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Homework Help: Capacitance in concentric shells

  1. Jun 5, 2010 #1
    1. The problem statement, all variables and given/known data

    Consider four concentric spherical metal
    shells of radii r, 2r, 3r, and 4r. The middle
    two shells are connected by wire, as shown.
    What is the total capacitance of the system?
    In your solution, clearly explain your method.

    2. Relevant equations

    u= 1/2cv^2

    3. The attempt at a solution
    So my teacher told us to assume we can hook it up to a battery(voltage) and that the two inner shells have the same charge and to evaluate the potential inner to outer, but I have no idea how to do this.
     
  2. jcsd
  3. Jun 5, 2010 #2

    vela

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    There's a number of details you'll have to figure out for yourself, but here's generally how to approach the problem. Assume there is a charge +Q on the inner sphere. Use Gauss's law to calculate the electric field between the spheres. You'll have to figure out what the charge of the other spheres as well. Once you know the electric field, you can integrate it to calculate the potential difference between the spheres.
     
  4. Jun 5, 2010 #3
    So when I do that I get

    E(r)= Q/4(pi)(R)2Epilsonknot


    E(2r)=Q/4(pi)(2R)2Epilsonknot


    E(3r)=Q/4(pi)(3R)2Epilsonknot

    E(4r)=Q/4(pi)(4R)2Epilsonknot


    Then I use V=Ed so I integrate this?

    What am I integrating though, the difference between the electric fields or the distance?
    I am stuck beyond this point and are my electric fields right?
     
  5. Jun 6, 2010 #4

    vela

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    I think you need to go back and review how the electric field and potential difference are related (beyond just plucking formulas out of the book) and then reread my previous post.

    Your electric fields are not correct. You want the electric field for the regions between the spheres. Also, you can't use V=Ed here.
     
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