Two identical capacitors C_{1} and C_{2} of equal capacitance are connected as shown in the circuit. Terminals a and b of the key k are connected to charge capacitor C_{1} using battery of emf V volt. Now disconnecting a and b the terminals b and c are connected. Due to this, what will be the percentage loss of energy? (1) 25 \% (2) 75 \% (3) 0 \% (4) 50 \%
Two identical capacitors C_{1} and C_{2} of equal capacitance are connected as shown in the circuit. Terminals a and b of the key k are connected to charge capacitor C_{1} using battery of emf V volt. Now disconnecting a and b the terminals b and c are connected. Due to this, what will be the percentage loss of energy? (1) 25 \% (2) 75 \% (3) 0 \% (4) 50 \%

Answer (4)
\mathrm{Sol},
Switch the key at point c
\begin{array}{l} \frac{q_{0}-q}{C}=\frac{q}{C} \\ 2 q=q_{0} \\ q=\left(\frac{q_{0}}{2}\right) \\ U_{f}=\frac{1}{2}\left(\frac{q_{0}}{2}\right)^{2} \times \frac{1}{C}+\frac{1}{2}\left(\frac{q_{0}}{2}\right)^{2} \times \frac{1}{C} \\ U_{f}=\frac{q_{0}^{2}}{4 C} \\ =\frac{\left(\frac{1}{2}-\frac{1}{4}\right) C V^{2}}{\frac{1}{2} C V^{2}} \times 100 \\ =50 \% \end{array}