Question

Two parallel plate capacitors of capacity $C$ and $3 C$ are connected in parallel combination and charged to a potential difference $18\, V$ The battery is then disconnected and the space between the plates of the capacitor of capacity $C$ is completely filled with a material of dielectric constant $9$ The final potential difference across the combination of capacitors will be______ $V$

Answer 1

The correct answer is 6.

Initial charge on C=18CV
initial charge on 3C=54CV
Let final common potential difference =V′
9CV′+3CV′=18CV+54CV
⇒12CV′=72CV⇒V′=6V