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Homework Help: Finding and using the Poynting vector

  1. Dec 2, 2008 #1
    1. The problem statement, all variables and given/known data

    Hi, this is a repeat post, I fear I put it in the wrong forum to start with (I put it in advance physics and I think it may be more of an introductory physics question). I'm really sorry if this contravenes any of the rules.

    A fat wire, radius a, carries a constant current I, uniformly distributed over it's cross section. A narrow gap in the wire, of width w<<a, forms a parllel-plate capacitor (see attachment, this question refers to question 3, sorry I couldn't cut and paste the picture).

    a)Find the electrical and magnetic fields inside the gap, as functions of the distance s from the axis and the time t. Assume the charge is zero at time t=0.

    b)Find the energy density uem in the gap and the Poynting vector, S in the gap. Note especially the direction of S

    c)Determione the total energy in the gap, as a function of time. Calculate the total power into the gap, by integrating the Poynting vector of the appropriate surface. Check that the power inout is equal to the rate of increase of energy in the gap.

    2. Relevant equations

    In the following:
    s=distance from the axis of the wire
    a=radius of the wire
    and I'm working in cylinrical polars.

    For a parallel plate capacitor,


    For an amperian loop (with Maxwells fix),

    [tex]\oint\underline{B}\cdot d\underline{l}=\mu_{0}I_{enc}+\mu_{0}\epsilon_{0}\int\frac{\partial\underline{E}}{\partial t} \cdot d\underline{a}[/tex]

    Energy density,


    Poynting Vector,

    [tex] \frac{1}{\mu_{0}}\underline{E}\times\underline{B}[/tex]

    3. The attempt at a solution

    For part a) I found the electric to be,


    and the magnetic field to be,

    [tex] \underline{B}=\frac{\mu_{0}}{2\pi}\frac{Is}{a^2}\widehat{\phi}[/tex]

    For part b) I found the energy density to be,


    and the Poynting vector to be,

    [tex]-\frac{\mu_{0}}{2\pi \epsilon_{0}}\frac{I^{2}t s}{a^{2}}\widehat{s}[/tex]

    For part c) I have no idea even where to begin.

    Please could someone tell me if parts a) and b) are correct and give me a hand for part c)? If you want me show more working just ask (I left it out to keep the post a bit smaller).

  2. jcsd
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