One volt can be defined as the amount of pressure that you need to create a flow of 1 ampere, which does 1 watt of work. As previously defined, 1 watt = 1 volt × 1 ampere, but the definition actually originated the other way around:
1 volt = 1 watt/1 ampere
It’s more meaningful this way, because a watt can be defined in nonelectrical terms. Just in case you’re interested, we can work backward through the units of the metric system like this:
1 watt = 1 joule/second
1 joule = a force of 1 newton acting through 1 meter
1 newton = the force required to accelerate 1 kilogram by 1 meter per second, each second
On this basis, the electrical units can all be anchored with observations of mass, time, and the charge on electrons.
Practically Speaking
For practical purposes, an intuitive understanding of electricity can be more useful than the theory. Personally I like the water analogies that have been used for decades in guides to electricity. Figure 1-77 shows a tall tank half full of water, with a hole punched in it near the bottom. Think of the tank as being like a battery. The height of the water is comparable to voltage. The volume of flow through the hole, per second, is comparable to amperage. The smallness of the hole is comparable to resistance. See Figure 1-79 on the next page.
Figure 1-77. If you want to get work out of a system…
Where’s the wattage in this picture? Suppose we place a little water wheel where it is hit by the flow from the hole. We can attach some machinery to the water wheel. Now the flow is doing some work. (Remember, wattage is a measurement of work.)
Maybe this looks as if we’re getting something for nothing, extracting work from the water wheel without putting any energy back into the system. But remember, the water level in the tank is falling. As soon as I include some helpers hauling the waste water back up to the top of the tank (in Figure 1-78), you see that we have to put work in to get work out.
Figure 1-78. . . . somehow or other you have to put work back into it.
Similarly, a battery may seem to be giving power out without taking anything in, but the chemical reactions inside it are changing pure metals into metallic compounds, and the power we get out of a battery is enabled by this change of state. If it’s a rechargeable battery, we have to push power back into it to reverse the chemical reactions.
Going back to the tank of water, suppose we can’t get enough power out of it to turn the wheel. One answer could be to add more water. The height of the water will create more force. This would be the same as putting two batteries end to end, positive to negative, in series, to double the voltage. See Figure 1-80. As long as the resistance in the circuit remains the same, greater voltage will create more amperage, because amperage = voltage/resistance.
What if we want to run two wheels instead of one? We can punch a second hole in the tank, and the force (voltage) will be the same at each of them. However, the water level in the tank will drop twice as fast. Really, we’d do better to build a second tank, and here again the analogy with a battery is good. If you wire two batteries side by side, in parallel, you get the same voltage, but for twice as long. The two batteries may also be able to deliver more current than if you just used one. See Figure 1-81.
Summing up:
Two batteries in series deliver twice the voltage.
Two batteries in parallel can deliver twice the current.
All right, that’s more than enough theory for now. In the next chapter, we’ll continue with some experiments that will build on the foundations of knowledge about electricity, to take us gradually toward gadgets that can be fun and useful.
Figure 1-79. Greater force generates more flow, as long as the resistance remains the same.
Figure 1-80. When you place two equal batteries in series, you double the voltage.
Figure 1-81. Two equal batteries that are wired in parallel will deliver the same voltage for twice as long as one.
2. Switching Basics and More
The concept of switching is fundamental in electronics, and I’m not just talking about power switches. By “switching,” I mean using one flow of electricity to switch, or control, another. This is such an important principle that no digital device can exist without it.
Today, switching is mostly done with semiconductors. Before I deal with them, I’ll back up and illustrate the concept by introducing you to relays, which are easier to understand, because you can see what’s going on inside them. And before I get to relays, I’ll deal with everyday on/off switches, which may seem very simple—but we have to nail down the basics.
Also in this chapter, I’ll deal with capacitance, because capacitance and resistance are fundamental to electronic circuits. By the end of the chapter, you should have a basic grounding in electronics and be able to build the noisemaking section of a simple intrusion alarm. This will be your first circuit that does something genuinely useful!
Shopping List: Experiments 6 Through 11
As in the previous shopping list, you should visit the various online suppliers for availability and pricing of components and devices. Manufacturers seldom sell small numbers of parts directly. Check the appendix for a complete list of URLs
