Tuesday, December 1, 2009

Conventional versus electron flow

The nice thing about standards is that there are so many of them to choose from.

Having a couple of motorcycles means that I also own a couple of battery chargers. During the winter, you need to keep them charged when not riding, since letting them go completely flat wrecks the battery. Even during riding season, the combination of big engines, small batteries, and periods when one does not ride much, you can don the helmet and leathers, stride confidently to the machine of choice, and get .... nothing. I was charging the 'buza's battery the other day, and asked the simple question: "which way does electricity flow"? Sure, in a DC system, electricity flows from the positive (red) to the negative (black) posts. That's odd. Why would negatively charged electrons move from the positive to the negative? Shouldn't they move from negative to positive? The amount of misinformation on the Internet regarding this simple question is staggering. Thankfully, Andrew Tanenbaum provides the answer in his discussion of "Conventional versus electron flow" in the very helpful All About Circuits site.

There are two ways to consider the direction of electricity flow, which are pretty much contradictory. The "conventional" view, where electricity flows from positive to negative, dates back to Franklin. He detected flow and assumed that there was a surplus of charge (hence positive) on one pole and a lack of charge on the other (hence negative). Of course, many years later, it was found that the true flow of electrons was the opposite, from negative to positive.

By the time the true direction of electron flow was discovered, the nomenclature of "positive" and "negative" had already been so well established in the scientific community that no effort was made to change it, although calling electrons "positive" would make more sense in referring to "excess" charge. You see, the terms "positive" and "negative" are human inventions, and as such have no absolute meaning beyond our own conventions of language and scientific description. Franklin could have just as easily referred to a surplus of charge as "black" and a deficiency as "white," in which case scientists would speak of electrons having a "white" charge (assuming the same incorrect conjecture of charge position between wax and wool).

However, because we tend to associate the word "positive" with "surplus" and "negative" with "deficiency," the standard label for electron charge does seem backward. Because of this, many engineers decided to retain the old concept of electricity with "positive" referring to a surplus of charge, and label charge flow (current) accordingly. This became known as conventional flow notation

He goes on to discuss the distinction in useful detail, concluding
I sometimes wonder if it would all be much easier if we went back to the source of the confusion -- Ben Franklin's errant conjecture -- and fixed the problem there, calling electrons "positive" and protons "negative."
Mystery resolved. Now, if I can only figure out how to get light bulbs out of sockets on ceiling fan/light systems without breaking them, I will be forever grateful. The vibration of the fan tends to wedge them in pretty tightly.....