39. A capacitor is formed from two concentric spherical conducting shells separated by vacuum. The inner sphere
has radius 12.5 cm, and the outer sphere has radius 14.8
cm. A potential difference of 120 V is applied to the
capacitor. (a) What is the energy density at r = 12.6 cm,
just outside the inner sphere? (b) What is the energy
density at r = 14.7 cm, just inside the outer sphere? (c) For
a parallel-plate capacitor the energy density is uniform in
the region between the plates, except near the edges of
the plates. Is this also true for a spherical capacitor?
40. Calculate the capacitance of a capacitor consisting of
two parallel plates separated by a layer of paraffin wax 0.5
thick, the area of each plate being 80 cm². The
dielectric constant constant for the wax is 2.0. If the
capacitor is connected to a 100 V source, calculate the
charge on the capacitor and the energy stored in the
capacitor
cm​