# Electrostatic energy of a dielectric sphere

by libelec
Tags: dielectric, electrostatic, energy, sphere
 P: 176 1. Find the electrostatic energy of a neoprene sphere or ratio R, charged with Q if: a) Q is uniformly distributed in surface 3. The attempt at a solution So, I can calculate the displacement field (so that I can use the expresion U= $$\int \vec{D}.\vec{E} dV$$), but only for those points in space where r>R. Then, given that D=$$\epsilon$$E, I can find the electric field for r>R. But I don't know how to get the electric field inside the sphere, since D=0 there because there're no free charges inside. It could also happen that E doesn't exist inside the sphere either, but I woundn't know the reason why. Any ideas? Thanks.
 P: 176 Anybody?
 P: 176 Does anybody know how I can get P of polarization?
HW Helper
P: 5,003
Electrostatic energy of a dielectric sphere

 Quote by libelec [ So, I can calculate the displacement field (so that I can use the expresion U= $$\int \vec{D}.\vec{E} dV$$), but only for those points in space where r>R.
How are you calculating the displacement field? And why can't you calculate it for $r\leq R$?

 Then, given that D=$$\epsilon$$E, I can find the electric field for r>R.
doesn't $\epsilon=\epsilon_0$ for $r>R$?

 But I don't know how to get the electric field inside the sphere, since D=0 there because there're no free charges inside. It could also happen that E doesn't exist inside the sphere either, but I woundn't know the reason why.
You've already given the reason why.....just assume that neoprene is a linear dielectric, then you know $\textbf{D}=\epsilon\textbf{E}$, so if $\textbf{D}=0$, then so does the electric field!
 P: 176 Yes, thank you. I thought I had to find the polarization vector P.

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