Electric field of a point outside a charged sphere

[dpg1276]

If a conducting sphere has a negative net charge and there is a positive point charge at the center of the sphere, what charge would you use to find the Electric Field at a point outside the sphere?

The equation is E=kq/r^2.

Would I use the net charge of the sphere, or do I need to calculate a new charge due to the positive one at the center?

 

[BruceW]

Are you talking about a hollow conducting sphere of net negative charge, with a positive charge inside the hollow? Luckily, the answer is very simple, just add up all the charges (including the positive one in the middle) and use this in the equation.

For example, if the net negative charge on the conducting sphere is exactly equal to -1 times the positive charge in the middle, then the charges cancel, and there is no electric field outside the sphere.

To find out why it works this way, you could either do an integral and Coulomb's law, or use Gauss' law, using the symmetry of the problem.