1. Chapter 22 Electric Fields
(22-1)
Chapter Electric Fields
In this chapter we will introduce the concept of an electric field As long as charges are stationary Coulomb’s law described adequately the forces among charges. If the charges are not stationary we must use an alternative approach by introducing the electric field (symbol ). In connection with the electric field, the following topics will be covered:
-Calculate the electric field generated by a point charge Using the principle of superposition determine the electric field created by a collection of point charges as well as continuous charge distributions Once the electric field at a point P is known we will be calculate the electric force on any charge placed at P Define the notion of an “electric dipole”. Determine the net force , the net torque, exerted on an electric dipole by a uniform electric field, as well as the dipole potential energy.

2. In chapter 21 we discussed Coulomb’s law that gives the force between two point charges. The law is written in such as way as to imply that q2 acts on at a distance r instantaneously (“action at a distance”)
Electric interactions propagate in empty space with a large but finite speed (c = 3108 m/s). In order to take into account correctly the finite speed at which these interactions propagate we have to abandon the “action at a distance” point of view and still be able to explain how does q1 know about the presence of q2 . The solution is to introduce the new concept of an electric field vector as follows: Point charge q1 does not exert a force directly on q2. Instead, q1 creates in its vicinity an electric field that exerts a force on q2 .
(22-2)

3. (22-3)

4. q qo r P
(22-4)

5. O O
(22-5)

6. d
+ q
- q
(22-6) -q
+q/2

7. (22-7)

8. P dq r dE dV
(22-8)

9. C A dq
(22-9)

10. P
electric field line
electric field lines P Q
(22-10)

11. q q
(22-11)

12. (22-12)

13. (22-13)

14. F+ F-
x-axis
(22-14)

15. A B  U
180˚
(22-15)

16. Fig.a
Fig.b
(22-16)