Consider the system pictured in Figure P19.21. A 15-cm

Chapter 19, Problem 19.54

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Consider the system pictured in Figure P19.21. A 15-cm length of conductor of mass 15 g, free to move vertically, is placed between two thin, vertical conductors, and a uniform magnetic fi eld acts perpendicular to the page. When a 5.0-A current is directed as shown in the fi gure, the horizontal wire moves upward at constant velocity in the presence of gravity. (a) What forces act on the horizontal wire, and under what condition is the wire able to move upward at constant velocity? (b) Find the magnitude and direction of the minimum magnetic fi eld required to move the wire at constant speed. (c) What happens if the magnetic fi eld exceeds this minimum value? (The wire slides without friction on the two vertical conductors.)