# Electric Field and spherical cavities

1. Oct 10, 2006

### stunner5000pt

Two spherical cavities of radii a and b are hollowed out from the interior of a neutral conducting sphere of radius R. At the center of each cavity is a point charge place, callthem qa and qb

a) Find teh surface charges sigma a, sigma b, and sigma R.

ok say for qa

$$\vec{E} = \frac{q_{a}}{4 \pi \epsilon_{0} a^2} \hat{a} = - \vec{\nabla} V \bullet \hat{n}$$

also $$\sigma = -\epsilon_{0} \vec{\nabla} V \bullet \hat{n} = \frac{q_{a}}{4 \pi a^2}$$

but i was told this answer should be negative.. where is my mistake?

#### Attached Files:

• ###### 1.JPG
File size:
10.6 KB
Views:
52
2. Oct 10, 2006

### OlderDan

The total charge inside a guassian surface surrounding one inner sphere has to be zero, The surface charge must have the opposite sign of the central charge.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook