Is the 'test charge' in electric potential energy defined as immobile?

[archaic]

Hello, quick question, when EPE was defined, did we decide that the "test charge" is immobile in the field? Otherwise a force equal and opposite to that of the electric field would just stop the charge from accelerating.

 

[kuruman]

A test charge $q$ is just an artifact that can be considered as "an electric field measuring device". If you can measure the force vector $\vec F$ on it at a given point in space, then $\vec F/q$ gives the electric field at that point in space.