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Q & A: spiralling electrons

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Most recent answer: 10/22/2007
Q:
This might seem like a stupid question, but why donít electrons orbits smaller and smaller till they crush the proton? Electron s have negative charge so it should attract to protons, despite the relatively large space between the two.
- Bill (age 16)
Vancouver, bc, canada
A:
Bill- Thatís not what Iíd call a stupid question. In fact, the answer is so strange that it cannot be expressed in the framework of classical physics. The answer lies in the quantum nature of the world, which becomes very evident on a the atomic scale. The states of the electrons are spread-out waves. If the wave scrunched in toward the positive nucleus, lowering its potential energy, it would be confined to a smaller region. It turns out, oddly, that the momentum associated with the state depends on how rapidly that wave changes from place-to-place. Scrunching in requires rapid variations, which means big momenta. The momenta point all different directions, averaging zero, but since the kinetic energy depends on the square of the momentum, it becomes big. So the kinetic energy of the electron goes up more than the potential goes down as the wave gets too concentrated near the nucleus. The minimum total energy is found at the ordinary atomic size- thatís why atoms have that size.

As I said, donít look for some classical version of this story. Ainít none.

Mike W.

(published on 10/22/2007)

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