# Potential and torque of electric dipole in E field

1. Sep 10, 2011

### nthnschager

1. The problem statement, all variables and given/known data

Q: A polar molecule has a dipole moment of magnitude 20 e*pm that makes an angle of 20 degrees with a uniform electric field of magnitude 3.0*10^3 N/C. FInd the magnitude of the torque on the dipole, and the potential energy of the system.

2. Relevant equations

torque = pE sin(theta)

where p is dipole moment and E is electric field mag.

3. The attempt at a solution

in the explanation in my book, (20 e*pm)(3*10^3)(sin20)
is reduced to (.02)(1.6*10^-19 C)(10^-9 m)(3*10^3 N/C)(sin20)
This might be a stupid question but why is 20 reduced to .02? I could not figure out what happened there.

Also, how do you minimize the potential energy when a dipole is is in an electric field? when U = -PEcos(theta), where U is potential energy, P is dipole moment, E is electric field, and theta is angle between direction of dipole moment and electric field. is pot. energy minimized when theta = zero or when theta = ninety?
In other words, I do not understand the concept of negetive potential energy. Would it be considered "minimized" when the potential energy is at zero or negetive?

Last edited: Sep 10, 2011