Magnitude of dipole moment

1. Jan 25, 2010

nothingatall

1. The problem statement, all variables and given/known data
A certain electric dipole is placed in a uniform electric field E of magnitude 20 N/C. Figure 22-59 gives the potential energy U of the dipole versus the angle θ between E and the dipole moment P. What is the magnitude of P?

2. Relevant equations
U=p(dot)E

3. The attempt at a solution
Somehow there is an integral needed to solve this but I don't know where to start. there is also a graph with this question.

2. Jan 25, 2010

minimark1234

i thought...
$$U=-\vec{p} \cdot \vec{E}$$
but idk if that really changes much.

3. Jan 26, 2010

nothingatall

It does, i was just forgetting to multiply it by the factor of 10^-26 hehe. Thanks.

4. Feb 10, 2011

slamin27

can't you just take Us to be the maximum, and the angle of 180 to be the angle that corresponds to the maximum potential, and plug these into U=-pEcos(theta), along with the magnitude of E?