A parallel-plate capacitor has an area of 0.12 m^2 and a separation of 1.2 cm. A battery charges the plates to a potential difference of 120V and is then disconnected. A dielectric slab of thickness 4.0 mm and dielectric constant 4.8 is then placed symmetrically between the plates. (a) What is the capacitance before the slab is inserted. (b) What is the capacitance with the slab in place? (c) With the slab in place, what is the potential difference across the plates?(d) How much external work is involved in inserting the slab?