Two long, smooth, horizontal conducting rails are a distance $l$ apart and are joined at one end by a capacitor of capacitance $C$, as shown in the figure. A uniform magnetic field $B$ is perpendicular to the plane of the rails. A rod of mass $m$ lies across the rails, and a constant horizontal force F pulls it along them, starting from rest. The resistance of the rod and the rails is negligible. How does the rod move?

Fig. 1: Two horizontal rails a distance l apart, joined at one end by a capacitor C. A rod of mass m lies across them and is pulled by a force F, in a magnetic field B perpendicular to the plane of the rails.