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Homework Help: Draw the field for two concentric conducting spheres

  1. Feb 18, 2014 #1
    1. The problem statement, all variables and given/known data
    We are given two concentric conducting shells centered around a common origin. Within the inner shell there is a positive point charge q and somewhere outside the two shells is another positive point charge q. The question wants the field lines for this system and then again for the same system with a wire connecting the two conducting shells

    2. Relevant equations
    I'm pretty sure I don't need any equations for this one.

    3. The attempt at a solution
    What I have currently is in the attached photo. I'm nearly sure this is wrong though because they can't be the same can they?
    I don't really get how having two shells makes the first system any different than having one shell.

    If you wouldn't mind taking a look and telling me if i'm on the right track or not that would be great!
    Thank You!

    Attached Files:

  2. jcsd
  3. Feb 18, 2014 #2


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    You are not far off. I don't know if you have been acquainted with Gauss's law already? Makes this a simple exercise.

    I take for granted that the spheres themselves are neutral. So the positive charge inside pulls the negative charge to the inside of the inner sphere and there remains a positive charge on the outside of the inner sphere. Has to be evenly distributed (because it is a conductor). Same story for the outside shell in the upper picture. So you draw radial field lines between the shells. Net result for the outside world indistinguishable from a positive charge at the center of the spheres. So the 0 field point should be half way (you draw it a bit to the right).

    In the bottom figure the wire ensures there is no field between the spheres (otherwise the potential difference over the wire would conduct charge until same potential).
    Net result to the outside world same as in top figure: a charge at the center of the spheres.

    Sketchy field lines at the left should - close to the sphere - be perpendicular to the surface: there can be no component parallel to the surface -- it's a conductor!
  4. Feb 19, 2014 #3
    Okay, that makes a lot of sense! I guess I didn't really think of how the wire was affecting the field. When you have two conductors connected by a wire can you almost think of the as one thick shell? Since there can never be a field inside a solid conductor?

    Thank you!
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