Physlet Illustration: Electric Charges and Potential

 

 

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In this simulation, two charges, are fixed at (-4, 0) and (+4, 0).  One has a charge of -4 mC. Equipotential lines are shown in the region surrounding the charges. Using the test point, which is dragable, determine the sign and magnitude of the second charge.

Hints:

  1. Drag the test point around and watch how the net potential at its location, due to the fixed charges, varies.
  2. As you drag the point, its position coordinates and the magnitude of the potential at its location are displayed.
  3. Knowing that the net potential at any point is the sum of the potentials from the two charges, what is the sign of the unknown charge?
  4. Can you find a place where the net potential at the test point is zero?
  5. Can you use the distances from the fixed charges to this zero point to determine the magnitude of the second charge? (This is easiest if you choose a point along the line joining the two charges.)

Reference

See Walker, Section 20-3

Illustration written by Steve Mellema and Chuck Niederriter