f the molecule moves as you predict in 4), decide if the electric potential energy of each atom increases, decreases, or stays the same as it moves. (Hint: consider the electrostatic potential. What happens to the potential as you move farther from the plate? What happens when you move closer to the plate?) 6) For an electric dipole (such as the polar molecule shown) we can define an electric dipole moment, . For a collection of charges the dipole moment vector is defined as: p = =1 , where r is a position vector of charge . The direction of is such that it points from the center of negative charge of the molecule toward the center of positive charge of the molecule. Draw the molecule to the left of the plate and sketch a reasonable electric dipole moment vector, . (N.B.: If you have taken chemistry, this definition may seem backwards to you. This is because chemists, who are interested in the behavior of negatively charged electrons, define the dipole moment differently from physicists!) 7) In working with electric dipoles, we often use energy rather than forces. The potential energy of an electric dipole in an electric field is: PEdipole = |p ||E | cos , where is the angle between the p vector and the local electric field E . Based on this equation, what it the lowest energy orientation of the water molecule? What is the highest energy orientation? Does this result make sense based on what you know about the electrostatic potential? Does this result match the answers you found for 4-6? Whole Class Discussion + +