Basic Electrical Properties

Although you may know quite a bit about electricity and its related phenomena from your high school physics classes, you may not know how these concepts are applied in the real world. This section attempts to start from very basic concepts, but apply these to real-world systems. Knowing the material in this section can save you a lot of headaches down the line when planning rentals, cabling, etc.

What is Electricity?

Before one can understand why electricity works the way it does, it's first necessary to understand what it is. Unfortunately, you can't see it, smell it, or taste it, so it can be a little difficult to describe. Electricity is (for our purposes) the flow of electrons through some material. Electrons are present in all matter, but in certain kinds of matter (e.g. metals), they are particularly easy to get moving. These materials that allow easy transmission of electrons are called conductors; materials that tend to oppose the flow of electrons are called insulators. Note that this is really a matter of degree: even a good insulator, like air, can "break down" suddenly and allow electrons to go through it. More on this later.

The degree to which a material will oppose electrons flowing through it is called resistance. All materials have some resistance, but in metals it is quite small. The path of an electron through a material can be envisioned as similar to the path of a bowling rolling down a tree-lined slope (this, like all other analogies used to describe electricity, is partially inaccurate, but will be sufficient to allow you to get a feeling for what is going on). The ball is rolling along swimmingly, but occasionally will bump into trees and slow down a bit. The impact also imparts some energy to the tree, and may make it start to vibrate.

The more "trees" there are in the way, the more resistance there is to the ball's rolling down the hill, because the ball is more likely to hit them. Similarly, in conductors electrons flow through the substance, occasionally wacking into other atoms and losing a bit of energy to those atoms. With electricity, this loss of energy into the material is felt as heat, since the electrons cause the material's atoms to vibrate, and heat energy is just atomic vibration. Good conductors like metal don't tend to lose a lot of energy to heat, but bad conductors, like the filament inside a lamp, produce so much heat they start to glow (albeit under very controlled conditions). If you are using metal cabling to get electricity to a lamp, the situation you want to avoid is where the cabling you are using has enough resistance that it heats up as well as the lamp!

There are three basic properties that determine resistance. The first is the intrinsic resistance (resistivity) of the transport material, which you generally can't do anything about (aside from using the correct material to begin with). The second is the cross-sectional area of the material, such as cabling. A good analogy here is to imagine the electrons as water, and think about trying to force them through a narrow pipe. The final property is length, which if we go back to the ball analogy is akin to having a long slope so the ball hits more trees. In general, you'll find that the only factor you really have control over is the thickness of the wire you are using, because the physical location of the components you are connecting can't change, and you can't use some "super-metal" that has less resistance than standard copper cabling.

There's one other factor you should be worried about with respect to cabling and resistance, and that's the "breakdown" effect previously mentioned. Copper cabling is provided with a rubber or similar type of sheath to prevent the electricity flowing through the wire from "escaping" to some other path, like, say, your body. This insulation is designer for certain types of use; make sure you are using the correct kind of cabling for the job! Also make sure that the insulation is intact and undamaged.

Now, as our electrons flow through our cabling, they cause some interesting effects, and this is where the analogies tend to break down. As the electricity flows through the wires, it actually creates a small magnetic field at right angles to the flow of electrons. [ILLUS] This is rather bizarre, but not terribly useful. It doesn't take long, however, to realize that if the wire is coiled, the small magnetic fields add together to create a larger one. If an iron (ferrous, for those science types) material is placed inside the coil, the naturally magnetic properties of the iron reinforce the electro-magnetic field. Presto! An electromagnet.

This strange property of electricity ends up being very useful, for reasons we will examine later. In short, the ability to create magnetic fields is also reversible, e.g. magnetic fields can create electric current as well. Using this property, one can construct ways to modify the electric current flowing through wires in interesting and useful ways. But before we get to that, we have to look at what makes electric current move in the first place.