PHYSICS · Multi-part · A Level · Physics · Fields and their consequences · Fields and their consequences
Problem
Two identical positive point charges, each of magnitude +Q, are fixed on the x–axis at x=0 and x=L, respectively. A third point charge of magnitude q can be placed at one of two candidate points on the x–axis: point A at x=2L (midpoint between the fixed charges) or point B at x=2L (to the right of both fixed charges).
Assume the charges are brought in from very far away and the positions of the two +Q charges never change.
(a) Without performing any explicit calculations, identify at which candidate point (A or B) the total electric potential energy of the three–charge system would be lower if q is positive. Justify your answer qualitatively by discussing the sign of each pairwise contribution.
(b) Repeat part (a) for the case in which q is negative. Explain any change in your reasoning.
(c) Suppose q is positive and placed at point A as originally proposed. A student claims that moving the charge a small distance to the left (toward x=0) will always decrease the total potential energy. Evaluate this claim and explain your reasoning without numerical calculation.