How are electrons able to move from one part of an object to another and what causes this to happen?
In: Physics
In physics, we separate our definitions a bit here to avoid confusion. A free electron would be an electron that is truly *free*. As in, it is not in a material and is nowhere near any atoms that it could possibly interact with. It’s just a particle all by its lonesome self in space.
What you are probably talking about is what’s called a *conducting* electron. So, how does an electron become a conducting electron? Well, atoms like to hold on to their electrons. But, if you give an atom enough energy it will let go of the electron. Think of it sort of like offering money for someone’s car: they need money for their car because when they go to buy a new car, they need to be able to pay for it. It’s the same idea for atoms. They want energy for their electrons, because if another electron happens to pass by, they can offer that energy to an electron and hope that it decides to stick to it.
So, you give an atom some energy and it pops out an electron. That electron now just begins to move around your material randomly. It sort of bounces of other electrons, sometimes vibrations in the material itself causes it to move. The main drive of the motion of electrons in a material that’s sitting there doing nothing is something called *diffusion*. Think about what happens when you spill a glass of water. The water wants to spread as evenly as possible. The electrons do the same. Since like charges repel, all of the conducting electrons try to get as far away from each other as possible, so sometimes this means moving around in the material to adjust for that.
You can also force the electrons to move to one side of the material by putting a positive charge at one side of the material. Electrons *love* positive charges so if you put a positive charge (or for more technicality here, a positive voltage) at one side of the material, most of the electrons stop caring about being so close to each otherand just want to get as close to the positive charge as possible.
But now you might ask: “couldn’t they just leave? What’s keeping the electrons in the material?” Well, without getting too complicated here I’ll try to answer that (the real answer deals with things called the lattice potential, energy bandage theory, etc. But there is a simpler more intuitive answer that still realistically explains what’s going on). The truth is, these electrons aren’t free when they are in the material. In fact, this simple model I explained is called the *nearly free* electron model for *conducting* electrons.
Imagine one of these electrons just decides to jump out of the material. This would take a bit of energy to do, sort of like how it takes us humans a lot of energy to escape from the earth. But it does happen! So, this electron escapes the material. At this point, it becomes a truly free electron. There is basically nothing to bump into, it’s not even thinking about the atom it left behind. But the material *knows* it’s missing an electron. How? Well, every atom in the material has the same number of protons as electrons so every atom has no net charge meaning that if you take the charge of a proton (+) and the charge of an electron (-) and add them together you get 0. But since the electron just left the material, it left behind a proton somewhere. The material, as a whole, now has a net (+) charge. So, every single free electron that happens to be passing by will want to come take that spot. After all, like charges attract. So materials are constantly losing and gaining electrons, just not that many as this takes a lot of energy. This, by the way, is why even when you rub your hair on a balloon and it becomes negatively charged, it won’t stay charged for long. Those electrons you put there are *itching* to fly away to other positively charged things.
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