(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
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)
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,
The work done is,
Substitute for F.
Therefore, the energy of this configuration is.