For the Love of Physics (25 page)

But there are lots of different kinds of magnets, not only magnetite. Iron has played such a big role in the history of magnetism, and remains such a key ingredient of many magnetically sensitive materials, that those materials that are most attracted to magnets are called ferromagnetic (“ferro” is a prefix indicating iron). These tend to be metals or
metal compounds: iron itself, of course, but also cobalt, nickel, and chromium dioxide (once used heavily in magnetic tapes). Some of these can be magnetized permanently by bringing them within a magnetic field. Other materials called paramagnetic become weakly magnetic when they’re placed in such a field and revert to being nonmagnetic when the field disappears. These materials include aluminum, tungsten, magnesium, and, believe it or not, oxygen. And some materials, called diamagnetic materials, develop fairly weak
opposing
magnetic fields in the presence of a magnetic field. This category includes bismuth, copper, gold, mercury, hydrogen, and table salt, as well as wood, plastics, alcohol, air, and water. (What makes certain materials ferromagnetic and some paramagnetic and others diamagnetic has to do with how the electrons are distributed around the nucleus—it’s
much
too complicated to go into in detail.)

There are even liquid magnets, which are not exactly ferromagnetic liquids, but rather solutions of ferromagnetic substances that respond to magnets in very beautiful and striking ways. You can make one of these liquid magnets fairly easily; here’s a link to a set of instructions:
http://chemistry.about.com/od/demonstrationsexperiments/ss/liquidmagnet.htm
. If you put this solution, which is fairly thick, on a piece of glass and put a magnet underneath, get ready for some remarkable results—a lot more interesting than watching iron filings line up along magnetic field lines as you may have seen in middle school.

In the eleventh century, the Chinese seem to have magnetized needles by touching them to magnetite and then suspending them from a silk thread. The needles would align themselves in the north-south direction; they aligned themselves with the magnetic field lines of the Earth. By the following century, compasses were being used for navigation both in China and as far away as the English Channel. These compasses consisted of a magnetized needle floating in a bowl of water. Ingenious, wasn’t it? No matter which way the boat or ship turned, the bowl would turn but the needle would keep pointing north and south.

Nature is even more ingenious. We now know that migrating birds have tiny bits of magnetite in their bodies that they apparently use as internal compasses, helping to guide them along their migration routes. Some biologists even think that the Earth’s magnetic field stimulates optical centers in some birds and other animals, like salamanders, suggesting that in some important sense, these animals can “see” the Earth’s magnetic field. How cool is that?

In 1600, the remarkable physician and scientist William Gilbert—not just any doctor, but physician to Queen Elizabeth I—published his book
De Magnete, Magneticisque Corporibus, et de Magno Magnete Tellure
(
On the Magnet and Magnetic Bodies, and on That Great Magnet the Earth
), arguing that the Earth itself was a magnet, a result of his experiments with a terrella, a small magnetite sphere meant to be a model of the Earth. It was maybe a little larger than a grapefruit, and small compasses placed on it responded just as they did on the surface of the Earth. Gilbert claimed that compasses point north because the Earth was a magnet, not, as some thought, because there were magnetic islands at the North and South Poles, or that compasses were pointing toward Polaris, the North Star.

Not only was Gilbert absolutely correct that the Earth has a magnetic field, but it even has magnetic poles (just like the poles in a refrigerator magnet), which do not quite coincide with the geographic north and south poles. Not only that, but these magnetic poles wander a bit, around 15 kilometers or so every year. So in some ways the Earth does act like a simple bar magnet—an ordinary rectangular magnetized piece of metal that you can buy in a hobby shop—but in other ways it’s completely different. It has taken scientists a very long time even to come up with a viable theory of why the Earth has a magnetic field. The fact that there’s a lot of iron in the Earth’s core isn’t enough, since above a certain temperature (we call it the Curie temperature) bodies lose their ferromagnetic quality, and iron is no exception; its Curie temperature is about 770° Celsius, and we know that the core is a whole lot hotter than that!

The theory is pretty involved, and has to do with the electric currents
circulating in the Earth’s core and the fact that the Earth is rotating—physicists call this a dynamo effect. (Astrophysicists use the theory of these dynamo effects to explain magnetic fields in stars, including that of our own Sun, whose magnetic field
completely reverses
about every eleven years.) It may seem amazing to you, but scientists are still working on a full mathematical model of the Earth and its magnetic field; that’s how complex the field is. Their work is made even thornier by the fact that there’s geological evidence that the Earth’s magnetic field has changed dramatically over the millennia: the poles have traveled much more than their annual stroll, and it appears that the magnetic field has also reversed itself—more than 150 times over the last 70 million years alone. Wild stuff, isn’t it?

We are able to chart the Earth’s magnetic field with some exactness now, thanks to satellites (such as the Danish Ørsted satellite) equipped with sensitive magnetometers. From this we know that the magnetic field reaches more than a million kilometers out into space. We also know that closer to Earth, the magnetic field produces one of the more beautiful natural phenomena in our atmosphere.

The Sun, you may remember, emits a huge stream of charged particles, mostly protons and electrons, known as the solar wind. Earth’s magnetic field directs some of those particles down into our atmosphere at the magnetic poles. When these fast-moving particles, with average speeds of about 400 kilometers per second, bang into atmospheric oxygen and nitrogen molecules, some of their kinetic energy (energy of motion) gets transformed into electromagnetic energy in the form of light—oxygen releases green or red and nitrogen blue or red. You’re probably guessing where I’m going—that’s right: this is what produces the spectacular light show known as the aurora borealis, the northern lights, in the Northern Hemisphere and the aurora australis, or southern lights, in the Southern Hemisphere. Why do you only see these lights when you are very far north or very far south? Because the solar wind predominantly enters the Earth’s atmosphere near the magnetic poles, where the magnetic field is the strongest. The reason the effects are stronger on some nights
than others is that whenever there are solar eruptions, there are more particles to make the light show. When there are huge solar flares, these effects can be massive, causing what we call geomagnetic storms, producing auroras far outside the normal areas and sometimes interfering with radio transmissions, computer functioning, satellite operations, and even causing power outages.

If you don’t live near the Arctic (or Antarctic) Circle, you won’t see these lights very often. That’s why, if you ever take an evening flight to Europe from the northeastern United States (and most flights are in the evening), you might want to try to get a seat on the left side of the plane. Since you’ll be seven miles up in the atmosphere, you might see some northern lights out your window, especially if the Sun has been particularly active recently, which you can find out online. I’ve seen it many times in just that way, so whenever I can, I sit on the left side of the plane. I figure I can watch movies whenever I want to at home. On planes I look for the northern lights at night and glories during the day.

We are truly indebted to Earth’s magnetic field, because without it, we might have suffered some serious consequences from the constant stream of charged particles bombarding our atmosphere. The solar wind might well have blasted away our atmosphere and water millions of years ago, creating conditions that would make the development of life much more difficult, if not impossible. Scientists theorize that just such a pounding by the solar wind due to Mars’s weak magnetic field is what accounts for the Red Planet’s thin atmosphere and comparative lack of water, an environment that human beings could inhabit only with the aid of powerful life support systems.

The Mystery of Electromagnetism

In the eighteenth century, a number of scientists began to suspect that electricity and magnetism were related in some way—even while others, such as the Englishman Thomas Young and the French scientist André-Marie Ampère, thought they had nothing to do with each other. William
Gilbert thought that electricity and magnetism were completely separate phenomena, but he nevertheless studied both simultaneously and wrote about electricity in
De Magnete
as well. He called the attractive force of rubbed amber the “electric force” (remember, the Greek word for amber was “
electron
”). And he even invented a version of the electroscope, the simplest way to measure and demonstrate the existence of static electricity. (An electroscope has a bunch of tinsel strips at the end of a metal rod. As soon as it is charged, the strips stand out away from one another: the laboratory equivalent of hat hair.)

The Bavarian Academy of Sciences invited essays on the relationship between electricity and magnetism in 1776 and 1777. People had known for some time that lightning discharges could make compasses go haywire, and none other than Benjamin Franklin himself had magnetized needles by using them to discharge Leyden jars. (Invented in the Netherlands at mid-century, the Leyden jar could store electric charges. It was an early version of the device we call a capacitor.) But while studies of electricity were exploding in the early nineteenth century, no scientist clearly linked electric current to magnetism until the Danish physicist Hans Christian Ørsted (born in 1777) made the absolutely crucial discovery bringing electricity and magnetism together. According to historian Frederick Gregory, this was probably the only time in the history of modern physics that such an enormous discovery was made in front of a class of students.

Ørsted noticed, in 1820, that an electric current flowing through a wire that was connected to a battery affected a nearby compass needle, turning it in a direction perpendicular to the wire and away from magnetic north and south. When he disconnected the wire, cutting the current flow, the needle returned to normal. It’s not entirely clear whether Ørsted was conducting his experiment intentionally as part of a lecture, or whether the compass happened to be right at hand and he simply observed the astounding effect. His own accounts differ—as we’ve seen more than once in the history of physics.

Whether it was an accident or purposeful, this may have been the
most important experiment (let’s call it that) ever carried out by a physicist. He concluded reasonably that the electric current through the wire produced a magnetic field, and that the magnetic needle in the compass moved in response to that magnetic field. This magnificent discovery unleashed an explosion of research into electricity and magnetism in the nineteenth century, most notably by André-Marie Ampère, Michael Faraday, Carl Friedrich Gauss, and finally in the towering theoretical work of James Clerk Maxwell.

Since current consisted of moving electric charges, Ørsted had demonstrated that moving electric charges create a magnetic field. In 1831 Michael Faraday discovered that when he moved a magnet through a conducting coil of wire, he produced an electrical current in the coil. In effect, he showed that what Ørsted had demonstrated—that electric currents produce a magnetic field—could be turned on its head: a moving magnetic field also produces electric currents. But neither Ørsted’s nor Faraday’s results make any intuitive sense, right? If you move a magnet near a conducting coil—copper works great because it’s so highly conductive—why on earth should you generate current in that coil? It wasn’t clear at first what the importance of this discovery was. Soon afterward, the story goes, a dubious politician asked Faraday if his discovery had any practical value, and Faraday is supposed to have responded, “Sir, I do not know what it is good for. However, of one thing I am quite certain; some day you will tax it.”

This simple phenomenon, which you can easily demonstrate at home, may not make any sense at all, but without exaggeration, it runs our entire economy and the entire human-made world. Without this phenomenon we would still live more or less like our ancestors in the seventeenth and eighteenth centuries. We would have candlelight, no radio, no television, no telephones, and no computers.

How do we get all this electricity that we use today? By and large we get it from power stations, which produce it with electric generators. Most fundamentally, what generators do is move copper coils through
magnetic fields; we no longer move the magnets. Michael Faraday’s first generator was a copper disk that he turned with a crank between the two arms of a horseshoe magnet. A brush on the outer edge of the disk ran to one wire, and a brush on the central shaft of the turning disk ran to a second wire. If he hooked the two wires up to an ammeter, it would measure the current being generated. The energy (muscle power!) he put into the system was converted by his contraption into electricity. But this generator wasn’t very efficient for a variety of reasons, not the least of which was that he had to turn the copper disk with his hand. In some ways we ought to call generators energy converters. All they are doing is converting one kind of energy, in this case kinetic energy, into electric energy. There is, in other words, no free energy lunch. (I discuss the conversion of energy in more depth in the next chapter.)

Electricity into Motion

Now that we’ve learned how to convert motion into electricity, let’s think about how to go in the other direction, converting electricity into motion. At long last, car companies are spending billions of dollars developing electric cars to do just that. They are all trying to invent efficient, powerful electric motors for these cars. And what are motors? Motors are devices that convert electric energy into motion. They all rely on a seemingly simple principle that’s pretty complicated in reality: if you put a conducting coil of wire (through which a current is running) in the presence of a magnetic field, then the coil will tend to rotate. How fast it rotates depends on a variety of factors: the strength of the current, the strength of the magnetic field, the shape of the coil, and the like. Physicists say that a magnetic field exerts a torque on a conducting coil. “Torque” is the term for a force that makes things rotate.