# Conducting Sphere and Shell. Please Help

1. Feb 24, 2005

### DR33

Here's the problem that I have been trying to solve:

A solid conducting sphere of radius 20cm is concentrically placed inside a spherical shell of inner radius 30 cm and outer radius 40cm. A Charge of 20uC is placed on the inner sphere, and a charge of -10uC is placed on the outer conductor. The dielectric constant of the medium between the 2 conductors is 5. Finc:
a) D, E and P at all points in space (all vectors)
b) the total energy in the system

Can anyone help me resolve this . Thakn you.

2. Feb 24, 2005

### CartoonKid

What do you mean by D, E and P?
E = Electric field?
D, P = ?
Thanks

3. Feb 24, 2005

### DR33

D = Electric Flux Density
P = Dipole Moment per unit Volume

4. Feb 24, 2005

### DR33

..

Anyone????

5. Feb 24, 2005

### CartoonKid

$$R_1 = 0.2m, R_2 = 0.3m, R_3 = 0.4m$$
For $$r < R_1$$,
Since the solid sphere is a conductor, the E field will be 0.
For $$R_1 < r < R_2$$,
$$\phi = \frac{q_{in}}{\varepsilon}$$
$$\phi = E\times 4\Pi r^2$$
So $$E = ...$$
For $$R_2 < r < R_3$$,
It's same with first one.
For $$r > R_3$$,
Use the method as for $$R_1 < r < R_2$$ with $$q_{in}$$ being changed to $$-10\mu C$$ (I don't know if the inner most charge has to be taken into account because it's bounded by the spherical conductor, correct me if I'm wrong)
As for the energy, would the inner charge contribute energy to the system?

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