Why are free electrons able to move around an entire object?



How are electrons able to move from one part of an object to another and what causes this to happen?

In: Physics

So not sure exactly what you mean by free electron. Do you mean electrons in a metal and an electric signal traveling through it or electron beams or what. If you specify I can maybe be more help.

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.

Electrons are able to move around objects when the force from the nucleus holding onto the electron is overcome by some external force. When the outermost electrons are held loosely enough by the nucleus, [they will move between atoms when subjected to an electrostatic or magnetic force.](https://www.edinformatics.com/math_science/why-do-electrons-flow.html#:~:text=When%20a%20negative%20charge%20is,a%20conductor%20electrons%20are%20repelled.&text=When%20electric%20voltage%20is%20applied,move%20toward%20the%20positive%20side.)

A full explanation of this must start with defining the atom. [This Northwestern University physics document defines atoms as the building blocks for everything in the universe](https://www.nrc.gov/reading-rm/basic-ref/students/science-101/what-is-an-atom.html). They are sphere-like in shape and are made up of three smaller subatomic particles.[ Protons and neutrons make up the center of the sphere called the nucleus and the electrons fly around outside the nucleus in a sort of cloud.](https://www.qrg.northwestern.edu/projects/vss/docs/Propulsion/1-what-is-an-atom.html) A common analogy is that electrons float around the nucleus of an atom kind of like vapor droplets in a rain cloud, moving around in space while staying within the cloud itself. [The big difference between electrons and the other subatomic particles is that they are free to move around outside the nucleus](https://www.britannica.com/science/electron), which makes it easier for them to move from atom to atom.

Electrons are able to move when they “break free” from the nucleus that was holding in the cloud. The only reason that they are able to do this, is because of an outside force influencing them. [Much like a moth is attracted to a flame](https://www.physicscentral.com/explore/action/dressed-to-impress.cfm), an electron is attracted to the opposite charge, typically the protons in the nucleus. But as this accredited textbook talks about, [when the attraction force to the protons of another atom is stronger than the attraction to an electron’s own nucleus, the other atom will pull the electron away from its atom.](https://www.physicsclassroom.com/class/estatics/Lesson-1/Charge-Interactions) This will happen when a positively charged piece of metal moves close to a neutral piece of metal, some of the electrons in the neutral piece will be pulled toward the positive piece. The outside force of the positive piece allows for electrons to move around the neutral piece because it is greater than the force of the nucleus trying to hold onto the electrons. To further visualize this, see the link attached below.

This concept can also be visualized by imagining there is a dog leashed to a tree. Left alone, the dog might simply be bored, walking or standing around the tree in any random location. In this scenario, think of the dog as the electron, the tree as the nucleus, and the leash indicates how tightly bound the electron is to the nucleus. As of now, the dog is free to move around the tree as he pleases, but if he were to smell a steak at the next tree over he would be immediately attracted to it. He would want to run in that direction as if the steak was a positive charge. If the dog’s attraction to the steak is stronger than the leash holding him back, he’ll break away from his tree and run to the next tree over. The same applies to electrons, they are attracted to opposite charges and when that attraction overcomes their attraction to the nucleus they break free.