Chapter 3: Q3.7P (page 129)
Find the force on the charge in Fig. 3.14. (The plane is a grounded conductor.)

Short Answer
Answer
The net force on q is .
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Chapter 3: Q3.7P (page 129)
Find the force on the charge in Fig. 3.14. (The plane is a grounded conductor.)

Answer
The net force on q is .
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Show that the average field inside a sphere of radius R, due to all the charge within the sphere, is
Where is the total dipole moment. There are several ways to prove this delightfully simple result. Here's one method:
(a) Show that the average field due to a single chargeqat point r inside thesphere is the same as the field at r due to a uniformly charged sphere with
, namely
Where r is the vector from r to
(b) The latter can be found from Gauss's law (see Prob. 2.12). Express the answerin terms of the dipole moment of q.
(c) Use the superposition principle to generalize to an arbitrary charge distribution.
(d) While you're at it, show that the average field over the volume of a sphere, dueto all the charges outside, is the same as the field they produce at the center.
A sphere of radiusR,centered at the origin, carries charge density
where k is a constant, and r, are the usual spherical coordinates. Find the approximate potential for points on the z axis, far from the sphere.
Solve Laplace's equation by separation of variables in cylindrical coordinates, assuming there is no dependence on z (cylindrical symmetry). [Make
sure you find all solutions to the radial equation; in particular, your result must accommodate the case of an infinite line charge, for which (of course) we already know the answer.]
A more elegant proof of the second uniqueness theorem uses Green's
identity (Prob. 1.61c), with . Supply the details.
In Ex. 3.2 we assumed that the conducting sphere was grounded ( ). But with the addition of a second image charge, the same basic modelwill handle the case of a sphere at any potential (relative, of course, to infinity). What charge should you use, and where should you put it? Find the force of attraction between a point charge q and a neutral conducting sphere.
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