The capacitor (of thickness d) is disconnected from a potential source of V and a dielectric of thickness t is inserted and it has relative permitivity Er. Find the new potential between the plates
This is the answer : Vf = V/d(d - t + t/Er)
The Attempt at a Solution
I don't get why this is it looks like they assumed that the electric field would remain constant and just added the potential drops. We know electric field isn't constant so what give?