1. Chapter 24 Electric Fields 第二十四章 電場

2. Action at a distance q1q1 q1q1 q2q2 q2q2

3. Another point of view -- Fields where E 2 is the electric field generated by charge q 2 and r is a position vector from the position of q 2 to any point in space.

4. Electric field lines

5. The concept of electric field lines was originally due to Faraday who envisioned that the space around a charge is filled with “lines of force”. The field lines are originated from positive charges and terminated at negative charges, and they are unbroken in vacuum.

6. The relationship between the electrical field and field lines Electric field lines with arrows indicate the local field directions, and their density in a plane perpendicular to the local field represents the magnitude of the local field. Suppose that there are N field lines originated from a positive charge q, then we have the density of field lines given by N / 4πr 2 at a distance r from the charge. Note that if N = q / ε 0 then we recover E = q / 4 πε 0 r 2.

7. Field lines repel each other

8. Field lines are smooth

10. The electric field due to a point charge

11. Sample problem What is the electric field at point P ?

12. Sample problem What is the electric field at point P ?

13. The electric field due to an electric dipole x y -a-aa P Electric dipole moment:

14. The electric field due to an electric dipole Along x-axis: For

15. The electric field due to an electric dipole In y-z plane: Note that for net charge  0 for net charge  0 but net dipole moment  0

16. Electric field due to a line of charge

20. Electric field due to a charge disk

21. A point charge in an electric field Millikan oil-drop experiment (1910-1913)

22. Cathode-ray tube

23. Deflecting charge particles

24. A dipole in an electric field

25. There is no net force on a dipole. But there is a torque.

26. Potential energy of a dipole in a uniform electric field

27. Home work Question ( 問題 ): 6, 8, 19 Exercise ( 練習題 ): 5, 10, 16 Problem ( 習題 ): 8, 18, 31, 32