- 140

- 1

A spherical capacitor has inner radius R and outer radius 3R, and contains to dielectrics, of permittivities e1, which extends from R to 2R, and e2, which extends from 2R to 3R. Assume charges of +q and -q on the inner and outer surfaces.

a) find E (electric field) and D (displacement field) in each region as functions of r.

b) find the capacitance of the capacitor.

c) what is the surface charge on the e1 dielectric at radius R?

for part a) D = (Einitial)(e) = (kq/r^2)e = q/(4pir^2) for both regions.

and E1 = D/e1 = q/(4e1pir^2)

E2 = D/e2 = q/(4e2pir^2)

for part b) V1 = q/(4piRe1) and V2 = Q(24piRe2)

so C1 = 4piRe1 and C2 = 24piRe2 and Ctotal = 24piRe1e2/(6e2-e1)

Having found all this I don't quite know how to approach part c)

A nudge in the right direction would be greatly appreciated.

Thanks