1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Simple electrical field problem

  1. Oct 12, 2006 #1
    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
  2. jcsd
  3. Oct 12, 2006 #2


    User Avatar
    Science Advisor
    Homework Helper

    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.
  4. Oct 12, 2006 #3

    Great. Thanks for the confirmation. He is fired. :biggrin:

    Attached Files:

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook