Teaching Physics with the Physics Suite Edward F. Redish

Three Charge Problems

Consider the forces in the following arrangements of charge. Answer each question, explaining clearly how you got your answer.

A. Suppose you have a particle with a negative charge -q exactly between two identical particles with equal, positive charge Q, as shown in the figure below.

1. If you moved the particle in the middle a tiny bit to the right, what direction would the total electric force exerted by the other two charges on it be?
2. Start with that particle back dead center again, and now move it a tiny bit up. In what direction would the total electric force exerted by the other two charges on it be?

B. In the figure below three charged particles lie on a straight line and are separated by distances d. Charges q1 and q2 are held fixed. Charge q3 is free to move but happens to be in equilibrium (no net electrostatic force acts on it). If charge q2 has the valueQ, what value must the charge q1 have?

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Page last modified February 18, 2006: E29