Grade upto college level Magnetism
work done in moving a magnetic dipole from an angle 
1 to an angle 2 w.r.t. the direction of magnetic field =
1 to an angle 2 w.r.t. the direction of magnetic field = [1 ]
[2 ] 
.d ... where is the torque reqd. to produce the given angular disp.
1 ][2 ] .d ... where is the torque reqd. to produce the given angular disp.= [1 ][2 ] M.B.sin.d
1 ][2 ] M.B.sin.d= - MB ( cos2 - cos1 )
2 - cos1 )= MB (cos1 - cos2 )
1 - cos2 )thus this work done is stored as the potential enrgy of the field-magnet system.
Thus .. U(2) - U(1) = MB (cos1 - cos2 )
2) - U(1) = MB (cos1 - cos2 )Now in HCV .... it is written that let us assume that the potential energy at = 90o as zero....and so the potential energy at an angle is
= 90o as zero....and so the potential energy at an angle is U() - U(90o) = MB ( cos90o - cos )
) - U(90o) = MB ( cos90o - cos ) = - MBcos
Now why shuold we suddenly consider potential energy at = 90o as zero ... ?????????
= 90o as zero ... ?????????
