TITLE: Energy of electron
QUESTION [0 upvotes]: If we have a reservoir of negative charges and we bring a test charge (electron) near that reservoir. Will all the potential energy of that test particle will be converted to the kinetic energy of the electron as the negative charges will repel the electrons. Or that test charges will posses any other type of energy as well?

REPLY [1 votes]: It looks to me like you have something specific in mind with your question, but you do not seem to disclose it.
If the test charge were repelled (by like charges, which are also negative), it would run off to infinity, where its potential energy with respect to what you call the "reservoir" is minimal. Hence, all the potential energy would be lost, either to its kinetic energy, or to radiation it has emitted while it was accelerated.
But if the "reservoir" consists of positive charges (as you have asked originally), the electron will be attracted. Depending on what theory you apply, different things would be predicted.
Classically, without radiation, the electron would cruise around and between the positive charges forever, like a planet in a system with multiple stars.
Classically with radiation, the electron would finally fall into the positive charges, presumably colliding with one of those. Therefore, it would lose all its potential energy, but not only to the increase of its kinetic energy, but also to radiation that goes off to infinity.
Quantum-mechanically (and this is what actually comes closest to reality), it will again lose energy to radiation, but this time not indefinitely, but actually only until it reaches the quantum-mechanical ground state of itself attracted by the positive charge configuration.
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