A dielectric slab is inserted between the plates of a capacitor. The charge on the capacitor is Q and the magnitude of the induced charge on each surface of the dielectric is Q’.
When a dielectric slab of dielectric constant K is introduced between the plates of the capacitor, the net electric field in the dielectric becomes
Where,
E0 is the electric field when there is vacuum between the plates.
K is the dielectric constant of the dielectric
The net electric field is due to charges +Q, -Q and due to induced charges +Q’,-Q’in opposite direction).
So, the net electric field becomes
Where,
Q’=induced charge due to dielectric
Q=charge on the capacitor in vacuum
K=dielectric constant
Since dielectric constant K>1
Q’<Q
Therefore, on inserting a dielectric slab between plates of capacitor the induced charge Q’ is less than Q.