Question

What will be the potential difference on the capacitor while placing dielectric between parallel plate capacitor in the following two cases:
1) If capacitor is already connected to some battery.
2) If the battery is switched off and after connecting dielectric again switched on. Explain with reason.

Answer 1

Thus when a dielectric is inserted in a charged capacitor (not connected to a power supply), the electric field would be decreased and so would the voltage (=Ed). Since $C=\frac{Q}{V}$, this means that C must be bigger when a dielectric is inserted.
For a parallel-plate capacitor containing a dielectric, the capacitance is:
$C=\frac{k{ϵ}_{0}A}{d}$
Where k is the dielectric constant.
In general, adding a dielectric to a capacitor increases the capacitance by a factor of k.