PhysicsLAB AP Free Response Question
2013 B6

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Two long, straight horizontal wires are near each other and parallel, with one directly above the other as shown in the figure.
Wire X is fixed in place and connected to a battery (not shown) so that it carries a current of 65 A. Wire Y, which is part of a second circuit, is free to move vertically and is suspended at rest by the magnetic force between the wires. The mass per length of wire Y is 5.6 x 10-3 kg/m . Neglect effects from the parts of the circuits that are not shown.
 (a) Calculate the magnitude of the magnetic field produced by wire X at the position of wire Y.

 (b) i. Calculate the magnitude of the current in wire Y.

(b) ii. Indicate the direction of the current in wire Y.
____ To the left
____ To the right
____ Neither left nor right, since there is no current

 (c) Now wire Y is moved to a new position that is closer to wire X, but wire Y is still below wire X and is still carrying the same current as determined in part (b). Wire Y is released from rest. Describe the initial motion of wire Y. Justify your answer.

 (d) Suppose wire Y is moved to a position 0.025 m above wire X. What changes in current, if any, must occur to maintain equilibrium?

(e) With wire Y still above wire X, the circuit connected to wire Y is removed. Wire Y, which is 1.2 m long, is then moved vertically up and away from wire X at a constant speed of 3.0 m/s.
 i. Calculate the magnitude of the induced emf in wire Y when the wires are 0.050 m apart.

ii. Indicate which end of wire Y is at a higher electric potential. Justify your answer.
____ The left end
____ The right end
____ Neither end, since they are at the same electric potential

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