The diagram below shows a particle with positive charge q accelerating between two conducting plates at potentials ${V_1}$ and ${V_2}$.

Which of the following is the kinetic energy gained by the charge in moving between the plates?

A.     ${V_2}q$

B.     ${V_1}q$

C.     $({V_1} - {V_2})q$

D.     $({V_2} - {V_1})q$

C

The vast majority of students incorrectly chose D as their answer. Kinetic energy gained is equal to potential energy lost. Furthermore, kinetic energy can only be positive, so, whether $q$ is negative or positive, the only correct response can be C.