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Homework Help: Electric potential

  1. Nov 30, 2009 #1
    1. The problem statement, all variables and given/known data
    What is the electric potential a distance x above the center of a ring of charge Q with a
    radius R? Derive the electric field from this relationship.

    2. Relevant equations

    V=k_e[tex]\int[/tex]dq/r = k_e[tex]\int[/tex]dq/([tex]\sqrt{}[/tex]a^2+x^2)

    3. The attempt at a solution

    V= k_e/([tex]\sqrt{}[/tex]a^2+x^2)[tex]\int[/tex] dq=k_eQ/([tex]\sqrt{}[/tex]a^2+x^2)

    E_x=-dV/dx= -k_eQ(d/dx)(a^2+x^2)^-1/2
    = -k_e(-1/2)(a^2+x^2)^-3/2(2x)
    E_x= k_ex/(a^2+x^2)^3/2 Q

    Is this right? If not what is?
  2. jcsd
  3. Nov 30, 2009 #2


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    Science Advisor
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    Gold Member

    Your expressions are correct although, strictly speaking, the given radius of the ring is R not a.
  4. Nov 30, 2009 #3
    opps...I think I was looking at another problem when I wrote that. Thanks
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