Two spherical, nonconducting, and very thin shells of uniformly distributed positive charge Q and radius d are located a distance 10d from each other. A positive point charge q is placed inside one of the shells at a distance d/2 from the center, on the line connecting the centers of the two shells, as shown in figure. What is the net force on the charge q?
A. 
to the left
to the left
B.
to the right
C.
to the leftD.
to the right
E.
to the left
(GR0177 #87)
Solution:
Inside thin shell, no charge → E = 0
Outside thin shell, E = kQ/r2 where k = 1/4πɛ0
Thus, we just have to consider the force exerted by the opposite sphere, to the left.
r = 10d − d/2 = 19d/2
E = kQ/(19d/2)2 = 4kQ/361d2 = Q/361πɛ0d2
F = qE = qQ/361πɛ0d2
Answer: A
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