# Finding net charge of a sphere

1. Jul 27, 2005

### mr_coffee

An unkown charge sits on a conducting solid sphere of radius 10 cm. If the E-field 15cm from the center of the sphere has the magnitude 3.0E3 N/c and is directed radiually inward, what is the net charge on the sphere? I used
E = q/(4PIEoR^2) plugged and chugged and got 3.34E-9 which isn't right of course, where did i screw up?

2. Jul 27, 2005

### Jelfish

Are you sure you're not doing the arithmetic incorrectly? I got a different numeric answer. Post what numbers you used in your calculation.

3. Jul 28, 2005

### Dr.Brain

For an outside point w.r.t the charged solid sphere , the solid sphere behaves like a point charge , so assume solid sphere to be a point charge with charge Q , now electric field at a distance 'r' outide this solid sphere(point charge) is given by:

$E= \frac{kQ}{r^2}$

You just need to fill in the values to calculate Q.Since the field is inwards, the charge will be negative.

BJ