Question

Condenser $A$ has a capacity of $15\mu F$ when it is filled with a medium of dielectric constant $15$. Another condenser $B$ has a capacity $1\mu F$ with air between the plates. Both are charged separately by a battery of $100V$. After charging, both are connected in parallel without the battery and the dielectric material being removed. The common potential now is :

• A. $400V$
• B. $800V$
• C. $1200V$
• D. $1600V$

Answer 1

The correct option is B $800V$

As battery is disconnected so the charges on two capacitor remain constant .

$Q_1=15\times {10}^{-6}\times 100=1500\mu C$ and $Q_2={10}^{-6}\times 100=100\mu C$

As dielectric is removed so the capacitance $15\mu F$ becomes $C_1=\frac{15}{15}=1\mu F$ and capacitance $1\mu F$ will be same $C_2=1\mu F$

Common potential $V_c=\frac{Q_1+Q_2}{C_1+C_2}=\frac{1500+100}{1+1}=800V$