1. -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!

2. -q+q

4. http://phet.colorado.edu/en/simulation/balloons

6. Why does the charged balloon stick to the neutral wall? http://phet.colorado.edu/en/simulation/balloons

7. Neutral atom + + -

8. The atom becomes polarized. + + -

9. + + -

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

11. The dipole makes an electric field at the charge. + + - What is the force on the charge?

12. + + -

13. + + - Substitute…

14. + + -

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

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

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

18. 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 ).

19. 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.

20. 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.

21. 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.

22. We will prove these later, using Gauss’ Law

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

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

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

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

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

28. Water Balloon - - - - - - - - + - + - + - + - + - + - + - + - + - + -

29. Water Balloon - - - - - - - - + - + - + - + - + - + - + - + - + - + - Water is polarized by rotating the molecules.