The absolute value of potential energy (and therefore total energy ) has no physical significance. It is the difference of potential energies that matters. One can, therefore, add or subtract a constant to the potential energy (provided we do it to potential energy at every position for a given force) without any change in the physical situation. By convention, for forces which fall off to zero at large distances, the potential energy at infinity is taken to be zero. With this choice, is the potential energy positive or negaive for (a) electron bound state, (b) planet-satellite system, (c) electron-electron system?