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 y-axis at y=+d and y=−d. A small positively charged bead of negligible mass is free to move without friction along the x-axis (where y=0).
(a) For a bead located at a position x>0 on the x-axis, qualitatively explain the direction of the net electrostatic force acting on it. Base your explanation on the symmetry of the charge configuration.
(b) Sketch a qualitative graph of the electric potential energy U(x) of the bead (vertical axis) versus its x-coordinate (horizontal axis). Clearly mark any equilibrium positions on your sketch.
(c) For each equilibrium point identified in part (b), state whether it is stable or unstable and justify your classification with reference to the shape of U(x) and the direction of the restoring or destabilizing force.
(d) If the bead were instead placed exactly at the origin (x=0) and released from rest, describe its subsequent motion, if any, and explain your reasoning.