This article covers Volts, Amps, Ohms, Watts, and some basic concepts about the flow of electricity, batteries, and resistance.
To start with, let's consider a very simple circuit. Suppose we have a flashlight using a single lantern battery (6 volts), a switch, and a light bulb. Here are some things you need to note about this circuit.
We'll come back to the flashlight, but at the moment we need to learn a few other concepts.
We measure the flow of electricity in units called AMPERES, which is often abbreviated to AMPS. So, you say, what's an amp? Well, the technical answer is that it is the flow of one coulomb of charge per second, and a coulomb is the charge on 6.25x10E18 (6.25 times ten to the eighteenth power) electrons. That's a lot of electrons, which makes for messy math if you use all those ten to the eighteenth type numbers. That's why we use amps.
A more practical answer is just that the flow of electricity is measured in amps, and if you really want to get into some self abuse, pick up a decent physics book and plow into it all. It's quite a mess down on the atomic level, and fortunately, as far as the electronics hobbyist is concerned, you don't really need to know any of it.
One thing you do need to know is the concept of resistance. Every material resists the flow of electricity to some degree or another (we'll ignore superconductors for the moment). True insulators and conductors do not exist. However, for all practical purposes, the amount that conductors resist the flow of electricity is negligable, so we ignore it in most cases. The resistance is measured in Ohms. Most metals make good conductors. My kitchen sink, for example, is made out of metal, and I measured about 0.7 ohms from one side to the other on it. That's pretty close to zero, so my sink is a pretty good conductor. Electricity flows fairly easily through my sink.
Wood is a very poor conductor. It's resistance is very high, so high that it can be considered to be an insulator. I was going to measure the resistance of my desk, but my meter only measures up to about 2 million ohms, and the resistance of my desk is higher than that.
Pure water is not that great of a conductor. Add a few impurities, though, and it's resistance drops dramatically. Your water will vary greatly, but I measure about 25,000 ohms on a cup of the sludge that Baltimore calls water (it's supposed to be clear, isn't it?). On dry skin, I measure about 250,000 ohms from one hand to the other. On wet skin, this drops down to about 20,000 to 25,000 ohms. Most plastics are very poor conductors. Glass is a poor conductor, and in fact I have seen glass used to make insulators on telephone poles. The ceramic that my coffee cup is made out of is a very poor conductor. Rubber is a poor conductor.
Another concept we deal with is charge seperation. This is measured in units called a VOLT. The amount of charge seperation applied across a material will cause some number of electrons to flow through the material. Don't panic if you didn't understand that. We'll get back to the flashlight to show you what this all means.
Fortunately, the relationship between volts, amps, and ohms in most materials is pretty simple. This is the formula known as Ohm's Law, and is simply V=IR, where V is the voltage, I is the current (I for Intensity, originally some French version like Intensitie or something) and R is the resistance.
Ohm's Law, V=IR, is VERY IMPORTANT. Memorize it. Most practical electronics problems can be solved with some variation on Ohm's Law.
One thing we haven't really looked at much is the battery. Batteries are fairly simple. A battery is nature's closest example of a voltage source. No matter what you connect across its terminals, the battery will attempt to keep the voltage at its terminals constant (assuming that it can supply enough current to do so). Nature doesn't really give us a good example of a current source (a device that will always try to supply a constant current, no matter what voltage is present at its terminals), although current sources can be constructed out of a few components, and are used in electronics. Don't worry about current sources for now, and more details on batteries can be found in another chapter.
We are going to assume that the wires are ideal conductors. Therefore they do not affect our calculations, and merely act as a current path, and so we can ignore them. We will also ignore any resistance in the switch. We will consider it to be a conductor when it is closed, and an insulator when it is open. This simplifies our problem a lot. All we have left to consider is a battery connected to a light bulb. Our battery is (for our purposes) a constant voltage source, 6 volts. That means that we have already determined the voltage across our light bulb (6 volts, since it is connected across the terminals of the battery). The resistance of our light bulb can be determined by the material it is made out of, how thick that material is, and how long it is. Let's assume that it is 600 ohms, just for our example. How much current is flowing?
We know V (6 volts) and R (600 ohms), so Ohm's Law tells is that V=IR, or I=V/R (rearranging). The current therefore is I=V/R=6/600=0.010 amps.
We will often use the different prefixes (milli, micro, etc) with the units of volt, amp, and ohm, so the above would often be written as 10 mA (ten milli-amps).
I never liked the water pipe analogy, but it helps some people, so here it is.
Voltage is kind of like how much electricity you have, and current is kind of like how much of it is moving past a certain point. Some people make the analogy between this and a water pipe. Voltage is like how much water you have (or how high the water pressure is), and current is like how fast the water is going through the pipe. All other things being equal, if you have more water, it will push faster through the pipe. Likewise, for any fixed resistance (like our light bulb), more voltage applied to it will cause a proportional increase in current. A wider pipe (less resistance) will let more water flow than a narrow pipe (more resistance).
The problem with the water pipe analogy (in my opinion) is that it encourages people to think of electricity as a liquid flowing through a pipe. While electricity does have some analogies to this, there are many cases where water does one thing and electricity does another. For example, electricity will travel up a water stream. This is a difficult concept for people who can't envision a pipe-like path for the electricity.
Sometimes it is more useful to look at the power in a device, rather than looking at the current through it or the voltage across it. The light from a light bulb is more a function of power than it is current. Power, measured in Watts, is simply the voltage multiplied by the current. We use P to represent power in formulas.
P=VI
You can play some algebra games using Ohm's Law, and you will find that P=V(squared)/R, and P=I(squared)R (sorry, I can't type the little two superscript where the word "squared" is written).
In our flashlight, P=VI=(6)(0.010)=0.06 watts (60 mW). Almost all of this energy is converted into heat in a flashlight bulb, by the way. The amount of energy converted into light is fairly small by comparison.