I see that electrons are quite small in size, moreover it is moving fast but I have a question why do free electrons not leave a conductor (like a wire) ? but they can only move at the edge of the conductor?
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$\begingroup$ They can't leave because they have no way to leave. $\endgroup$– An_ElephantCommented May 4, 2023 at 16:57
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4$\begingroup$ They (electrons) can leave under the right conditions: en.wikipedia.org/wiki/Thermionic_emission $\endgroup$– Solomon SlowCommented May 4, 2023 at 17:31
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$\begingroup$ You might be mixing ideal conductor from electrostatics (which has all charge on its surface) with actual QM description of conductors. $\endgroup$– Roger V.Commented May 5, 2023 at 8:00
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$\begingroup$ @SolomonSlow Also cathode ray and corona discharge. $\endgroup$– R.M.Commented May 5, 2023 at 15:55
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$\begingroup$ does this help? $\endgroup$– Amit VermaCommented Sep 17 at 9:56
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4 Answers
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Fundamentally they can't leave because they are attracted to the fixed positively charged atomic nuclei in the material.
This results in the work function, meaning an amount of energy required to free an electron from the material. Given a big enough stimulus (for example in the photoelectric effect) the work function can be overcome and the electron can be freed.
answered May 4, 2023 at 17:10
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$\begingroup$ Can you explain this more clearly? $\endgroup$ Commented May 4, 2023 at 17:13
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3$\begingroup$ @newbieatphysics, positive and negative charges attract each other. It takes energy to separate them from each other. Can you make your question more specific if you need more explanation? $\endgroup$ Commented May 4, 2023 at 17:14
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1$\begingroup$ aren't electrons attracted to the nuclei inside the conductor?, I want you to clarify the difference in the attraction of the nuclei inside and at the surface of the conductor. I find this quite confusing $\endgroup$ Commented May 4, 2023 at 17:17
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7$\begingroup$ The only difference is if you move a nanometer away from one of the internal nuclei, you'd be near some other nucleus. If you move a nanometer away from one of the surface nuclei, you might be in free space with no other positive charge nearby. (ignoring surface defect states and other complications) $\endgroup$ Commented May 4, 2023 at 17:19
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1$\begingroup$ Another way to imagine it is by analogy with gravity: The electrons are sitting in a well of low electric potential (around the positively charged nuclei), and require a great deal of energy to escape that well. This is similar to the large amount of energy that NASA or SpaceX require for their rockets to escape the Earth's gravitational well. $\endgroup$– KevinCommented May 5, 2023 at 20:01
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Suppose the conductor is copper.
A copper atom has $29$ protons in its nucleus and $29$ orbiting electrons.
In the solid state each copper atom gives up one election to form a copper ion which on average is located at a fixed position, the 28 electrons being bound to the copper nucleus to form a lower energy state with the process forming what is called metallic bonds.
The free (unbound / liberated) electrons are free to move within the copper lattice (boundary of the solid) but are bound by the overall effect of the copper ions.
The motion of the free electrons with the copper can be liken to that of gas molecules within a box and they have a kinetic energy distribution like that of the molecules in a gas.
To remove the a free electron from its bound state within the copper requires a certain amount of energy which is called the work function energy.
If a free electron has sufficient kinetic energy (greater than the work function energy) when it is near the surface of the copper there is a possibility of it escaping from the copper.
If the
The kinetic energy of the free electrons can be increased by heating the copper and when the temperature is sufficiently high a significant number of electron can escape from the copper, the process being called [thermionic emission(https://en.wikipedia.org/wiki/Thermionic_emission).
Another mechanism by which the free electrons can gain sufficient energy to escape in numbers is by irradiating the copper with electromagnetic radiation, the process being called photoelectric emission.
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$\begingroup$ How is the free electron at a lower energy than the bound one in the material? It's at a higher energy level, by at least as much as the work function. There's no "lip of the bath" and lower energy state beyond. If you positively ionize a metal material (for example in the anode plate of a vacuum tube) and bombard it with electrons, it will readily re-capture those electrons. $\endgroup$ Commented May 5, 2023 at 15:50
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$\begingroup$ I'm ready to upvote for the more detailed explanation in your first paragraph compared to my answer, but I think the bathtub analogy is misleading. $\endgroup$ Commented May 5, 2023 at 15:51
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$\begingroup$ @ThePhoton Yhanks for your perceptive comment and I have removed the paragraph that you did not like. $\endgroup$– FarcherCommented May 5, 2023 at 17:34
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An exact answer requires quantum mechanics, but here is a sort of rationale.
Imagine a big flat sphere of conductor, that is negatively charged. The movable electrons spread out on the surface. They repel each other so each of them tries to get as far as it can from the others, and that's how they do it.
So each of them gets most of its repelling charge from the electrons that are close to it, to the sides. And they get most of their attractive force from the immovable charged atoms which are a little bit inside. So there's a larger force pulling them in than pushing them out. That force can be overcome, but it takes a lot to do it.
Now imagine the same sphere with a thin conducting needle sticking out of it. Electrons will climb the needle because they can, and because that puts them farther away from the other electrons.
By the time they reach the tip of the needle, they have other electrons behind them pushing them forward, and not much force pulling them back. They can escape much easier.
So if you want to make a conducting surface that can build up a large charge, you want to make it nice and smooth.
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Due to quantum mechanical properties of conductor material and charges. Charge typically needs a lot of energy in order to leave the surface of the conductor. You may be interested in a thing called vaccum level.
