The concept of electric potential energy (EPE) relies on the assumption that the "test charge" is immobile within the electric field. This immobility is crucial because if the test charge were to move, an opposing force would counteract the electric field, preventing acceleration. The test charge, denoted as ##q##, serves as a tool for measuring the electric field rather than as an active participant. By measuring the force vector ##\vec F## on the test charge, one can calculate the electric field at that specific location using the formula ##\vec F/q##. Understanding this distinction is essential for accurately defining and applying electric potential energy concepts.
