Sunday, May 05, 2002

Static electricity basics

I've been trying to come up with a blog-item length approach to this for a while. Let's see if this does any good. It will be simplified, perhaps too much - I'm sure people can find something to disagree with.

What is electricity? For our purposes today, we'll say it's a means of transmitting energy via electric charge differences.

OK, what's an electric charge? Physicists don't attempt to define this really. They just note that the smallest units of charge are protons and electrons, with arbitrarily labeled "positive" and "negative" charges respectively. Since electrons are easier to shake loose than protons are, for practical purposes electricity results from moving electrons.

Charged particles have their own electric fields, which exert forces when they interact with the electric fields of other objects. These forces are inversely proportional to the square of the difference between the particles - they get stronger rapidly as the particles get closer together. They tend to repel objects of like charge, and attract those of opposite charge. A result is that protons and electrons tend to stick together tightly into aggregates which as a whole carry no net charge, and getting electrons concentrated takes some "pressure".

If you've ever been zapped by static electricity, you have evidence that it's no big trick to get electrons loose. There will always be some running around, and this rate increases with temperature. But they are randomly distributed such that we can't take advantage of them for electric power.

It also makes a difference what substance we're talking about. The freedom of electrons to move within a particular substance varies over a broad range. The substances in which electrons move freely can be called "conductors", and are usually metallic. Substances that hold on to their electrons tightly are called "insulators". There are substances with intermediate properties known as "semiconductors", but that's another post.

The trick with electricity is to get these movable electrons directed and packed together long enough to make the resulting forces to do work for us.

A measure of the "packing" is the electric potential. This is sometimes called "voltage" because it is measured in volts (named for Alessandro Volta). Volts are a measure of the amount of energy stored per unit of charge.

These forces created by "packing" these electrons are significant. One of the reasons why electricity must be used as generated is that the demand is so large relative to the size of the devices it is practical to build to hold the electrical energy. Such devices, called capacitors, can literally blow up from the stresses if misused, and they're expensive in very large sizes. Thus when energy must be stored, it stored in a form other than electricity, then turned into electricity upon demand.

So how do you "pack" the electrons, and how do you get them to go where you want and do work? That'll be another in a series of blog items that'll be sure to thrill you. Stay tuned...

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