Two circular plates `A` and `B` of a parallel plate air capacitor have a diameter of `0.1m` and are `2xx10^(-3) m` apart. The plates `C` and `D`, of a similar capacitor have a diameter of `0.12m` and are `3xx10^(-3)m` apart. Plate `A` is earthed and plates `B` and `D` are connected together as shown in figure. Plate `C` is connected to the positive pole of a `120V` battery whose negative terminal is earthed. Calculate.
(a) the combined capacitance of the arrangement and
(b) the energy stored in it.