A parallel-plate capacitor with a solid dielectric of relative permittivity εr=4.0\varepsilon_r = 4.0εr=4.0 completely filling the space between its plates is connected to a DC power supply and fully charged. The capacitor is then isolated from the supply.
The solid dielectric slab is subsequently removed, leaving air (εr≈1.0\varepsilon_r \approx 1.0εr≈1.0) between the plates, while the plate separation is simultaneously tripled.
If the initial electric field strength between the plates was EiE_iEi, what is the final electric field strength EfE_fEf?
12Ei12 E_i12Ei
4Ei4 E_i4Ei
43Ei\frac{4}{3} E_i34Ei
13Ei\frac{1}{3} E_i31Ei
