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So I was taught that:

Kinetic Energy (of electron) = Energy (of photon) - Ionization Energy

If E(photon) = IE, then KE=0 of the electron.

What does this physically/theoretically mean?

My current thoughts/interpretation is that enough energy/force is applied to ionize the electron so it is 'sufficiently far' from the atom, and then I guess it just moves with whatever speed it is moving at with natural kinetic laws, since no more energy/force is being applied....?

Any clarifications would be greatly appreciated, thanks in advance.

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  • $\begingroup$ It simply means that it's speed after ionization will be zero, anyway you should be aware of that this is idealization that the electron left the potential well of the atom, thus in quantum physics this little bit different.. $\endgroup$ – TMS Dec 2 '12 at 21:11
  • $\begingroup$ hmm, so is it like the speed of the electron decreases as it is pulled away from the atom..? Because it wouldn't make much sense for it to just stop after being ionized, right? $\endgroup$ – student Dec 2 '12 at 22:04
  • $\begingroup$ Why not? imagine you are rolling a ball "across" a road of sand, if you give it a very big speed at start it will decrease due to friction with sand until it reaches the other side, and then will continue to move at constant speed (supposing that out of the road is very smooth), now if you give it less energy but at the same time enough to reach the other side of road, it's speed will continuously decrease but it will stop once reaches the edge. $\endgroup$ – TMS Dec 2 '12 at 22:12
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    $\begingroup$ Well, atoms are always continuously moving with some speed, because they are never at 0K temperature. They are always moving/vibrating with some KE /viceversa. But so in the case of an electron, if it just 'stopped', is it at absolute zero temperature..? Doesn't it have SOME KE/movement? $\endgroup$ – student Dec 2 '12 at 22:17
  • $\begingroup$ Yes you absolutely right, this why I said from the beginning that this an idealization, because this thermal vibration of atoms/electrons is usually negligibly small comparing to ionization energy. $\endgroup$ – TMS Dec 2 '12 at 22:21
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This is the analogue of a projectile getting launched at exactly the escape velocity, something you may remember from studying gravity in freshman physics.

Here we're talking about the photoelectric effect. The electron jumps out of the material into air or vacuum, overcoming the force of attraction that tries to keep it bound inside the material (the force of attraction comes from the surface field, the image force effect, etc.).

If the electron jumps out with too little energy, it cannot escape, but gets pulled right back in. On the opposite extreme, if the electron jumps out with much more than enough energy to escape, it will not only break free of the material but also still energy left over, i.e. it will travel away from the material with a large kinetic energy.

You are asking about the borderline case. Here the electron has just barely enough energy to escape the material, with no energy left over. So it will slow down as it moves away from the material, and get slowed down more and more as it gets farther and farther away. It will never quite come to a stop, but its velocity will approach zero.

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  • $\begingroup$ Hi, Steve, I am learning about the same subject. I might not be getting your answer correctly. But if what you are saying is that when KE in that formula is equal to zero, the electron will still be traveling at escape velocity, which I guess is enough to ensure that the electron doesn't get pulled back into the atom, IT STILL HAS SOME VELOCITY AND THEREFORE K.E.. why is it not shown in the formula? $\endgroup$ – most venerable sir Aug 7 '15 at 14:09

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