Question

The adjoining figure shows two identical parallel plate capacitors connected to a battery with switch S close. The switch is now opened and the plates are filled with a dielectric of dielectric constant 3. The ratio of the total electrostatic energy stored in both the capacitors before and after the introduction of the dielectric is

(a)  2 : 5

(b)  3 : 5

(c)  5 : 2

(d)  5 : 3

Answer 1

Correct option  (b) 3 : 5 

Explanation:

When the switch is closed, each capacitor has energy

1/2CV²

So total energy = 1/2CV² + 1/2 CV² = CV²

When switch is opened, capacitance of A becomes KC, when dielectric material was introduced its energy

becomes 1/2(KC)V².

 Energy of B becomes Q²/2(KC)

where Q is the charge on it.