1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Parallel plate capacitor with layers of dielectrics in between

  1. Aug 14, 2014 #1


    User Avatar
    Gold Member

    Consider a parallel plate capacitor with layers of dielectric between its plates somehow that the interfaces between them are parallel to the plates of the capacitor. If the surface charge density on the plates of the capacitors be [itex] \sigma [/itex] , gauss's law gives [itex] D=\sigma [/itex] which is the same inside all dielectric layers.
    But as boundary conditions for interfaces between dielectrics, we have [itex] | D_i-D_j |=\sigma_b^{ij} [/itex] and the fact that the displacement field is the same inside all dielectrics, gives [itex] \sigma_b^{ij} =0 [/itex]. But I know that there should be a surface density of bound charges on the interfaces which tells me something is wrong in the above argument.
    What is that?
  2. jcsd
  3. Aug 14, 2014 #2


    User Avatar
    Science Advisor
    Gold Member

    The divergence of the displacement field gives you the free charge, not the bound charge. So the continuity of the normal displacement field tells you that there is no free charge at the interface, not bound charge.

    The bound charge is the negative divergence of the polarization density, the difference between the vacuum displacement field and the displacement field.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook