Question
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 field acts perpendicular to the page. When a 5.0-A current is directed as shown, in the figure, the horizontal wire moves upward at constant velocity in the presence of gravity. (a) What force 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 field required to move the wire at constant speed. (c) What happens if the magnetic field exceeds this minimum value? (The wire slides without friction on the two vertical conductors.)
Explanation / Answer
PART A:
The forces acting on the wire is
1. Force due to the B field
2. Its weight
It can move upward at constant velocity if the magnetic field is out of the page (so the force by it is upward) and the force due to the field is as strong as its weight.
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In that case,
BIL = mg
--> B = mg/IL
Thus,
B = 1.96 T [ANSWER, PART B]
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if the magnetic field exceeds this, the wire would accelerate upward [no longer constant speed]. [ANSWER, PART C]