FIGURE 2-1: A common model of an atom.
Examining the Elements
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.
Minding Your Charges
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.
If you’ve been paying attention, you may have wondered how it can be that the nucleus of an atom can stay together if it consists of two or more protons that have positive charges. After all, don’t like charges repel? Yes they do, but the electrical repellent force is overcome by a much more powerful force called, for lack of a better term, the strong force. Thus, the strong force holds protons (and neutrons) together in spite of the protons’ natural tendency to avoid each other.
The strong force doesn’t affect electrons, so you never see electrons clumped together the way protons do in the nucleus of an atom. The electrons in an atom stay well away from each other.
If one were so inclined, one might liken the strong force to the patriotic force that binds the citizens of a nation together in spite of their differences. It’s this force that keeps a country together in spite of the fact that its political parties seem to hate each other. Let’s hope the strong force remains strong.
Conductors and Insulators
Some elements don’t hold on to their outermost electrons very tightly. These elements frequently lose electrons or pick up extra electrons, and so they frequently get bumped off of neutral and become either negatively or positively charged. Such elements are called conductors. The best conductors are the metals silver, copper, and aluminum.
Other elements hold on to their electrons tightly. In these elements, it’s hard to pry loose an electron or force another electron in. These elements almost always stay neutral. They’re called insulators.
In a conductor, electrons are constantly skipping around between nearby atoms. An electron jumps out of one atom — call it Atom A — into a nearby atom, which I’ll call Atom B. This creates a net positive charge in Atom A and a net negative charge in Atom B. But almost immediately, an electron will jump out of another nearby atom – call it Atom C — into Atom A. Thus, Atom A again becomes neutral, and now Atom C is negative.
This skipping around of electrons in a conductor happens constantly. Atoms are in perpetual turmoil, giving and receiving electrons and constantly cycling their net charges from positive to neutral to negative and back to positive.
Ordinarily, this movement of electrons is completely random. One electron might jump left, but another one jumps right. One goes up, another goes down. One goes east, the other goes west. The net effect is that although all the electrons are moving, collectively they aren’t going anywhere. They’re like Keystone Kops, running around aimlessly in every direction, bumping into each other, falling down, picking themselves back up, and then running around some more. When this randomness stops and the Keystone Kops get organized, the result is electric current, as explained in the next section.
Understanding Current
Electric current is what happens when the random exchange of electrons that occurs constantly in a conductor becomes organized and begins to move in the same direction.
When current flows through a conductor such as a copper wire, all those electrons that were previously moving about randomly get together and start moving in the same direction. A very interesting effect then happens: The electrons transfer their electromagnetic force through the wire almost instantaneously. The electrons themselves all move relatively slowly — on the order of a few millimeters a second. But as each electron leaves an atom and joins another atom, that second atom immediately loses an electron to a third atom, which immediately loses an electron to the fourth atom, and so on trillions upon trillions of times.
The result is that even though the individual electrons move slowly, the current itself moves at nearly the speed of light. Thus, when you flip a light switch, the light turns on almost immediately, no matter how much distance separates the light switch from the light.
Here are a few additional points that may