Two identical positive point charges are fixed on the -axis
PHYSICS · Multi-part · AP · Physics 2: Algebra-based · Electric Force Field and Potential · Electric Force Field and Potential
Problem
Two identical positive point charges are fixed on the x-axis, separated by a distance d. A student claims that a positive test charge released from rest at the midpoint between the two fixed charges will remain at rest indefinitely because the net electric field at the midpoint is zero.
(a) Describe qualitatively the net electric force experienced by the test charge if it is displaced by a very small distance Δx along the x-axis toward the charge at +2d.
(b) Based on your description in part (a), evaluate the student’s claim. Is the equilibrium at the midpoint stable, unstable, or neutral for motion confined to the x-axis? Provide a clear justification.
(c) Would your conclusion about stability change if the fixed charges were identical in magnitude but opposite in sign? Explain briefly.