Question

A 10 micro farad capacitor and a 20 microfaradcapacitor are connected in series across a 200V supply line.the charged capacitors are then disconnected from the line and reconnected with their positive plates together and negative plates together and no external voltage is applied.The potential difference across each capicitor is

Answer 1

When connected in series the total capacitance of the system is $\frac{20\times 10}{20+10}=\frac{200}{30}=\frac{20}{3}\text{μF}$. So total charge in the capacitor is $Q=VC=200\times \frac{20}{3}=\frac{4000}{3}\text{μC}$. Now when positive plate is connected to other positive plate and negative to negative then the connection will be series, so total capacitance will be $20+10=30\text{μC}$ so total potential difference is $V=\frac{Q}{C}=\frac{4000/3}{30}=400\text{V}$.