Example A loop with radius R,carrying a current of I.Magnetic field B is parallel to the loop,the diameter is fixed as the axis.Find (1)the torque on the loop (2)the work done by the magnetic torque,after the loop rotates n/2. Solution: dF IBdl sin a IBR sin a da B dM =dF.Rsin a IBR-sin a daExample、A loop with radius R, carrying a current of I。Magnetic field B is parallel to the loop, the diameter is fixed as the axis. Find（1）the torque on the loop（2）the work done by the magnetic torque,after the loop rotates /2.  R  dl B I dF  IBdl sin   IBR sin  d Solution: dM  dF Rsin IBR  d 2 2  sin