AP® Physics 1 & 2

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Test for Positive Charge


A metal sphere is placed on an insulating stand for the following demonstration: A physics instructor brings a negatively-charged rod close to the sphere but does not touch it. While holding the rod close to the sphere, the instructor touches the other side of the sphere with her finger.

Upon seeing the demonstration, Student A argues that since the instructor brought the negatively-charged rod so close to the sphere, the sphere is now also negatively charged.

Student B argues that the negatively-charged rod forced electrons out of the sphere into the instructor’s finger and that the sphere is now positively charged.

Which of the following actions would be best to prove Student B is correct?


Test if the metal sphere attracts a negatively-charged pith ball.


Test if the metal sphere repels a positively-charged pith ball.


Touch the negatively-charged rod to the metal sphere and see if there is a spark.


Have another student touch the metal sphere with a finger and see if there is a spark.