Comment by marshray

10 years ago

Oh sure, you can find explanations of good conductors and semiconductors all over the web.

But what I have never found is an explanation of poor conductors: How does electric current through a (non-super-) conductor convert energy to heat?

7 comments

marshray

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jeffwass  10 years ago

Do you mean "why is there resistance"?

In 1900, 3 years after JJ Thomson discovered the electron, physicist P. Drude came up with a simple approach for modelling metals. He assumed electrons would scatter off the atomic ions, all behaving classically like balls. Even this incredibly simplistic model, which ignores the important effects of both the quantum nature of electrons, and the crystalline lattice ordering of atoms in a metal, can predict Ohm's Law. Ie, that current density is proportional to the applied electric field. https://en.m.wikipedia.org/wiki/Drude_model

That's probably the simplest explanation, and adding in more accurate assumptions to the model still shows that Ohm's Law still follows in the classical regime.

Though clearly the band gap (which involves insulators and semiconductors) has important effects when you look at the the impact of the atomic lattice on quantum states. And the 2nd-order electron-lattice-electron interaction that allows Cooper pairing and superconductivity.

You can also get strange things in nanoscale systems, such as non-Ohmic ballistic electronic current, where resistance is only due to the contacts of a device but not the length.

bkcooper  10 years ago

How does electric current through a (non-super-) conductor convert energy to heat?

You have a coupling to other, non-electron/hole degrees of freedom in the system. The stronger that coupling is, the easier it is to transfer energy between the systems and the worse your conductor tends to be.

  • acqq  10 years ago

    As much as I know about the subject, I like this answer more than all others here. The "bumping" interpretations don't appear to match what we know about the atoms and the particles for many decades and look to me like an explanation for small kids.

marshray  10 years ago

These are two great responses, but they seem to be saying opposite things.

https://news.ycombinator.com/item?id=10877121

One says electrons bump into electrons, the other says electrons bump into non-charge-carriers. Someone on Twitter said it was phonons.

I guess I can kind of imagine a pachinko machine, but this is all very unsatisfying. :-)

Edit: From the Wikipedia link: "Charged particles in an electric circuit are accelerated by an electric field but give up some of their kinetic energy each time they collide with an ion"

I mean, electrons are very very lightweight and tend to move quite slowly for typical currents. How much kinetic energy can they give?

  • ZenoArrow  10 years ago

    > "I mean, electrons are very very lightweight and tend to move quite slowly for typical currents. How much kinetic energy can they give?"

    I only have a layman's understanding of this whole process, so I could be missing some important details, but I believe it's necessary to look at the atoms as a whole rather than just the electrons...

    Consider, the electrons (from the outermost orbit of the nucleus) are travelling from atom to atom (the ease by which they can come and go determines how conductive the material is).

    During this process, when an atom has more electrons than its stable state it is negatively charged, and when it has less electrons than its stable state it is positively charged. The greater the polarisation between the positively-charged atoms and the negatively-charged atoms, the more electrons can move through the material. I believe the potential difference between the poles is voltage, and the volume of electrons flowing at a given time is current, but I could be wrong on that.

  • tedsanders  10 years ago

    Electrons lose their energy to other things.

    Here's one quick explanation why:

    If electrons lost their momentum and energy only to other electrons, then in aggregate, electrons would keep their momentum and energy. Since electrons do not permanently keep their momentum and energy in normal conductors, they therefore must lose momentum and energy to non-electrons.

    Also, if you look at the Wikipedia link on the comment that says claims electrons lose energy to electrons, you'll see that Wikipedia says electrons lose energy to ions, not electrons.

    (Also, as a condensed matter physicist, I always feel a little dirty/inaccurate saying that electrons carry charge currents. I prefer the vaguer charge carrier, because charge carrier can refer to an ensemble of electrons.)