Two small spheres each carrying a charge $q$ are placed $r$ metre apart. If one of the spheres is taken around the other one in a circular path of radius $r$, the work done will be equal to

Energy is stored in what form in capacitor ?

$100$ capacitors each having a capacity of $10\,\mu F$ are connected in parallel and are charged by a potential difference of $100\,kV$. The energy stored in the capacitors and the cost of charging them, if electrical energy costs $108\;paise\;per\;kWh$, will be

The plates of a parallel plate capacitor have an area of $90 \,cm ^{2}$ each and are separated by $2.5\; mm .$ The capacitor is charged by connecting it to a $400\; V$ supply.

$(a)$ How much electrostatic energy is stored by the capacitor?

$(b)$ View this energy as stored in the electrostatic field between the plates, and obtain the energy per unit volume $u$. Hence arrive at a relation between $u$ and the magnitude of electric field $E$ between the plates.

The energy stored in the electric field produced by a metal sphere is $4.5\, J$. lf the sphere contains $4\,\mu C$ charge, its radius will be…….$mm$ : [Take : $\frac{1}{{4\pi {\varepsilon _0}}} = 9 \times {10^9}\,N – {m^2}\,/{C^2}\, ]$