1. Not finding help here? Sign up for a free 30min 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!

Electric field between two spheres

  1. Mar 17, 2013 #1
    1. The problem statement, all variables and given/known data

    If the inner sphere of radius a has charge +Q and the outer sphere of radius b has charge -Q/2. What's the electric field between them?

    3. The attempt at a solution

    If I use Gauss' law then I would have E*4*pi*a^2 = Q/ε then just solve for E. Is that correct? It seems like the outer sphere would affect the E-field on the inner sphere. By the way, they don't say that these are conducting spheres.

    Thanks for any help.
     
  2. jcsd
  3. Mar 17, 2013 #2

    rude man

    User Avatar
    Homework Helper
    Gold Member

    You have the field at r = a correct, but what about a < r < b? No, the outer sphere does no affect the E field on the inner sphere. Believe in Dr. Gauss! And also no, it doesn't matter if the sphers are conducting or insulators in this case.
     
  4. Mar 17, 2013 #3
    Thanks for the reply. Is this the E-field between the two spheres:

    E = Q/(ε*4*pi*r^2) for a < r < b ?

    I'm still confused how the outer sphere doesn't affect the E-field between them..
     
  5. Mar 18, 2013 #4

    rude man

    User Avatar
    Homework Helper
    Gold Member

    Right.

    For the same reason that, if you go inside the Earth, the only part exerting gravity on you is the part below you.

    At a point r in your sphere, some of the charges outside r will set up a + field and others will set up a - field. Some will push a test charge at r one way, others the opposite way. The net result is complete cancellation of each others' fields. It's not an easy task to do that integration, so again - believe Dr. Gauss!
     
  6. Mar 18, 2013 #5

    SammyS

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    There are several additional pieces of information needed before this problem could possibly be solved.

    Questions:
    Are the spheres concentric? (Do they have a common center?)

    Is the outer sphere actually a spherical shell? -- That's was is implied, seemingly.

    Is the charge distributed uniformly? -- or at least in some sort of symmetrical manner

    Suppose you have a uniformly charged spherical shell. What is the electric field inside the shell?
    This situation is often covered even before introducing Gauss's Law. ​
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted