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Homework Help: Electrostatic energy of a dielectric sphere

  1. Oct 10, 2009 #1
    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= [tex]\int \vec{D}.\vec{E} dV[/tex]), but only for those points in space where r>R. Then, given that D=[tex]\epsilon[/tex]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.
  2. jcsd
  3. Oct 11, 2009 #2
  4. Oct 11, 2009 #3
    Does anybody know how I can get P of polarization?
  5. Oct 12, 2009 #4


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    Homework Helper
    Gold Member

    How are you calculating the displacement field? And why can't you calculate it for [itex]r\leq R[/itex]?

    doesn't [itex]\epsilon=\epsilon_0[/itex] for [itex]r>R[/itex]?

    You've already given the reason why.....just assume that neoprene is a linear dielectric, then you know [itex]\textbf{D}=\epsilon\textbf{E}[/itex], so if [itex]\textbf{D}=0[/itex], then so does the electric field!
  6. Oct 12, 2009 #5
    Yes, thank you. I thought I had to find the polarization vector P.
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