PhysicsLAB AP Free Response Question
2009 B2

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Two small objects, labeled 1 and 2 in the diagram above, are suspended in equilibrium from strings of length L. Each object has mass m and charge +Q. Assume that the strings have negligible mass and are insulating and electrically neutral. Express all algebraic answers in terms of m, L, Q, and fundamental constants.
(a) On the following diagram, sketch lines to illustrate a 2-dimensional view of the net electric field due to the two objects in the region enclosed by the dashed lines.

 (b) Derive an expression for the electric potential at point A, shown in the diagram at the top of the page, which is midway between the charged objects.

(c) On the following diagram of object 1, draw and label vectors to represent the forces on the object.

 (d) Using the conditions of equilibrium, write—but do not solve—two equations that could, together, be solved for  and the tension T in the left-hand string.

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