MadSci Network: Physics

Re: How exactly does resistance cause heat increase?

Date: Thu Nov 2 20:31:15 2000
Posted By: William Beaty, Electrical Engineer / Physics explainer / K-6 science textbook content provider
Area of science: Physics
ID: 971549229.Ph

Hi Alex!

There are two mental models for resistive heating: the old "orbiting classical particles" model, and the new "Quantum Atoms" model. I don't know enough about the second one to talk about it, so my explanation will focus on orbiting particles, not "Fermi surfaces" and wave-functions.

Your teacher is right, the electrons never touch the nucleus. They are exactly like the electrons surrounding any atom: they "orbit" without falling in. In fact, an electric current in a metal wire is made of electrons supplied by the metal atoms. The flowing electrons aren't really injected into the wire from outside. Instead, electric fields produced by the power supply cause the wandering electrons (always found within the metal) to begin flowing.

This might be hard to understand unless you realize that metals aren't made of separate atoms. In metals, the outer electrons of each metal atom leave the individual atoms, and they "orbit" among all the atoms as a whole. A metal object is like a jar of water. Metals contain an electron sea or "electric fluid." A metal object is like one giant atom with many nucleii but only one electron cloud. During an electric current, it is the shared "electron fluid" which moves along. And in an electric circuit, the power supply is not the source of the electrons. The power supply is simply an electron pump.

So, why does the metal get hot during electric current? Because the flowing electrons "collide" with other electrons! Metals are different than other materials in that they store heat energy as wiggling electrons. (In non-metals, the heat energy takes the form of wiggling atoms.) If you try to force the metal's electrons to flow along, those electrons will speed up at first. But then they will be deflected by other electrons which had been trying to orbit around the non-moving atoms. Your flowing electrons get slowed by the non-flowing electrons, but the non-flowing electrons get sped up... but those non-flowing electrons weren't moving in the direction of the electric current. They fly in all directions with no average drift and no overall electric current. Speeding them up doesn't push them in the direction of the current. Speeding them up makes the crowd wiggle faster. Speeding them up is the same thing as heating the metal.

To repeat: the electrical force from the battery keeps trying to make electrons flow along, but the "collisions" with other electrons keep deflecting them and slowing down the overall flow. The overall flow gets turned into random motion, like that of the whizzing atoms in a hot gas. But there will always be SOME overall flow as long as the electric force is there, and a stronger force makes a faster flow. That's where Ohm's Law comes from. Ohm's Law says that electric current is directly proportional to the voltage difference, but there's a simpler way to say it: the harder you push, the faster it flows.

> Also when the electrical energy is transfered to
> heat energy do the electrons just disappear and if not
> do they carry on and still produce a current.
They don't disappear. Are you thinking that electrons are particles of energy, and that they have to change into heat enery? Nope. Electrons aren't particles of energy, they are particles of matter. An electric current is not a flow of energy, it is a flow of matter. A flow of electrical energy is called "electric power," not "electric current." Here's an analogy: when you use a circular drive belt to send energy from an engine to a spinning fan, is the energy made of rubber molecules? Nope. The rubber belt moves slowly in a circle while the mechanical energy flows almost instantly along the belt, going from engine to fan. Electric circuits are the same: the electrons fill the whole circle, and they flow slowly like a belt while the electrical energy flows rapidly from one component to another. More about this stuff is here:


Note that electrons can "collide" without touching. They repel each other because of alike charges, and this means that they can push against each other. If you throw one electron at another, the first one pushes the second one away, while the second one pushes back upon the first and slows it. But they never touch, and the "pushing" is done by the electrical repulsion forces. It's just like a real collision, but the particles never actually make contact.

Current Queue | Current Queue for Physics | Physics archives

Try the links in the MadSci Library for more information on Physics.

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci

MadSci Network,
© 1995-2000. All rights reserved.