What happened when a charge Q is placed at a distance r from the center of an isolated metal sphere (net charge is zero) with radius R.
The following applet shows the field line distribution between charge Q and sphere.
You can drag the charge Q to different location and watch how the field distribution changed!
Red and green circle shown inside the metal are the image charge for this problem.
The electric field inside the metal is zero. All the electric field outside the sphere can be calculated frome elctric field due to charge charge Q and those two image charges.
Because net charge on the sphere is zero, so the sum of those two image charges is zero,too.
The image charge(green) at the center of the sphere is equal to Q*R/r;
The distance between red and green charge is R*R/r;
You can change the situation from [b]isolated spher[/b]e to [b]grounded sphere[/b] case.
Then, charge will flow to sphere to make the potential become zero.
The net charge will not be zero when the sphere is grounded.
The applet will show electric field, pontential contiur or charge distribution on the surface of the sphere if you click on those checkboxs.
If the charge Q is move inside the sphere, then one of the image charge(red) will be located outside the sphere.
You can study many related physics with this applet. I hope you can enjoy it as much as I do.

Virtual Physics laboratory Original URL

Fu-Kwun Hwang, Dept. Of Physics, National Taiwan Normal University.