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Homework Help: Taking small element for integration purpose in SOLID sphere?

  1. Nov 28, 2013 #1
    1. The problem statement, all variables and given/known data

    The Question originally is to find the m of a solid uniformly charged solid sphere which is rotating uniformly with ω

    2. Relevant equations

    Now my question to you is how to take the small element?

    3. The attempt at a solution

    i take a small disc with radius rsinθ.

    Now a= ∏(rsinθ)^2

    i= q/t= dq/(2∏/w)

    where dq= rho times dv= 2∏rsinθ times rdθ.

    now this gives the relevant answer but why is it that i cant take a small element of a small sphere?

    because i cant decribe m for that too?

    It's just that i dont recall taking a disc as a small element in case of solid sphere :~(
  2. jcsd
  3. Nov 28, 2013 #2

    rude man

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    Please quote the problem verbatim. If it's not in English then I would need a better translation.
    What is m? Usually mass, obviously not in this case.
  4. Nov 28, 2013 #3
    I am so sorry for not posting here accurately.

    The question is to find the magnetic dipole moment of a solid sphere rotating about its axis with w angular velocity. The total charge on the sphere, Q is uniformly distributed@ rho ρ volumetric distribution.

    It's really not the doubt actually.

    It's actually about the small element which must be taken which i guess is quite important considering if we want to find the com of a solid hemisphere.right?
  5. Nov 29, 2013 #4

    rude man

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    Looks like a double integration is necessary.

    Consider a thin slice dz of the sphere with normal along the spin axis = z axis. The sphere's center is at the origin of an xyz coordinate system. Then consider a thin annular volume of radius r, width dr and thickness dz within this slice.

    What is the differential current di due to this annular volume? Then, what is the differential magnetic moment dm due to this di?

    dm now needs to be integrated from r = 0 to r = R where R is the radius of the slice, giving a magnetic moment dμ.

    Note that R = R(z): R(+/-a) = 0 and R(0) = a where a is the radius of the sphere.

    That takes care of the slice of thickess dz and radius R.

    Now you have to do a second integration along z from z= -a to z = a, adding all the dμ. Then you have integrated all the dμ moments into one magnetic moment μ.
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