Capacitance C of a parallel plate capacitor is C=Q/V, where Q is the charge on the plates and V is the potential difference between the plates. The potential of the capacitor is then: V=Q/C.
The capacitance C is given by the equation C=k*ε₀*(A/d) where k is the dielectric constant, ε₀ is the permittivity of free space, A is the surface of the plate and d is the distance between the plates.
For vacuum and air, k=1 so the capacitance is C=ε₀*(A/d).
But when we insert a dielectric between the plates like teflon which has k=2.1 at T=20°C the capacitance becomes:
C₁=k*ε₀*(A/d)=2.1*ε₀*(A/d)=2.1*C.
We can see that the capacitance increases by k=2.1 which is the dielectric constant of teflon.
So the voltage V becomes:
V₁=Q/C₁=Q/(2.1*C)=(1/2.1)*(Q/C)=(1/2.1)*V=0.48*V.
V₁=0.48*V.
Potential difference decreases by 0.48 because we inserted teflon between the plates.
