•• A metal cross- bar of mass m rides on a parallel pair of long hori- zontal conducting rails separated by a distance L and connected to a device that supplies constant current I to the circuit, as shown in Figure 26-42. The circuit is in a region with a uniform magnetic field B whose direction is 74 Constant current xL x source x x x FIGURE 26-42 Problems 74 and 75 downward. vertically There is no friction and the bar starts from rest at t 0. (a) In which direction will the bar start to move? (b) Show that at time t the bar has a speed of (BIL/m)t.

•• A metal cross- bar of mass m rides on a parallel pair of long hori- zontal conducting rails separated by a distance L and connected to a device that supplies constant current I to the circuit, as shown in Figure 26-42. The circuit is in a region with a uniform magnetic field B whose direction is 74 Constant current xL x source x x x FIGURE 26-42 Problems 74 and 75 downward. vertically There is no friction and the bar starts from rest at t 0. (a) In which direction will the bar start to move? (b) Show that at time t the bar has a speed of (BIL/m)t.