A conductor of length l lies along the x axis with current I in the + X cap direction (phi = 0). Find the work done in turning it at a constant angular speed to phi = pi/2, with one end fixed at origin o, if the uniform magnetic field is B = B_0 Z cap. Solution Force per unit length: dF/dL = i L X B Evaluate the cross-product then set up the integral: F = i B L sin ½ dL F = i ½ B L² Work done W = Fds W = F½ = i ½ B L² = ¼ L²B i GET THE VALUE OF WORK DONE.