# Potential of a dielectric sphere and point charge

I've seen in Jackson's 4.9 problem and also in a similar example given in the book (pag. 157) they use the azimuthal symmetry solution for Laplace equation which gives the ordinary Legendre polynomials. The 4.9 problem states:

Find a potential at all points in space for a point charge $$q$$ located in a free space a distance $$d$$ from the center of a solid dielectric sphere of radius $$a$$ $$(a and dielectric constant $$ϵ/ϵ_0$$.

And part of the solution is to find the potential inside and outside the sphere:

$$\Phi_{\mathrm{in}}(\vec{r})=\sum_{l=0}^{\infty} A_{l} r^{l} P_{l}(\cos \theta)$$ $$\Phi_{\text {out }}(\vec{r})=\Phi_{q}+\sum_{l=0}^{\infty} B_{l} r^{-(l+1)} P_{l}(\cos \theta)$$

where $$\Phi_{q}$$ is the potential produced by the point charge.

Why the azimuthal symmetry is assumed? Why not working with the general solution involving spherical harmonics?

• If the point charge is located on the $z$ axis you will have symmetry in $\phi$. Do you agree? Jan 20 at 22:46