So say we have a uniform field of magnitude X. Let's say the force that the field exerts is dependent on a particle's mass, M. Now say we move the particle D distance against the force of the field, XM. Our potential energy will be equal to XMD, because we exerted XM force over distance D, hence work was converted into potential energy. Now we suddenly turn off this field, like one would turn off an electric field by discharging a capacitor. What happens to the potential energy?