Problem

(a) Using the law of cosines, show that Eq. 3.17 can be written as follows: whe...

(a) Using the law of cosines, show that Eq. 3.17 can be written as follows:

where r and θ are the usual spherical polar coordinates, with the z axis along the line through q. In this form, it is obvious that V = 0 on the sphere, r = R.

(b) Find the induced surface charge on the sphere, as a function of θ. Integrate this to get the total induced charge. (What should it be?)

Reference equation 3.17

Step-by-Step Solution

Solution 1

Surface charge density on a solid is the total amount of charge per unit area.

The expression for potential using law of cosines as follows:

Here, r and are the spherical polar coordinates, q is the charge, is the permittivity, and R is the radius of sphere.

(a)

Picture 1

From the fig,

We know that,

Substitute for.

The potential of this configuration is,

Substitute for, for r, and for.

Substitute for b.

The potential is zero when r=R.

Therefore, the potential is zero when r=R.

(b)

Induced surface charge density is,

But

Substitute for V.

Induced surface charge is,

Substitute for.

For a>R,

Substitute for and for.

(c)

The force of image charge on q is,

Substitute for b and for.

The work done is,

Substitute for F.

Therefore, the energy of this configuration is.

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