Mass and Potential Energy Suppose now at the far end of the tube we have a hydrogen atom at rest. As we shall see later, this atom is essentially a proton having an electron bound to it by electrostatic attraction. It is known that a flash of light with total energy 13.6eV is just enough to tear the electron away, so in the end the proton and electron are at rest far away from each other. The energy of the light was used up dragging the proton and electron apart—that is, it went into potential energy. (It should be mentioned that the electron also loses kinetic energy in this process, 13.6 ev is the net energy required to break up the atom.) Now, the light isabsorbed by this process, so from our argument above the right hand end of the tube must become heavier. That is to say, a proton at rest plus a (distant) electron at rest weigh more than a hydrogen atomby E/c2, with Eequal to 13.6eV. Thus, Einstein’s box forces us to conclude that increased potentialenergy in a system also entails the appropriate increase in mass.
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