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Homework Help: Charged Sphere with off-center cavity Electric field

  1. Jan 24, 2012 #1
    1. The problem statement, all variables and given/known data
    Consider a sphere uniformly charged over volume, apart from a spherical
    off-center cavity. The charge density is ρ, radius of the sphere is a, radius of the cavity is
    b, and the distance between the centers is d, d < a-b. (a) Find the total charge and the
    dipole moment (with respect to the center of the large sphere) of this configuration. (b) Use
    superposition principle to find the electric field inside the cavity. (c) Show that far from the
    sphere the field is that of a charge plus dipole correction. Check that the charge and the
    dipole moment correspond to that of part (a).

    2. Relevant equations

    E_sphere=Qr/4piεR^2 for r<R
    superposition principle

    3. The attempt at a solution
    total charge I'm fairly certain is (4/3)piρ(a^3-b^3) just the large sphere minus the cavity.
    The dipole moment i attempted to use a sum p=q_a(0-0)+q_b(d-0) and got

    p=(4/3)pi*ρ*b^3*d (from the center of the cavity towards the center of the large sphere)

    for b) I tried to find the Electric field due to the large sphere ((4/3)piρa^3)*(r/4piεa^2) and the field from the small sphere ((4/3)piρb^3)*(r/4piεb^2) but im not sure what coordinate system i should be using, nor how to superimpose/sum the fields correctly.

    c) We were not taught nor can i find anything in the book about a dipole correction, so I'm lost for this part.
  2. jcsd
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