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!

Homework Help: Potential of a dipole in E field

  1. Oct 5, 2013 #1
    1. The problem statement, all variables and given/known data

    I'm going over some EM notes and I found a derivation for the potential energy of a dipole in an E field which first found the torque on the dipole and then integrated.
    I had a go at a derivation that goes the other way, starting from the potential of the 2 charges. I've ran into a bit of a problem midway through and I can't see how to get past it.

    2. Relevant equations

    Want to show:
    U = -p.E

    3. The attempt at a solution
    U = q(VB-VA)

    now I don't know how to get from here to what I want to show. Both of the potentials look like [itex]\frac{q}{4πε0r}[/itex] , where the r is different in either potential.
    The only thing I can think of is to use an approximation to r, but that doesn't give an acosθ term in the numerator.
  2. jcsd
  3. Oct 6, 2013 #2


    User Avatar
    Homework Helper

    The charges of the dipole are in an external electric field. By changing the angle, that potential energy gained from the field changes, the potential energy of their interaction does not. You ignored the contribution of the external field to the potential energy.

    As for the potential energy of the interaction between two point charges, it is [itex]\frac{q_1 q_2}{4 \pi \epsilon_0 r}[/itex] where r is the distance between the charges.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted