A parallel plate capacitor has area of each plate A, the separation between the plates is d. It is charged to a potential V and then disconnected from the battery. The amount of work done in filling the capacitor completely with a dielectric constant k is :
When dielectric was not filled C=ε0Ad.
It was changed till V ∴Q=ε0AVd.
energy stored E1=12QV=12ε0AV2d
Now when battery is disconnected Q remains constant. When it is filled with dielectric K
We get C=Kε0Ad. as Q is constant
V decreases by k
∴V1=12⋅Kε0Ad(VK)2=1ε0AV22dk
∴work done by us=−(V2−V1)=ε0AV22d(1−1k)