Test Charge

Animation 1

Animation 2

Animation 3


One fixed charge and one “test” charge are shown in each animation. The fixed charge is labeled by an “f” and the test charge is labeled by a “t”. You can click-drag on the "test" charge to change its position. There are three animations depicted.


In which animation is the “test” charge a true “test” charge?


Instructor Resources

Reference: See Giancoli-PA: 16-7, Giancoli-SE: 21-6.
Answer: Animation 3.  As the “test” charge is moved in each animation, the field changes or remains the same. The animation where the test charge leaves the original field unaffected is the correct animation. This is of course the definition of the test charge to begin with. It is important for students to grasp the fact that in the course of measuring the field of the fixed charge, one may disturb the field if one uses a real charge. Note that this is also Problem 9.1.2 here used without major modification as an in-class exercise.
Script Author: Mario Belloni