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[E&M] Concentrical metalic spheres

  1. Aug 11, 2015 #1
    The problem statement, all variables and given/known data

    I two concentric metalical spheres r1 =/= r2.
    What to do to have:
    a) potential energy = 0 and electric charge =/= 0
    b) potential energy =/= 0 and electric charge =0.

    In eighter one of the spheres.

    Relevant equations
    Gauss law.

    3. The attempt at a solution
    a)
    If I charge the outside sphere, I will have potential energy = 0 on it and electric charge =/= 0.

    b)
    I tought If I charge the one inside with Q, I will have a potential =/= 0 on the bigger sphere and 0 charge.

    Is this correct? The problem does not give correct answer.
     
  2. jcsd
  3. Aug 13, 2015 #2
    Do any of the spheres have an initial charge on them? And are you at a point in between the two spheres?
     
  4. Aug 14, 2015 #3
    I'm outside and I can charge eighter the bigger or the smaller or both of them. I have to have the specs from eighter a) or b) on 1 of them.
     
  5. Aug 14, 2015 #4
    Well, when you have a net positive or negative charge the pontential will approach zero only at infinity (unless otherwise specified such as the ground on a circuit). If you charge the outside sphere, why would potential energy be zero on it? May you show us what equation you are using for electric potential energy?
     
    Last edited: Aug 14, 2015
  6. Aug 14, 2015 #5

    SammyS

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    Electric potential

    or

    Potential energy stored in the system?
     
  7. Aug 15, 2015 #6
    Electric potential
     
  8. Aug 15, 2015 #7

    haruspex

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    That makes more sense. Electric potential is different from electrical potential energy.
    So, what equation do you have for finding the electric potential at a given distance from a uniformly charged sphere?
     
  9. Aug 15, 2015 #8
    k * q1 / r ( where K is 1/4 pi epsilon )
     
  10. Aug 15, 2015 #9

    SammyS

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    What does "r" represent ?
     
  11. Aug 15, 2015 #10
    the distance from where the electric potential is 0 to where you want to measure the electric potential.
     
  12. Aug 15, 2015 #11

    haruspex

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    No. Check your course notes.
     
  13. Aug 16, 2015 #12
    that's what I have written in the manual. Electric potential V = Q/(4*pi*epsilon) * 1/r. Should I upload a picture of the manual?
     
  14. Aug 16, 2015 #13

    haruspex

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    I'm not querying your equation. It's your definition of r that is wrong.
     
  15. Aug 16, 2015 #14
    then I'm lost. whats the definition? I always tought it is the distance between the place where electric potential is 0 to where you want to measure the electric potential..
     
  16. Aug 16, 2015 #15

    haruspex

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    The equation you quoted can be applied in two contexts.
    In the most basic one it gives the potential at distance r caused by a point charge Q, if taking the potential at infinity to be zero.
    It also works if the charge is spread uniformly over a spherical shell or through a solid sphere. In these cases, r is the distance from the centre of the sphere, and the equation only works if r is greater than the radius of the sphere.
     
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