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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a) Using the law of cosines, show that Eq. 3.17 can be written as follows:
Where and are the usual spherical polar coordinates, with the z axis along the
line through . In this form, it is obvious that on the sphere, .
b) Find the induced surface charge on the sphere, as a function of . Integrate this to get the total induced charge. (What should it be?)
c) Calculate the energy of this configuration.
A stationary electric dipole is situated at the origin. A positive
point charge q(mass m) executes circular motion (radius s) at constant speed
in the field of the dipole. Characterize the plane of the orbit. Find the speed, angular momentum and total energy of the charge.
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.
In Prob. 2.25, you found the potential on the axis of a uniformly charged disk:
(a) Use this, together with the fact that to evaluate the first three terms
in the expansion (Eq. 3.72) for the potential of the disk at points off the axis, assuming .
(b) Find the potential for by the same method, using Eq. 3.66. [Note: You
must break the interior region up into two hemispheres, above and below the
disk. Do not assume the coefficients are the same in both hemispheres.]
Two long straight wires, carrying opposite uniform line charges,are situated on either side of a long conducting cylinder (Fig. 3.39). The cylinder(Which carries no net charge) has radius ,and the wires are a distance from the axis. Find the potential.

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