# What keeps electrons on wires?

1. Oct 5, 2007

### Zorodius

say you have a current carrying wire pointing in the +x direction, while there's an external magnetic field pointing in +y. Then the electrons in the wire feel a force towards +z.

Well, what stops them from flying off the wire? If you make the current or the magnetic field strength strong enough, can you overcome whatever it is, and tear the electrons off the wire?

2. Oct 5, 2007

### malawi_glenn

They dont have enough kinetic energy to overcome the potential from the atoms.

But or course in certain circumstances electrons can be ripped of the conductor, look how an X-ray tube works for example.

3. Oct 6, 2007

### Zorodius

could I understand this classically, or would I need quantum mechanics to understand it?

Is the potential you describe due to an attractive nuclear force?

If such a force acts on the electrons to hold them on the wire, why are they still free to move through the body of the conductor?

4. Oct 6, 2007

### malawi_glenn

The electrons are not totaly free. They are bound to the crystal (the wire), but not as bound as the non-valence electrons are. The electron in the wire as (mean)velocites about 10^6 m/s, but the drift velocity (the velocity the electrons will get EXTRA due to the applied electric field) is only about 1mm/s !!

And you sould also perhaps look at the Hall Effect, try google it and see if you get wiser.

5. Oct 6, 2007

### Zorodius

interesting, thanks!

6. Oct 6, 2007

My understanding is that wires have resistance and that current travels through or over the surface of the wire. If there is less resistance over the surface, then where do you think they will go. That is not to say that electrons do not flow through the wire. In the 60's we did experiments, 'exploding wires', where the wire could not handle the current. It produced miniature 'ball lightning'.

7. Oct 6, 2007

### Staff: Mentor

Remember, Zorodius, an electron on a wire is still part of an atom. It is attracted to the atom's protons by the same magnetic force that any other atom has. But the thing about metals is that when you put a lot of atoms together, they act like one group of combined charges, allowing the electrons to be mobile as long as they keep the overall charge roughly neutral.

Now what if we make the overall charge negative? Have you heard of a van de Graff generator? It uses a belt and brushes (I think...) to strip electrons and put them on a metal sphere much like your shoes strip electrons off a carpet when you shuffle your feet. Those electrons do want to jump off the metal sphere and they collect on the surface of the sphere, slowly dissipating into the atmosphere. Give them an easier path (like through your finger), and they will jump off all at once, bringing the charge on the metal sphere back into equilibrium.