PhysicsLAB AP Free Response Question
2011 B2 Form B

You are to determine the magnitude and direction of the electric field at a point between two large parallel conducting plates. The two plates have equal but opposite charges, but it is not known which is positive and which is negative. The plates are mounted vertically on insulating stands.
 
 
(a) A small ball of known mass m, with a small charge +q of known magnitude, is provided. The ball is attached to an insulating string. The additional laboratory equipment available includes only those items listed below, plus stands and clamps as needed. Choose the equipment you would use to make measurements needed to determine the magnitude and direction of the electric field between the two plates.
 
 

 (b) Sketch a diagram of the experimental setup and label the pieces of equipment used.

 (c) Outline the experimental procedure you would use, including a list of quantities you would measure. For each quantity, identify the equipment you would use to make the measurement.

 (d) i. Explain how you would calculate the magnitude of the electric field.

 (d) ii. Explain how you would determine the direction of the electric field.

 (d) iii. Explain how you would determine which plate is positive.





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