Lag1989 task 13.36

There is a dielectric between the plates of a flat capacitor. A potential difference U₁ = 200V is applied to the plates. The distance between the plates is d = 1 mm. If, after disconnecting the voltage source, remove the dielectric from the capacitor, then the potential difference between the plates will increase to U₂ = 800V. Find: a) the dielectric constant ε of the dielectric; b) surface density σ ´of bound charges.