Electronics for begginers – Introduction to electronics – Part 4

25 Mar

Benjamin Franklin’s Positive Charges

Now, there is a tricky, if not crude, subtlety with regard to the direction of current flow that can cause headaches and confusion later on if you do not realize a historical convention initiated by Benjamin Franklin (often considered the father of electronics). Anytime someone says “current I flows from point A to point B,” you would undoubtedly assume, from what I just told you about current, that electrons would flow from point A to point  B, since they are the things moving. It seems obvious.

Unfortunately, the conventional use of the term current, along with the symbol I used in the equations, assumes that positive charges are flowing from point A to B! This means that the electron flow is, in fact, pointing in the opposite direction as the cur-rent flow. What’s going on? Why do we do this?

Trivia: Do you know what an ac to dc converter is? If you don’t, check out by clicking here.

The answer is convention, or more specifically, Benjamin Franklin’s convention of assigning positive charge signs to the mysterious things (at that time) that were moving and doing work. Sometime later a physicist by the name of Joseph Thomson performed an experiment that isolated the mysterious moving charges. However, to measure and record his experiments, as well as to do his calculations, Thomson had to stick with using the only laws available to him—those formulated using Franklin’s positive currents. However, these moving charges that Thomson found (which he called electrons) were moving in the opposite direction of the conventional current I used in the equations, or moving against convention.

What does this mean to us, to those of us not so interested in the detailed physics and such? Well, not too much. I mean that we could pretend that there were positive charges moving in the wires, and electrical devices and things would work out fine. In fact, all the formulas used in electronics, such as Ohm’s law (V = IR), “pretend” that the current I is made up of positive charge carriers. We will always be stuck with this convention. In a nutshell, whenever you see the term current or the symbol I, pretend that positive charges are moving. However, when you see the term electron flow, make sure you realize that the conventional current flow I is moving in the opposite direction.

Next chapter available here.


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