The basic equation is as follows: U=∑kqiqj/Rj with (i≠j) My question is how is this definition useful? What makes you want to say that is the potential energy of the system? For example, I thought that it is useful because if all particles change in position then you could evaluate that equation to find the total work done. However, this wouldn't be true because all of the charges would move in that hypothetical situation. So why is this useful? How is it derived, or proven? Thanks.