For the Love of Physics (22 page)

Because of electricity, horses run, dogs pant, and cats stretch. Because of electricity, Saran Wrap crumples, packing tape attracts itself, and the cellophane wrapping never seems to want to come off of a box of chocolates. This list is hardly exhaustive, but there’s really nothing that we can imagine existing without electricity; we could not even think without electricity.

That holds true when we turn our focus to things even smaller than the microscopic cells in our bodies. Every bit of matter on Earth consists of atoms, and to really understand electricity we have to go inside the atom and briefly look at its parts: not all of them now, because that gets incredibly complicated, but just the parts we need.

Atoms themselves are so tiny that only the most powerful and ingenious instruments—scanning tunneling microscopes, atomic force microscopes, and transmission electron microscopes—can see them. (There are some astonishing images from these instruments on the web. You can see some at this link:
www.almaden.ibm.com/vis/stm/gallery.html
.)

If I were to take 6.5 billion atoms, roughly the same as the number of people on Earth, and line them up in a row, touching one another, I would have a line about 2 feet long. But even smaller than every atom, about ten thousand times smaller, is its nucleus, which contains positively charged protons and neutrons. The latter, as you might imagine from their name, are electrically neutral; they have no charge at all. Protons (Greek for “first one”) have about the same mass as the neutrons—the inconceivably small two-billionths of a billionth of a billionth (2 × 10
^–27
) of a kilogram, approximately. So no matter how many protons and neutrons a nucleus has—and some have more than two hundred—it remains a real lightweight. And tiny: just about a trillionth of a centimeter in diameter.

Most important for understanding electricity, however, is that the proton has a positive charge. There’s no intrinsic reason for it to be called positive, but since Franklin, physicists have called the charge left on a glass rod after it’s been rubbed with silk positive, so protons are positive.

Even more important, it turns out, is the remainder of the atom, consisting of electrons—negatively charged particles that swarm around the nucleus in a cloud, at some distance by subatomic standards. If you hold a baseball in your hand, representing an atomic nucleus, the cloud of electrons around it would range as far as
half a mile
away. Clearly, most of the atom is empty space.

The negative charge of an electron is equal in strength to the positive charge of the proton. As a result, atoms and molecules that have the same number of protons and electrons are electrically neutral. When they are not neutral, when they have either an excess or deficit of electrons, we call them ions. Plasmas, as we discussed in
chapter 6
, are gases partially or fully ionized. Most of the atoms and molecules we deal with on Earth are electrically neutral. In pure water at room temperature only 1 in 10 million molecules are ionized.

As a consequence of Franklin’s convention, when some objects have an overabundance of electrons, we say that they are negatively charged, and when they have a deficit of electrons, we say they have a positive charge. When you rub glass with a piece of silk you “rub off” (sort of) lots of electrons, so the glass ends up with a positive charge. When you rub amber or hard rubber with the same piece of silk, they collect electrons and develop a negative charge.

In most metals large numbers of electrons have escaped their atoms altogether and are more or less freely wandering around between atoms. These electrons are particularly susceptible to an external charge, either positive or negative, and when such a charge is applied, they move toward or away from it—thus creating electric current. I have a lot more to say about current, but for the time being I’ll just point out that we call these materials conductors, because they easily conduct (allow the movement of) charged particles, which in this case means electrons. (Ions can also create electric currents but not in solids, and thus not in metals.)

I love the idea of electrons just ready to play, ready to move, ready to respond to positive or negative charges. In nonconductors, there’s very little action of this sort; all the electrons are well fixed to their
individual atoms. But that doesn’t mean we can’t have some fun with nonconductors—especially your garden-variety, rubber, nonconducting balloon.

You can demonstrate everything I’m talking about here by supplying yourself with a little pack of uninflated rubber balloons (thinner ones work better, like the ones you can twist into animals). Since most of you don’t have glass rods sitting around, I had hoped that a water glass or wine bottle or even a lightbulb might substitute, but despite my best efforts, they don’t. So why not try a large plastic or hard rubber comb? It will also be helpful to have a piece of silk, maybe an old tie or scarf, or a Hawaiian shirt your significant other has been trying to get you to throw out. But if you don’t mind getting your hair mussed—for the cause of science, who would mind?—you can make use of your own hair. And you’ll need to tear up some paper into, say, a few dozen or so pieces. The number doesn’t matter, but they should be small, about the size of a dime or penny.

Like all static electricity experiments, these work a lot better in winter (or in afternoon desert air), when the air is dry rather than moist. Why? Because air itself is not a conductor—in fact, it’s a pretty good insulator. However, if there is water in the air, charge can bleed away for complicated reasons which we will not discuss. Instead of allowing charge to build up on a rod or cloth or balloon, or your hair, humid air gradually bleeds charge away. That’s why you only have a problem getting shocked on doorknobs when the air is really dry.

Invisible Induction

Assemble all your materials, and get ready to experience some of the wonders of electricity. First charge up your comb by rubbing it hard on your hair, making sure your hair is very dry, or rubbing it with the piece of silk. We know from the triboelectric series that the comb will pick up negative charge. Now, stop for a moment and think about what’s going to
happen as you bring the comb close to the pile of paper bits, and why. I could certainly understand if you say “nothing at all.”

Then put the comb a few inches above your little mound of paper pieces. Slowly lower the comb and watch what happens. Amazing, isn’t it? Try it again—it’s no accident. Some of the bits of paper jump up to your comb, some stick to it for a bit and fall back down, and some stay fast. In fact, if you play around with the comb and the paper a bit, you can make the pieces of paper stand on edge, and even dance on the surface. What on earth is going on? Why do some pieces of paper stick to the comb, while others jump up, touch, and fall right back down?

These are excellent questions, with very cool answers. Here’s what happens. The negative charge on the comb repels the electrons in the paper atoms so that, even though they’re not free, they spend just a little more time on the far side of their atoms. When they do so, the sides of the atoms nearest the comb are just a tiny bit more positively charged than they had been before. So, the positive-leaning edge or side of the paper is attracted to the negative charge on the comb, and the very lightweight paper jumps up toward the comb. Why does their attractive force win out over the repulsive force between the comb’s negative charge and the electrons in the paper? It’s because the strength of electrical repulsion—and attraction—is proportional to the strength of the charges, but
inversely proportional
to the square of the distance between them. We call this Coulomb’s law, named after the French physicist Charles-Augustin de Coulomb, who made this important discovery, and you will notice its astonishing similarity to Newton’s law of universal gravitation. Note that we also call the basic unit of charge the coulomb, and the positive unit of charge is +1 coulomb (about 6 × 10
¹⁸
protons), while the negative charge is –1 coulomb (about 6 × 10
¹⁸
electrons).

Coulomb’s law tells us that even a very small difference in the distance between the positive charges and the negative charges can have a large effect. Or put differently, the attractive force of the nearer charges overpowers the repelling force of the more distant charges.

We call this entire process
induction
, since what we are doing when we bring a charged object toward a neutral one is
inducing
charge on the near and far sides of the neutral object, creating a kind of charge polarization in the pieces of paper. You can see several versions of this little demonstration in my lecture for kids and their parents called “The Wonders of Electricity and Magnetism” on MIT World, which you can find here:
http://mitworld.mit.edu/video/319
.

As for why some bits of paper fall right back down while some stay stuck, this is also interesting. When a piece of paper touches the comb, some of the excess electrons on the comb move to the paper. When that happens, there still may be an attractive electric force between the comb and the piece of paper, but it may not be large enough anymore to counter the force of gravity, and thus the piece of paper will fall down. If the charge transfer is high, the electric force may even become repelling, in which case both the force of gravity and the electric force will accelerate the piece of paper downward.

Now blow up a balloon, knot the end so it stays blown up, and tie a string to the end. Find a place in your house where you can hang the balloon freely. From a hanging lamp, perhaps. Or you can put a weight of some kind on the string and let the balloon hang down from your kitchen table, about six inches to a foot. Charge the comb again by rubbing it vigorously with the silk or on your hair—remember, more rubbing produces a stronger charge. Very slowly, bring your comb close to the balloon. What do you think is going to happen?

Now try it. Also pretty weird, right? The balloon moves toward the comb. Just like with the paper, your comb produced some kind of separation of charge on the balloon (induction!). So what will happen when you move the comb farther away—and why? You know, intuitively, that the balloon will return to its vertical position. But now you know why, right? When the external influence disappears, the electrons no longer have any reason to hang out a little more on the far side of their respective atoms. Look what we were able to deduce just from this little bit of
rubbing a comb and playing with little pieces of paper and a drugstore balloon!

Now blow up some more of the balloons. What happens when you rub one vigorously on your hair? That’s right. Your hair starts to do weird things. Why? Because in the triboelectric series human hair is way at the positive end, and a rubber balloon is on the seriously negative side. In other words, rubber picks up a lot of the electrons from your hair, leaving your hair charged positively. Since like charges repel, what else can your hair do when each strand has a positive charge and wants to get away from all the other like-charged hairs? Your strands of hair are repelling one another, making them stand up. This is of course also what happens when you pull a knit hat off of your head in winter. In rubbing your hair, the hat takes lots of electrons away, leaving the strands of your hair positively charged and aching to stand up.

Back to the balloons. So you’ve rubbed one vigorously on your hair (rubbing it on your polyester shirt may work even better). I think you know what I’m going to suggest, right? Put the balloon against the wall, or on your friend’s shirt. It sticks. Why? Here’s the key. When you rub the balloon, you charge it. When you hold the balloon against the wall, which is not much of a conductor, the electrons orbiting the atoms in the wall feel the repulsive force of the balloon’s negative charge and spend just a wee bit more time on the side of the atom farthest away from the balloon and a little bit less on the side closest to the balloon—that’s induction!

The surface of the wall, in other words, right where the balloon is touching it, will become slightly positively charged, and the negatively charged balloon will be attracted. This is a pretty amazing result. But why don’t the two charges—the positive and negative charges—just neutralize each other, with charges transferring, making the balloon immediately fall off? It’s a very good question. For one thing the rubber balloon has picked up some extra electrons. They don’t move around very easily in a nonconductor like rubber, so charges tend to stay put. Not only that,
you’re not rubbing the balloon against the wall, making lots and lots of contact. It’s just sitting there, doing its attractive thing. But it’s also held there by friction. Remember the Rotor carnival ride back in
chapter 3
? Here the electric force plays the role played by the centripetal force of the Rotor. And the balloon can stay on the wall for some time, until the charge gradually leaks off the balloon, generally onto moisture in the air. (If your balloons don’t stick, the air is either too humid, making the air a better conductor, or your balloons might be too heavy—I suggested thin ones for just this reason.)

I have a ball sticking balloons on the kids who come to my public lectures. I have done this for years at kids’ birthday parties, and you can have great fun with it too!

Induction works for all kinds of objects, conductors as well as insulators. You could do the comb experiment with one of those helium-filled Aluminized Mylar balloons you can buy in grocery or dollar stores. As you bring the comb near the balloon, its free electrons tend to move away from the negatively charged comb, leaving positively charged ions nearer the comb, which then attract the balloon toward it.

Even though we can charge rubber balloons by rubbing them on our hair or shirt, rubber is, in fact, a nearly ideal insulator—which is why it’s used to coat conducting wires. The rubber keeps charge from leaking out of the wires into moist air or jumping to a nearby object—making sparks. After all, you don’t want sparks jumping around in flammable environments, like the walls of your house. Rubber can and does protect us from electricity all the time. What it cannot do, however, is protect us from the most powerful form of static electricity you know: lightning. For some reason people keep repeating the myth that rubber sneakers or rubber tires can protect us from lightning. I’m not sure why these ideas still have any currency, but you’re best off forgetting them
immediately!
A lightning bolt is so powerful that it doesn’t care one bit about a little bit of rubber. Now you
may
be safe if lightning hits your car—probably not, in reality—but it doesn’t have anything to do with the rubber tires. I’ll get to that a little later.