When the battery across the plates of a charged condenser is disconnected and a dielectric slab is…
When the battery across the plates of a charged condenser is disconnected and a dielectric slab is introduced between its plates then the energy stored
becomes infinity
does not change
increases
decreases
Solution
Since the battery is disconnected, the charge on the capacitor remains constant. When the dielectric slab is introduced, the capacitance increases $\mathrm{k}$ times.
Energy stored, $\mathrm{U}_{\mathrm{i}}=\frac{\mathrm{q}^2}{2 \mathrm{C}}$
$\mathrm{U}_{\mathrm{f}}=\frac{\mathrm{q}^2}{2 \mathrm{kC}} < \mathrm{U}_{\mathrm{i}}$