The capacitance of a parallel plate capacitor is 400 pico farad and its splates are separated by 2mm of air
(i) What will be the energy when it is charged to 1500 volts ?
(ii) What will be the potential difference with the same charge if separated is doubled ?
(iii) How much energy is needed to double the distance between its plates ?

Explanation

(i) `U_(i) = (1)/(2) CV^(2) = (1)/(2) xx 400 xx 10^(-12) xx ( 1500 )^(2)`
`= 4.5 xx 10^(-4)` joules
(ii) When d becomes half then for same charge potential difference becomes double i.e., 3000 volt
(iii) `U_(f) = (1)/(2) xx 200 10^(-12) xx ( 3000)^(2)`
` 9 xx 10^(-4) `joules
So,` Delta U = U _(f) - U_(i)`
`= 9 xx 10^(-4) - 4.5 xx 10^(-4)`
`= 4.5 xx 10^(-4) `joules