2 point charges with sphere inbetween

Cairrd
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I am having a little trouble knowing how to approach the following question:

Twi point charges q and -q are placed 2d apart. Show that a conducting sphere of small radius r (where r << 2d) is placed midway between the charges, show the force on each is increased by a factor of approx (1 + 16 (r^3/a^3)).



Anyone want to help me here?

Thanks
 
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Use the method of images on the conducting sphere, and then treat the two images as a dipole when you calculate the total force on one charge due to the other charge and the image charges.

Also, I assume you mean d, not a, in your factor.
 
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