# Induction of spheres /shells

1. Mar 25, 2012

### kikonenko

Hi, i'm a physics student, preparing for an exam (it happens to be tomorrow heh), and i'm having trouble with induction of conductors. I need to get it right so i can solve more difficult problems, but the more i try the worse it gets. Basically i'd like to know how things work in sets of shells / spheres where one is grounded, or connected to another sphere, you know these kind of problems. i know the basics, shielding prevents outside charge to induce inner conductors, E-field inside the conductor itself is zero, and net charge remains the same while no contact between conductors/earth is made.

Since i'm lost, i tried with some solved examples in my book (no explanation, just numbers)

Here's a pic of a problem http://imagebin.org/205050 [Broken]

I don't get why when the second shell is grounded, its outside surface charge isn't automatically equaled to zero and instead replaced by q' . And what happens from step 2 to step 3 (grounded) in the bigger shell's surfaces? they go from -Q-3q (in) & Q+2q(out) to q'-q & -q' . I guess they are returning to their initial state before any induction was made, and from there they add up the new inductions as a result of q' in the small shell.

Then, they solve it by equaling 0 (ground) = V from inf to outer shell surface + V outer shell to nested shell , and the result is q' =2/3q. That makes sense, but it also makes sense to me to think that as soon as earth makes contact with the shell, any charges on it are grounded and induced charge = 0. (considering that the inside surface charge of the nested shell is also grounded when the outside surface is grounded. I don't know that for sure, please tell me).

Thanks in advance for any explanation.

Last edited by a moderator: May 5, 2017