如图1所示,在匀强磁场中有一U形导线框abcd,线框处于水平面内,磁场与线框平面垂直,R为一电阻,ef为垂直于ab、cd的一根质量为m的导体杆,它可在ab、cd上无摩擦地滑动并保持良好接触,杆ef及线框的电阻都可不计,现给ef一个向右的初速度v_0. As shown in Figure 1, in a uniform magnetic field there is a U-shaped lead frame abcd, the wire frame is in a horizontal plane, the magnetic field perpendicular to the plane of the frame, R is a resistance, ef is perpendicular to ab, cd a mass m Conductor rod, it can ab, cd friction-free sliding and maintain good contact, the rod ef and wire frame resistance can be excluded, now to ef a right initial velocity v_0.