A 900 pF capacitor is charged by 100V battery.
(a) How much electrostatic energy is strored by the capacitor ? The capacitor is disconnected from the battery and connected to another 900 pF capacitor. How much is the electrostatic energy stored in the system ?

Explanation

(a) `U = (1)/(2) CV^(2) = (1)/(2) xx900 xx 10^(-12) xx ( 100)^(2)`
`= 4.5 xx 10^(-6) `
`= 4.5 mu J`
(b) Common potential , `V' '= ( 900 xx 100 + 900 xx 0)/( 1800 ) = 50` volt
`U' = (1/(2) (C_(1) + C_(2))xx V^('2)`
`= (1)/(2) xx 1800 xx ( 50 )^(2) xx 10^(-12)`
`= 2.25 muJ`