申論題

9. Figure 1 shows a wood cylinder of mass m = 0.25 kg and length L =0.10 m, with N = 10 turns of wire wrapped around it longitudinally, so that the plane of the wire coil contains the long central axis of the cylinder. The cylinder is released on a plane inclined at an angle 6 to the horizontal, with the plane of the coil parallel to the incline plane. If there is a vertical uniform magnetic field of magnitude B = 0.50 T, what is the least current I through the coil that keeps the cylinder from rolling down the plane?