1、Chapter 28 Electric Potential Energy and Potential 128-1 Potential Energy The gravitational force is conservative. For conservative force we could define a potential energy. The electrostatic force is also conservative, and therefore there is a potential energy associated with the configuration of a

2、 system in which electrostatic forces act.228-2 Electric Potential energyabC When the trial charge q0 moves from point a to point b in the external electric field , along a path C, the work done by electrostatic force is:31. In field of a point charge q :The work is independent on the path.42. In fi

3、eld of a collection of point chargesThe field strength is, Then, is the region of the charge distribution.The work does not depend on path also.5 Therefore, the electrostatic force is among the conservative forces. We can define the “ electric potential energy ” to measure the work done by electrost

4、atic force :Corollary: This is the mathematical expression of the fact that the electrostatic force is conservative.6Generally, we choose a reference point, Ub=0. (rb ) This is electric potential energy of the trial charge q0 in the electrostatic field of point charge q.In field of a point charge q

5、:73. Conservation of energy in electrostatics For an isolated system, E=K+U=constant.Thus, K+ U=0q1q2vTwo charges have the same sign, U0.q1q2vTwo charges have the same sign, U0, K R1, q2 q1, this is the concept of “ground”.31 Near a sharp conductor the electric field may be large enough to ionize mo

6、lecules in the surrounding air,Corona Discharge3228-7 The Electrostatic Accelerator33Homework: E-13, 29, 45P-4, 12, 13Intensive readings: 28-2, 3, 4, 6, 7, 834End35Proof:In Cartesian coordinate system, we have, On the other hand,So,36By comparison, we get: or, We can get the formula for computing electric field in other coordinates similarly. F