PhysicsLAB AP Free Response Question
2009 B3

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A metal rod of mass 0.22 kg lies across two parallel conducting rails that are a distance of 0.52 m apart on a tabletop, as shown in the top view above. A 3.0-ohm resistor is connected across the left ends of the rails. The rod and rails have negligible resistance but significant friction with a coefficient of kinetic friction of 0.20. There is a magnetic field of 0.80 T perpendicular to the plane of the tabletop. A string pulls the metal rod to the right with a constant speed of 1.8 m/sec.
 (a) Calculate the magnitude of the current induced in the loop formed by the rod, the rails, and the resistor.

 (b) Calculate the magnitude of the force required to pull the rod to the right with constant speed.

 (c) Calculate the energy dissipated in the resistor in 2.0 seconds.

 (d) Calculate the work done by the string pulling the rod in 2.0 seconds.

 (e) Compare your answers to parts (c) and (d). Provide a physical explanation for why they are equal or unequal.

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