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

Explanation

(a) `U=(1)/(2) CV^(2)=(1)/(2)xx900 xx10^(-12)xx(100)^(2)`
`= 4.5xx10^(-6)`
`4.5 mu ` J
(b) Common potential ` V =(900xx100+900xx0)/(1800) = 50 ` volt
`U=(1)/(2)(C_(1)+C_(2))xxV^(2)=(1)/(2)xx1800xx(50)^(2)xx10^(-12)=2.25 mu` J