MadSci Network: Physics
Query:

Re: How does AC current

Date: Thu Aug 19 03:16:17 1999
Posted By: William Beaty, Electrical Engineer / Physics explainer / K-6 science textbook content provider
Area of science: Physics
ID: 930838765.Ph
Message:

Hi Jason!

This is one of those questions which never seems to be answered in any textbooks. I go into it in depth on my Electricity Misconceptions page

The brief answer: current does not flow. "Electric current" is defined as meaning "charge flow". Charges flow, but "current" is not a substance, therefore whenever a book contains the phrase "flow of current" or "current flow", it is making a mistake. The name of the stuff that flows in rivers is "water", not "current," and the name of the stuff that flows in wires is "charge".

This however doesn't answer your question. How can we imagine the flow of charges in a wire? It's not that hard once we realize an important fact: all wires are already full of charge. Matter in general is full of charge (it is made of electrons and protons after all,) but metals are different because some of the electrons of the metal atoms are "orbiting" throughout the metal as a whole, and aren't staying next to individual atoms. Metals are like tanks of "liquid charge." When we make a wire into a circle, we essentially have formed a "drive belt" made of movable electron-stuff. Push the electrons along in one part of the wire, and this will make all of the electrons move forward like the rim of a wheel.

See where this is headed? If we force the "wheel" of electrons to move back and forth instead of moving continuously in a circle, we have created an AC circuit. The electrons wiggle back and forth and do not go anywhere.

There's one more part left to this picture. What happens if our circle of wire is very long? If we then wiggle the electrons in one segment of wire, it will take awhile for the wave to reach other parts of the circle. In this case let's imagine that the AC circuit is a circle of hose full of air. If we wiggle the air in one part, SOUND WAVES flow outwards into all other parts of the tube. How can AC move forwards while it also goes back and forth? In air, sound waves move forwards while the air sits in one place and vibrates. In wires, waves of changing electron density, as well as waves of electromagnetic field energy are flowing rapidly along the wire. At the same time the electrons themselves sit in one place and wiggle.

There are other parts to this story as well (for example, at higher frequencies only the electrons in the surface of the wire will wiggle, and the rest of the electrons deeper inside will not participate in the alternating current.) These are typically treated in "E and M" courses in college physics or engineering, but once your're armed with a couple of analogies, you can explain quite a bit of the physics of propagating waves on wires.

Also see: ELECTRICITY IS NOT ENERGY!


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, webadmin@www.madsci.org
© 1995-1999. All rights reserved.