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Homework Help: Magnetic dipole moment help

  1. Dec 25, 2006 #1
    1. The problem statement, all variables and given/known data
    A phonograph record of radius R carrying a unifrom surface charge sigma is rotating at a cosntant angular velocity omega. Find its magnetic dipole moment


    2. Relevant equations
    [tex] m = int I \bullet da [/tex]


    3. The attempt at a solution
    Need to find the current first

    well the sruface charge density is
    [tex] K = \sigma \omega R [/tex]

    [tex] K = \frac{dI}{dl_{perp}} [/tex]

    where dl perp is the infinitesimal widthruning parallel to the flow of current
    so here dl perp refers to circumference
    [tex] I = \int K \bullet 2 \pi r dr [/tex]??

    is this even close to being right??
    I fear i may have made some geomterical error..

    thanks for your help!
     
    Last edited: Dec 25, 2006
  2. jcsd
  3. Dec 25, 2006 #2
    ahh i got the answer using the equation

    [tex] \vec{m} = \frac{1}{2} \int_{S} \vec{r}' \times \vec{K}(\vec{r'}) da' [/tex]

    [tex] da = r dr d\theta [/tex]

    thanks anyway!
     
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