Question

When dielectric medium of constant k is filled between the plates of a charged parallel-plate condenser, then the energy stored becomes, as compared to its previous value,

• A. $K^{-3}times$
• B. $K^{-2}times$
• C. $K^{-1}times$
• D. $Ktimes$

Answer 1

Answer: (C)

As no cell is connected to the capacitor so the charge (Q) remains constant.
If the capacitance of capacitor before inserting the dielectric is $C$ then capacitance after inserting dielectric becomes $KC$
Before inserting dielectric the energy is $U=\frac{Q^2}{2C}$
After inserting dielectric the energy is $U^{\prime }=\frac{Q^2}{2KC}=U{K}^{-1}$