Energy of a Dipole in a Constant E Field

[saxyliz]

1. The problem statement, all variables and given/known data
The graph shows the potential energy of an electric dipole which is in a constant electric field; only the electric force is acting on the dipole. Consider a dipole that oscillates between plus and minus 65 degrees.

http://session.masteringphysics.com/problemAsset/1000040726/3/knight_Figure_29_10.jpg

a) What is the dipole's mechanical energy?
b) What is the dipole's kinetic energy when it is aligned with the electric field?

2. Relevant equations
U = -pEcos\theta


3. The attempt at a solution
I know that when the graph is at the trough it's -pE and at it's peak it's +pE. But I don't know how to factor in the 65 degrees. Please help!

[rl.bhat]

At 180 degrees U = 2 μJ. Substitute in the relevant equation and find pE.
At θ = 0, all the potential energy in converted to K.E. Similarly you can find P.E. at 65 degrees by substituting the value of pE and θ in the equation.

[saxyliz]

At 180 degrees U = 2 μJ. Substitute in the relevant equation and find pE.
At θ = 0, all the potential energy in converted to K.E. Similarly you can find P.E. at 65 degrees by substituting the value of pE and θ in the equation.

Thank you so much! That really cleared it up for me!