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Homework Help: Divergence and Curl

  1. Mar 5, 2009 #1
    My notes say that if we know the divergence and curl of a field then that uniquely determines the field.

    Can somebody give me an example of how, given only the div and curl of a field, we can deduce the field?

    I considered the electric field where we have,
    [itex]\nabla \cdot \vec{E}=\frac{\rho}{\epsilon_0}, \nabla \times \vec{E}=0[/itex]
    but we can't actually establish E using only vector calculus can we? we need other techniques do we not? perhaps i'm just being silly?
    Last edited: Mar 5, 2009
  2. jcsd
  3. Mar 5, 2009 #2
    I think the boundary condition needs to be specified.
  4. Mar 5, 2009 #3
    The electrostatic case is easy to understand. To find the electric field at a point, you need to integrate the electric field contributions from each infinitesimal charge. Since the local charge density is found from the divergence of the E-field, you are essentially integrating the divergence over the volume of interest.

    In practice, the charge density is not always known a priori. Sometimes the electric field or potential is specified along some boundary, which is where Neumann and Dirichlet boundary conditions come into play.
  5. Mar 6, 2009 #4
    yeah. can you just confirm a couple of things:

    (i) the RHS of the divergence equation is easy enough to integrate given a charge density. To integrate the LHS however, I would use divergence theorem and then take it from there using an appropriate Gaussian surface yes?

    (ii)In the method you just gave for finding the E field in post 3, you have only used the divergence equation, how would the curl one come into play?

  6. Mar 6, 2009 #5
    to make sure it's electrostatic
  7. Mar 6, 2009 #6
    i.e. a non zero curl results in a non continuous tangential component of electric field at a conductor's surface and so the tangential component of electric field outside the conductor isn't necessarily 0 (as is guaranteed if the curl were 0) and so charges are moving on the surface - hence it's no longer electrostatics!

    is that what you mean?
  8. Mar 7, 2009 #7
    em...i would rather say a nonzero curl is resulted from a changing magnetic field, by faraday's law.So it won't be not electrostatics.
  9. Mar 7, 2009 #8
    Actually electric fields and magnetic fields are the same thing if you consider standing still the same as moving in the time dimension.
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