Electric Potential 3.1 Potential and Potential Energy In the introductory mechanics course,we have seen that gravitational force from the Earth on a particle of mass m located at a distance r from Earth's center has an inverse- square form: F.=-GMmp (3.1.1) where G=6.67x10-1N.m2/kg2 is the gravitational constant and r is a unit vector pointing radially outward.The Earth is assumed to be a uniform sphere of mass M.The corresponding gravitational field g,defined as the gravitational force per unit mass,is given by g= F-_GMi (3.1.2) m r2 Notice that g only depends on M,the mass which creates the field,and r,the distance from M. Figure 3.1.1 Consider moving a particle of mass m under the influence of gravity (Figure 3.1.1).The work done by gravity in moving m from A to B is g-尾-（学-[6 (3.1.3) The result shows that W.is independent of the path taken;it depends only on the endpoints 4 and B.It is important to draw distinction between W.,the work done by the 3-2Electric Potential 3.1 Potential and Potential Energy In the introductory mechanics course, we have seen that gravitational force from the Earth on a particle of mass m located at a distance r from Earth’s center has an inverse￾square form: 2 ˆ g Mm G r F = − r G (3.1.1) where is the gravitational constant and is a unit vector pointing radially outward. The Earth is assumed to be a uniform sphere of mass M. The corresponding gravitational field 11 2 2 G 6.67 10 N m /kg − = × ⋅ rˆ g G , defined as the gravitational force per unit mass, is given by 2 ˆ g GM m r = = − F g r G G (3.1.2) Notice that g G only depends on M, the mass which creates the field, and r, the distance from M. Figure 3.1.1 Consider moving a particle of mass m under the influence of gravity (Figure 3.1.1). The work done by gravity in moving m from A to B is 2 1 1 B B A A r r g g r B A r GMm r W d dr GMm r r GMm r − ⎛ ⎞ ⎛ ⎞ = ⋅ = ⎜ ⎟ = = ⎜ − ⎝ ⎠ ⎝ ⎠ ⎡ ⎤ ∫ ∫ ⎢ ⎥ ⎣ ⎦ F s G G ⎟ (3.1.3) The result shows that Wg is independent of the path taken; it depends only on the endpoints A and B. It is important to draw distinction between , Wg the work done by the 3-2