[例四]带电圆环(R1.R2.)顺时针旋转求 p 解 2 od e d q cord R Ⅰ=dI=| gordo R OO(R-Ro) Bmn=S=00(R2-R)(R2-R1)=( R2-R) 2 ob d (R2-R)对否? 2 R1 R2 o  [例四]带电圆环（ ）顺时针旋转 ，求 Pm  R1 .R2 . .  ( ) 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 P m = −    对否？ 解一： dq = 2rdr r r q I d 2 d d    r = =