⑩天掌 Teaching Plan on Advanced Mathematics 解取r为积分变量, +1 r+dr ∈ ke 取任一小区间r,r+M,功元素d=mr, 所求功为=m=q kq a b 如果要考虑将单位电荷移到无穷远处 ke qTianjin Polytechnic University Teaching Plan on Advanced Mathematics 解 取r 为积分变量， o r • + q a  b • •• • • • • + 1 r r [a,b],  r + dr 取任一小区间[r,r + dr], 功元素 , 2 dr r kq dw = 所求功为 dr r kq w b a = 2 b a r kq       = − 1 . 1 1       = − a b kq 如果要考虑将单位电荷移到无穷远处 dr r kq w a + = 2 +       = − a r kq 1 . a kq =