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Question about the E-field along center axis of charged ring

  1. Feb 7, 2014 #1

    I'm having trouble understanding the derivative notation. I know that "dE" and dQ" have to do with the derivative, but what exactly does that mean in the context of this problem? For some reason I find it difficult to understand why this derivative notation is used in so many physics questions. How do I know what variables have derivative notation?
  2. jcsd
  3. Feb 9, 2014 #2


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    Hi Loopas! :smile:
    It isn't a derivative … in fact, you're going to integrate it.

    dq is the charge on a tiny piece of the ring.

    This is a standard procedure: you split the body into slices that are so small that you can regard all measurements on them as constant.

    (for example, if you know the density of a body as a function of position, you say consider a small element of volume dxdydz, it can be assumed to have a constant density ρ, so the mass is ρdxdydz, and the total mass is ∫ ρdxdydz)

    In this case, the charge is dq, the total charge is ∫ dq, the Ex for that charge is kxdq/(x2+a2)3/2, and so the total Ex is ∫ kxdq/(x2+a2)3/2 :wink:

    (btw, in this case, all measurements on the small charge really are constant, so ther's actaully no assumption here … but usually there is)
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