# Simple electrical field problem

1. Oct 12, 2006

### cyberstudent

A metal sphere of radius R with a charge q is surrounded by a concentric metal sphere as shown in Figure P.23. The outer surface of the the sperical shell has a charge of + 30.0*10^-6C and the inner surface of the shell has a charge of +25.0*10^-6C.
(a) Find q
(b) Sketch qualitative graphs of (i) the radial electric field component Er and (ii) the electric potential V as functions of r.

I hired an online tutor who told me that q = -(25+30) micro coulombs
= -55 micro coulombs.

I disagree.

Shouldn't q=-25 micro coulombs? There is no electric field inside the shell, since it is a conductor. The charge residing on the outer surface of the shell therefore should not have any effect on the sphere inside the concentric shell.

Should I fire my tutor or can I have confidence in his answers?

P.S. Diagram attached

#### Attached Files:

• ###### 23.gif
File size:
41.9 KB
Views:
60
2. Oct 12, 2006

### OlderDan

I can't see your diagram yet, but it sounds like you are correct. The charge on the outermost surface has no effect. The net charge from the inner surface of the outer shell and everything inside is zero.

3. Oct 12, 2006

### cyberstudent

Thanks

Great. Thanks for the confirmation. He is fired.

#### Attached Files:

• ###### mydiagram.JPG
File size:
37.9 KB
Views:
55
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook