Doug Lowe - Electronics All-in-One For Dummies

One of the most amazing things about electricity is that it is, literally, everywhere. By that I don’t mean that electricity is commonplace or plentiful, or even that the universe has an abundant supply of electricity. Instead, what I mean is that electricity is a fundamental part of everything.

To get an idea of what I mean, consider a common misconception about electric current. Most of us think that wires carry electricity from place to place. When we plug in a vacuum cleaner and turn on the switch, we believe that electricity enters the vacuum cleaner’s power cord at the electrical outlet, travels through the wire to the vacuum cleaner, and then turns the motor to make the vacuum cleaner suck up dirt and grime and dog hair. But that’s not the case. The truth is that the electricity was already in the wire. The electricity is always in the wire, even when the vacuum cleaner is turned off or the power cord isn’t plugged in. That’s because electricity is a fundamental part of the copper atoms that make up the wire inside the power cord. Electricity is also a fundamental part of the atoms that make up the rubber insulation that protects you from being electrocuted when you touch the power cord. And it’s a fundamental part of the atoms that make up the tips of your finger which the rubber keeps from touching the wires.

In short, electricity is a fundamental part of the atoms that make up all matter. So, to understand what electricity is, we must first look at atoms.

As you probably learned in grade school, all matter is made up of unbelievably tiny bits that are called atoms . They’re so tiny that the period at the end of this sentence contains several trillion of them.

It’s hard for us to comprehend numbers as large as trillions. For the sake of comparison, suppose you could enlarge the period at the end of this sentence until it was about the size of Texas. Then, each atom would be about the size of — you guessed it — the period at the end of this sentence.

The word atom comes from an ancient Greek fellow named Democritus. Contrary to what you might expect, the word atom doesn’t mean “really small.” Rather, it means “undividable.” Atoms are the smallest part of matter that can’t be divided without changing it to a different kind of matter. In other words, if you divide an atom of a particular element, the resulting pieces are no longer the same thing.

For example, suppose you have a handful of some basic element such as copper and you cut it in half. You now have two pieces of copper. Toss one of them aside, and cut the other one in half. Again, you have two pieces of copper. You can keep doing this, dividing your piece of copper into ever smaller halves. But eventually, you’ll get to the point where your piece of copper consists of just a single copper atom.

If you try to cut that single atom of copper in half, the resulting pieces will not be copper. Instead, you’ll have a collection of the basic particles that make up atoms. There are three such particles, called neutrons, protons, and electrons.

The neutrons and protons in each atom are clumped together in the middle of the atom, in what is called the nucleus . The electrons spin around the outside of the atom.

When I first learned about atoms as a kid, I was taught that the electrons orbit around the nucleus much like planets orbit around the sun in our solar system. Even today, kids are taught this. School children are still being taught to create models of atoms using Styrofoam balls and wires, like the one shown in Figure 2-1.

That turns out to be a really bad analogy. Instead, the electrons whiz around the nucleus in a cloud that’s called, appropriately enough, the electron cloud. Electron clouds have weird shapes and properties, and strangely enough, it’s next to impossible to figure out exactly where in its cloud an electron actually is at any given moment.

FIGURE 2-1:A common model of an atom.

Several times in this chapter, I use the term element without explaining it. So here’s the deal: An element is a specific type of atom, defined by the number of protons in its nucleus. For example, hydrogen atoms have just one proton in the nucleus, an atom with two protons in the nucleus is helium, atoms with three protons are called lithium, and so on.

The number of protons in the nucleus of an atom is called the atomic number . Thus, the atomic number of hydrogen is 1, the atomic number of helium is 2, lithium is 3, and so on. Copper — an element that plays an important role in electronics — is atomic number 29. Thus, it has 29 protons in its nucleus.

What about neutrons, the other particle found in the nucleus of an atom? Neutrons are extremely important to chemists and physicists. But they don’t really play that big of a role in the way electric current works, so we can safely ignore them in this chapter. Suffice it to say that in addition to protons, the nucleus of each atom (except hydrogen) contains neutrons. In most cases, there are a few more neutrons than protons.

The third particle that makes up atoms is the electron. Electrons are what we’re most interested in when we work with electricity because they are the source of electric current. They’re unbelievably small; a single electron is about 200,000 times smaller than a proton. To gain some perspective on that, if a single electron were the size of the period at the end of this sentence, a proton would be about the size of a football field.

Atoms usually have the same number of electrons as protons, and thus an atom of the element copper has 29 protons in a nucleus that is orbited by 29 electrons. When an atom picks up an extra electron or finds itself short of an electron, things get interesting because of a special property of protons and electrons called charge, which I explain in the next section.

Two of the three particles that make up atoms — electrons and protons — have a very interesting characteristic called electric charge . Charge can be one of two polarities : negative or positive. Electrons have a negative polarity, while protons have a positive polarity.

The most important thing to know about charge is that opposite charges attract and similar charges repel. Negative attracts positive and positive attracts negative, but negative repels negative and positive repels positive.

As a result, electrons and protons are attracted to each other, but electrons repel other electrons and protons repel other protons.

The attraction between protons and electrons is what holds the electrons and the protons of an atom together. This attraction causes the electrons to stay in their orbits around the protons in the nucleus.

Here are a few more enlightening details about charge:

Charge is a property of one of the fundamental forces of nature known as electromagnetism . The other three forces are gravity , the strong force , and the weak force.

As I say in the previous section, an atom normally has the same number of electrons as protons. This is because the electromagnetic force causes each proton to attract exactly one electron. When the number of protons and electrons is equal, the atom itself has no net charge. It is then said to be neutral. However, it’s possible for an atom to pick up an extra electron. When it does, the atom has a net negative charge because of the extra electron. It’s also possible for an atom to lose an electron, which causes the atom to have a net positive charge because it has more protons than electrons.