Chapter 8, Problem 53P

A parallel-plate capacitor has charge of magnitude $9.00\mu F$ on each plate and capacitance $3.00\mu F$ when there is air between the plates. The plates are separated by 2.00 mm. With the charge on the plates kept constant, a dielectric with $\kappa =5$ . is inserted between the plates, completely filling the volume between the plates, (a) What is the potential difference between the plates of the capacitor, before and after the dielectric has been inserted? (b) What is the electrical field at the point midway between the plates before and after the dielectric is inserted?