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Name: David R.
Status: student
Grade: 9-12
Country: USA
Date: Winter 2011-2012

I came upon the following question and answer on Newton and would like to ask a follow-up: Here is my follow-up: I am interested in what happens when the energy of collision between a proton and an electron is too high to form a hydrogen atom and too low to form a neutron. In particular, I am wondering whether the particles could just bounce off each other. This is puzzling in light of the fact that the force between the proton and the electron is attractive. It seems that there would have to be a repulsive force involved for there to be a rebound.

David, When an electron and proton come together, excess energy is often released as extra particles, often photons. Producing an electron/anti-electron pair is also quite common. The extra particles can carry away the energy as both mass (E=mc^2) and as kinetic energy (the motion of the particles). If the electron and proton do not join, they can still each affect the motion of the other.

Dr. Ken Mellendorf Physics Instructor

It can be pretty hard to tell the difference between a repulsive rebound and an attractive sling-around, when watching from a larger-scale point-of-view. They both do hyperbolic trajectories, and from a great distance you can't really see the now-small offset between the points of closest approach and the mass-center of the encounter. What you see instead is that the particles went in on straight lines, and came back out on complimentary straight lines, all those lines being radial from the apparent center of the encounter.

I think even the statistics, i.e., how often it happens vs. the rebound-angle that happened, are very similar.

I think such close encounters have another effect - they radiate photons and lose a corresponding amount of kinetic energy. So maybe it is not _only_ bouncing off.

Jim Swenson

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