A:a2÷2
B:a2
C:2a2
(2015•四川达州)—Excuse me, could you please tell me the way to the ? I have had too much water.—Go along the street and you will find one.
如图所示,在磁感应强度为B的匀强磁场中,有半径为r的光滑半圆形导体框架,OC为一能绕O在框架上滑动的导体棒,OC之间连一个电阻R,导体框架与导体棒的电阻均不计,若要使OC能以角速度ω匀速转动(未离开框架),则外力做功的功率是( )