An electric dipole is in a uniform electric field For which

An electric dipole is in a uniform electric field. For which orientation is the potential energy of the dipole least? Explain your answer.

A) The positive end of the dipole is in the direction of the electric field.

B) The line connecting the two charged particles is perpendicular to the electric field.

C) The negative end of the dipole is in the direction of the electric field.

Solution

the energy of an electric dipole with dipole moment p in an electric field E is

U=pE=|p||E|cos

This expression is minimized when cos=1, which is when the angle between the dipole moment vector (which for a physical dipole points from the negative to the positive charge) and the field is 0. This is precisely the condition for the dipole to be aligned with the electric field. Ans: A) The positive end of the dipole is in the direction of the electric field