefront of light diverging from point source

A cube of side $L$ has point charges $+q$ located at its seven vertices and $-q$ at remaining one vertex. The electric field at its centre is found to be $|\mathbf{E}|=\alpha\left(\frac{q}{4 \pi \varepsilon_0 L^2}\right)$.

The magnitude of constant $\alpha$ is

A capacitor is fully charged with a battery and then disconnected. A dielectric is then inserted into the capacitor. How do charges on surface of the dielectric and outer surface of the plates of the capacitor would change respectively?

A conducting wire of cross-sectional area $1 \mathrm{~cm}^2$ has $3 \times 10^{23}$ charge carriers per $\mathrm{m}^3$. If wire carries a current of 24 mA , then drift velocity of carriers is