例四]带电圆环(R1R2O)顺时针旋转()求P 解1 da=o. 2/rdr R R 2丌 R R gordo R R O(R 元o P=IS=0(R2-R)z(R2-12(R2-R2)2 (R2-R2)对否? 2[例四]带电圆环（ ）顺时针旋转（ ）求 Pm  R1 .R2 . .   R1 R2 o  解1： dq =   2rdr r r q I d 2 d d     r = = ( ) 2 1 d d 2 1 2 2 2 1 2 1 R R I I r r R R R R = − = =     ( ) 2 1 2 1 2 Pm = IS =   R2 − R ( ) 2 1 2  R2 − R 2 2 1 2 2 ( ) 2 = R − R  ( ) 2 2 1 2 R2 R q Pm = −    对否？