申論題

6. A copper rod of mass m rests on two horizontal rails with a distance d apart from each other and carries a current i from one rail to the other as shown in Figure 5. The coefficient of static friction between rod and rails is u. A uniform magnetic field B is applied to the system and puts the rod on the verge of sliding, what is the angle of magnetic field with respect to the direction of tails, if the applied magnetic field is at minimum?