A microscopic dust grain of mass carries a charge of
PHYSICS · Multi-part · A Level · Physics · Fields and their consequences · Fields and their consequences
Problem
A microscopic dust grain of mass 4.0×10−6kg carries a charge of q=1.6×10−6C. The grain is initially at rest at point P, where the electric potential is +50V relative to a distant reference. The grain then moves without friction to point Q, where the potential is −150V. Assume the grain’s charge remains constant and that no forces other than the electrostatic force do work on it.
(a) Determine the speed of the grain when it reaches point Q if its charge is +q.
(b) Repeat the calculation in part (a) if the charge on the grain were −q instead, and state the direction of motion relative to the electric field lines.
(c) For the case of part (a), calculate the work done by the electric field on the grain during the transit from P to Q. State whether the field does positive or negative work and justify your answer.