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quasar_4
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Homework Statement
I am trying to find the force acting on a charge placed a distance r0 from the center of a spherical cavity of radius a. The entire cavity is immersed in a dielectric material, epsilon2.
Homework Equations
The Attempt at a Solution
Here's my quandary: I can find the field inside the cavity with no charge. This is fine. But now that there's this point charge floating around, I have
[tex] \nabla \cdot D_{in} = \rho [/tex]
[tex] \nabla \cdot D_{out} = 0 [/tex]
Well, the form of the potential is obviously
[tex] \Phi_{in} = \sum_{l=0}^{\infty} A_l r^l P_l(\cos{\theta}) [/tex]
[tex] \Phi_{out} = -E0 r \cos{\theta}+ \sum_{l=0}^{\infty} B_l r^{-(l+1)} P_l(\cos{\theta}) [/tex]
where E0 is the applied field in the dielectric (without loss of generality, we can choose E0 to be in the z-direction).
I applied boundary conditions -- the normal component of D and the tangential component of E should be continuous at r = a (because there's no charge *there*). You get some nice equations, which you can easily solve for the coefficients A1 and B1 (all other terms in the Legendre polynomials vanish except l =1). I get:
[tex] A_1 = \frac{-3 \epsilon_2 E0}{2 \epsilon_2+\epsilon_0} [/tex]
[tex] B_1 = \frac{E0 a^3(\epsilon_0-\epsilon_2)}{2 \epsilon_2 + \epsilon_0} [/tex]
Great, I can plug these into my potential expressions...
But I don't see how my potential is even affected by this new charge q! I get the same result as for an empty cavity. That can't be right. So, do I need to use a totally different technique? I thought maybe the method of images is the way to go, but I don't understand how this works for a spherical set-up... where's the image charge if q is inside a sphere?
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