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Homework Help: How do I calculate the electric potential

  1. Apr 22, 2007 #1
    How do I calculate the electric potential "V" of a point charge "q" at radius "r" if I am told that the electrical potential is zero at a distance "d" other than infinity? I believe that there is some arbitrary constant that must be found but I don't know if, where, or how that fits in.

    Formulas I know:
    V=K(q/r)
    V=U/Q (where U is the potential energy)

    Thank you very much if you can help!
     
  2. jcsd
  3. Apr 22, 2007 #2
    If V at r = (Kq)/r, and at infinity V = 0. What is the difference ? Ask yourself why at infinity, the V = zero.
     
    Last edited: Apr 22, 2007
  4. Apr 22, 2007 #3
    The V=0 at infinity because as r goes to infinity, the function becomes infinitely small.

    But what if V=0 at 1meter? Then how would I find V at .5 meters? for some reason the equation V=K(q/r) doesn't work by itself in this case. The only way I can reconcile this is if there is some other part to the equation that we leave out because V=0 is usually at infinity. Any ideas?!
     
  5. Apr 22, 2007 #4
    Is this a book question or are you having a problem with a concept? Where did you get V = 0 at 1, at r =1 ? Doesn't V = 0 at r =1 a contradiction if the equation is Kq/r? wouldn't V = kq at 1?
     
    Last edited: Apr 22, 2007
  6. Apr 22, 2007 #5
    Exactly my problem. My professor assigned a few problems where V=0 at different distances (not infinity). Supposedly it doesn't matter where you set the zero of electric potential (just like it really doesn't matter where you choose the zero of gravitational potential), I just can't figure out how the equation should be set up.
     
  7. Apr 22, 2007 #6
    This is how you find teh potential due to a uniformaly charged sphere, or simply a charge :

    DV=change in V= potential difference=V2-V1=(Kq)/r2 - (Kq)/r1
    = (Kq)[1/r2 - 1/r1]
    at r1= infinity , V1= 0 Which is how you get a voltage value at a distance outside a charge, you assume r1 is infinate distance. If you really say your teacher said you can get a zero voltage. My oh my:frown:
     
    Last edited: Apr 22, 2007
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