Whats the physics in this

[arkain101]

I was curious about the physics explanation of a wire heating up and melting from the reaction of electric current passing through the wire.

 

Whats the process electrons, electricity, goes through to cause atoms to gain kinetic energy?

 

This is just a curiousity question, that I thought may have quite a detailed and complex answer and reason.

[trapit]

The source for the energy is the potential difference between the ends.

[arkain101]

Right.

 

So do electrons flow through the wire by just hovering their way over atoms towards the attraction source and this bouncing through material causes atoms to jiggle faster.

 

Or is it that electrons actually bond and tear away through atoms as they pass through the wire?

 

I was curious how passing electrons in a wire cause atoms to "heat up".

[Qfwfq]

In a metallic conductor the first is basically true. In a semiconductor atomic orbital levels are also involved but a correct expanation is quite complicated, involving quantum physics. It can however be simplified by talking about charge carriers, which can be electrons or holes.

 

To really go into these things, if you're up to it, the subject to inquire about is called solid state physics.

[arkain101]

alright, Yah I assumed it would be a complicated answer.

 

I suppose I can just happily assume its because of electron mass, and resistance causes sort of a friction reaction, bam, heat..?

[Erasmus00]

I suppose I can just happily assume its because of electron mass, and resistance causes sort of a friction reaction, bam, heat..?

 

The easiest way of thinking about it (which has the disadvantage of being a little wrong) is to think of the electrons in your resistor as traveling in fits and starts down the wire, and constantly colliding with ions in the crystal lattice.

 

Before you apply the electric field, the electrons collide with the ions, sometimes bouncing off fast sometimes slow, and move around randomly. After you apply your field, the electrons begin to pick up an overall drift, because they are being acclerated by the field. Because they are being accelerated by the field, they have more energy on average when they slam into the lattice, so the lattice picks up more energy. One way of saying the lattice picks up more energy on average is to say it gets hotter.

-Will

[Qfwfq]

Reasonably good explanation. It isn't really all that "wrong" either. Coulomb scattering is, after all, a collision of sorts and it even has it's treatment in RQFT.

[CraigD]

So do electrons flow through the wire by just hovering their way over atoms towards the attraction source and this bouncing through material causes atoms to jiggle faster.

 

Or is it that electrons actually bond and tear away through atoms as they pass through the wire?

First, let’s be clear on how direct electric currents (DC) in solid metallic conductors occur:

Electrons are added to atoms at the negative end of the conductor;

those atoms – now negatively charged ions – give electrons to their immediate neighbors;

this is repeated, until the atoms at the positive end of the conductor give electrons to whatever is in contact with it.

- Or –

Electrons are removed from atoms at the positive end of the conductor;

those atoms – now positively charged ions – take electrons from their immediate neighbors;

this is repeated, until the atoms at the negative end of the conductor take electrons from whatever is in contact with it.

 

Many nice graphical illustrations of this, such as this one, are available.

 

So, the electrons emerging from the positive end of the conductor are not (with occasion exceptions when the current has been flowing for a long time, and the distance between the positive and negative ends of the conductor is sufficiently short) the same ones disappearing into it at the negative end.

 

The time it takes between one end of the conductor getting or losing electrons, and the other end doing the same, is related to what is called signal speed. For ordinary metal conductors, signal speed is usually between 25 and 90 % the speed of light (around 10^8 m/s). Individual electrons wander their way through a conductor in an unpredictable way, with a typical average “drift speed” of a tiny fraction of the signal speed – around 10^-3 m/s). They wander this way even when no ordinarily detectable current is present.

 

With this in mind, we can consider your original question about wires heating up.

 

When an electron moves from one atom to another, it may find itself briefly in a different, higher energy, quantum physical electron orbit than is normal for the atom. When it changes to its normal, lower energy orbit, it releases a photon of light. For most metals, most currents, and most orbit shifts, most such photons are in the form of heat. Some, of them are in the visible spectrum, however, which is why a hot exposed wire, such as the ones inside an electric toaster, have a visible glow.

 

Many of these photons are not radiated, but absorbed and re-radiated by other atoms in the wire. Provided this doesn’t happen at such a rate that the wire disintegrates into a gas, the only kinetic (heat) energy it can gain is in the form of a small portion from the increased movement of electrons, and a larger portion due to the “rattling” motion of the much more massive atomic nuclei as they exchange photons of a different, magnetic kind with their electrons.

[Qfwfq]

First, let’s be clear on how direct electric currents (DC) in solid metallic conductors occur:

Electrons are added to atoms at the negative end of the conductor;

those atoms – now negatively charged ions – give electrons to their immediate neighbors;

this is repeated, until the atoms at the positive end of the conductor give electrons to whatever is in contact with it.

- Or –

Electrons are removed from atoms at the positive end of the conductor;

those atoms – now positively charged ions – take electrons from their immediate neighbors;

this is repeated, until the atoms at the negative end of the conductor take electrons from whatever is in contact with it.

:naughty:

 

In a metallic conductor the conduction electrons aren't in a bound state of one atom or another. That description is more fitting for semiconductors. Erasmus gave a correct explanation for metallic conductors, the interaction between electrons and atoms is more like Coulomb scattering.