-q+q s We can define a vector called the dipole moment ( 电矩 ). Magnitude: Direction: from the negative (-) charge to the positive (+) charge. NOTE: This p has nothing to do with momentum!

-q+q

Why does the charged balloon stick to the neutral wall?

Neutral atom + + -

The atom becomes polarized

+ + -

We can approximate the polarized atom as a dipole This is the polarizability, which is different for every material.

The dipole makes an electric field at the charge What is the force on the charge?

+ + -

+ + - Substitute…

+ + -

+ + - The charge is attracted to the polarized atom, no matter if the charge is positive or negative!

Wall Balloon (1) The charges on the balloon polarize the wall.

Wall Balloon (2) The polarized atoms in the wall attract the balloon.

The wall and the balloon are both insulators ( 绝缘材料, or 电价值 ). In an insulator, the charges are not free to move around. Atoms can be polarized, but they do not change their position. A material in which charges are free to move around is called a conductor ( 导体 ).

Example: Salt water (ionic solution 离子溶液 ) At equilibrium, the net field in a conductor must be zero. The ions move with a speed v = uE, where u is called the mobility.

Example: A metal In a metal, one or two electrons from every atom is free to move around. They form a sea of electrons. The metal becomes polarized.

Example: A metal In a metal, one or two electrons from every atom is free to move around. They form a sea of electrons. Net field inside is zero. The metal becomes polarized.

We will prove these later, using Gauss’ Law

Permanent dipoles in an electric field + - Water is a permanent dipole O H H

+ - In a uniform field, the net force on a dipole is zero:

+ - But the net torque around the dipole’s COM is not zero:

+ - This torque will rotate the dipole until it is parallel with the field.

+ - The dipole-field system has a potential energy:

Water Balloon

Water Balloon Water is polarized by rotating the molecules.