B=dt =R = d=dt(Blx)=R = Bl v=R, as before, so W p = B 2l3v=R. [Alternatively, the work can be calculated from the conservation of energy: I = Bl v=R, P diss = I 2R =(Blv)2=R, and W app = P disst =[(Bl v) 2=R](l=v).] Problem 14. A square wire loop 3 .0 m