Electrons flying off a conductor

[Noesis]

There is something I don't quite understand about conductors with excess charge.

I understand the rationale that says that the electrons will all be on the surface, since they are repelled by the inner electrons orbitting the atoms, and by other excess electrons...but why do they not fly off of the conductor?

We see a continuous surface on a conductor, but we know the truth is that there is a discrete amount of atoms that compose any material.

What prevents an electron from continuously being repelled and just jetting right off of the conductor?

 

[ZapperZ]

There is something I don't quite understand about conductors with excess charge.

I understand the rational that says that the electrons will all be on the surface, since they are repelled by the inner electrons orbitting the atoms, and by other excess electrons...but why do they not fly off of the conductor?

We see a continuous surface on a conductor, but we know the truth is that there is a discrete amount of atoms that compose any material.

What prevents an electron from continuously being repelled and just jetting right off of the conductor?

Under certain circumstances, THEY DO!

It depends on the amount of coulombic forces/electric field. There is a potential barrier at the surface of a conductor which is equivalent to the work function of the material. This is what most electrons have to overcome to escape. Under certain circumstances, the electrons can tunnel through this barrier via quantum tunneling. This occurs when you put a metal in a large external electric field, or in your case, a lot of repulsive electric field from other charges.

... or you can heat up the material and help the electrons escape via thermionic emission.

Note (and this is one of my pet peeve): in a material, the behavior of that material is no longer dictated that much by the "individual atoms". It is more of a collective behavior, rather than the individual atoms, that influences the material's characteristics. So in a metal, for example, you have a "conduction band", which consists of a sea of electrons that don't really belong to any particular atoms that made up that conductor. This is a collective behavior, not the behavior of individual atoms. Individual atoms no longer play that much of an influence anymore. This is true for optical, thermodynamics, and electrical properties of the material.

Zz.

 

[Noesis]

Wow...thanks for the quick and informative response Zz.

I must admit a good bit of that did indeed go over my head, as I've only started to learn about the amazingness that is physics. I understand just about all of it..but the first paragraph 's me.

How does it depend on the coloumbic forces/electric field? What exactly is this potential barrier?

Do you mean potential as in it could possibly exist, or in the electric potential sense?

And what in the world is the work function? The amount of work it would take to put the conductor together?

I have no idea what quantum tunneling is...but seeing as I've never taken quantum mechanics I assume I'll learn about that later.

I apologize for the barrage of questions...but even though what you say is a mystery to me as of now...it's incredibly interesting.

Thanks again for the help.

 

[Hootenanny]

How does it depend on the coloumbic forces/electric field?

The electric field is defined as the electric force per unit charge. For example if you have an electric field strength of 1 N/C at a certain point, then if you placed a particle with a charge of 1 coloumb at this point is would experience a force of 1 Newton.

The more homeogenous charge you have in the conductor the greater this force (becuase like charges repel). The electric field must be great enough to overcome the potential barrier / work function of the metal.

Do you mean potential as in it could possibly exist, or in the electric potential sense?

Electrical potential.

And what in the world is the work function? The amount of work it would take to put the conductor together?

Simply put, the work function is the energy required to remove an electron from the surface of a metal and move it to an infitite distance away from the surface.

I've tried to address your questions as best I could in the abscence of Zapper. I tried to keep my responses as simple as possible to avoid confusion, if you required any more detail, I will be happy to oblige.

~H

 

[Noesis]

Thank you Hootenanny.

That did make more sense of things.

But what I don't quite understand...is why is there a work function at all?

The electric field in the conductor is zero. And correct me if I'm wrong, but I take it that the electric field in there is zero because all of the electric fields produced by the protons in the nuclei are effectively canceled out by the electrons that are orbitting them. And these orbitting electrons exert a repelling force on the excess electrons thus exposing them to the surface. Other excess electrons also do the same.

So since the electric field in the conductor is zero, there is no force for the electrons to do work against. The only thing that could keep it on the conductor would be a coloumbic force from a proton, and they are all tied up.

Now I know there is some flaw in my reasoning, or else electrons would be flying around everywhere, but that is my problem in understanding the work function.

Thank you very much.

 

[Noesis]

Still curious

 

[ZapperZ]

The reason why there is a work function (or even an electron affinity) can be quite complicated. This involves an area of physics called surface science. I will simply describe two possible origin of the work function.

1. Coulombic charge. When an electron tries to leave a conductor, it is going to leave a neutral object and causing it to be positively charged momentarily. So that electron has to overcome this electrostatic attraction.

2. If you have studied electrostatics, you'll encounter an image charge problem. Even when an electron managed to just barely escape the metal, it will encounter its image charge inside the metal surface. This will cause the electron to get pulled back into the metal. So unless the electron come off the surface with enough kinetic energy to get far enough away from the surface until the image force becomes too small, it will be pull back into the metal and not able to escape.

Zz.

 

[Noesis]

Well that is indeed absolutely wild...thank you for the responses.

I hope to learn more about it later...as it seems right now it is a bit over my head.

Good to have a vague idea about it now though.

 

[rcgldr]

If you're referreing to a static charge, there's a maximum amount of charge per mass of a metal, exceed this and electrons are forced away.