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Homework Help: Magnetic Field and Poynting Flux in a Charging Capacitor

  1. Dec 10, 2009 #1
    This is a two part question. I completed the 1st part, but im having a difficult time on the second part. I have one try remaining.

    Magnetic Field and Poynting Flux in a Charging Capacitor-

    When a circular capacitor with radius and plate separation is charged up, the electric field , and hence the electric flux , between the plates changes. According to Ampère's law as extended by Maxwell, this change in flux induces a magnetic field that can be found from

    integral of B * dl = Mu 0 (i + Epsilon 0 (delta flux/delta t)) = ampere maxwell law

    We can solve this equation to obtain the field inside a capacitor:

    B(r) = Mu 0 (ir/2piR^2) theta

    where r is the radial distance from the axis of the capacitor.

    Part A.) Find an expression for the magnitude of the Poynting vector S on the surface that connects the edges of the two circular plates.

    S = 1/Mu 0 (E X B) = S(t) = (i^2/(2pi^2R^3epsilon 0))t

    Part B.) Calculate the the total amount of energy U that flows into the space between the capacitor plates from t= 0 to t= T, by first integrating the Poynting vector over the surface that connects the edges of the two circular plates, and then integrating over time.

    Here is where im unsure where to go.

    I believe we will need to integrate S*Area, where the area is 2piRd, twice like they said, once respecting to the d distance, and again w/ respect to time. Im having a tough time with the setup, and which we are constants/variables in the integration.

    Thanks in advance

    Attached Files:

  2. jcsd
  3. Dec 10, 2009 #2


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    Homework Helper

    Is the current i supposed to be constant? If so, since S(t) = (i^2/(2pi^2R^3epsilon 0))t represents energy flow per unit area, multiplying it by 2piRd would give you the rate of energy flow. Integrating the resulting equation with respect to t would give you U.
  4. Dec 10, 2009 #3

    Yes, thats what they want us to do, but first integrate respect to distance, then integrate again with respect to time. Im confused how to go about this. What do i integrate with respect to for the two different integrations?
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