PHYSICS · Multi-part · A Level · Physics · Fields and their consequences · Fields and their consequences
Problem
A proton is fired directly toward a large, positively charged metal plate that is held at a higher electric potential than the launch point. The proton’s initial speed is non-zero.
Assume the proton’s motion occurs entirely along a straight line perpendicular to the plate and that no forces other than the electrostatic force act on the proton.
(a) Describe whether the electric potential energy of the proton–field system increases, decreases, or remains constant as the proton approaches the plate. Justify your answer without using equations.
(b) Explain qualitatively how the proton’s kinetic energy and speed change as it moves toward the plate. In your explanation, explicitly connect the behavior of kinetic energy to the change in electric potential energy identified in part (a).
(c) Suppose the proton momentarily comes to rest at a distance d from the plate before reversing direction and moving away. Describe the energy transformations that occur from the launch moment until the proton returns to its original position. Identify where along the path the kinetic energy is maximum and where it is minimum, and justify your reasoning.