Science Matters (7 page)

6
A.M. ANOTHER WEEKDAY
. The clock-radio blares, but you lie in bed a few minutes longer, listening to the news and weather and gathering energy to face the day. Turn on the light, start the coffee, wake the children, shower and dress. Grab the orange juice from the refrigerator—a note on the door reminds you that the kids have basketball practice after school. Eat a piece of toast, maybe some cereal. Brush your teeth, feed the cat, turn on the answering machine, and out the door to work. By 7 A.M. you’re on your way to another busy day.

Gravity is not the only natural force you experience daily, nor even the strongest. In just one hour you’ve had dozens of run-ins with electricity and magnetism. A magnet clings to your refrigerator door, easily overcoming the gravitational force that the entire Earth, pulling down, exerts on it. Static cling holds your clothes together, and you have to exert a force to pull them apart. These effects are not caused by gravity.

Electricity and magnetism are familiar forces and they appear everywhere in nature. Four laws of nature, Maxwell’s equations,
summarize everything we know about the phenomena of electricity and magnetism. The most important statement made in these equations is:

Lightning, static cling, friction, radio transmission, and the little magnets you use to hold notes on your refrigerator are all siblings.

Once Isaac Newton demonstrated the power of the scientific method in mechanics, it was natural that the method would be applied to other areas. Eighteenth-century researchers with now familiar names like Alessandro Volta and André-Marie Ampère studied electrical and magnetic phenomena as curiosities of the laboratory. They constructed batteries, examined the effects of electric sparks, passed current through various materials, and performed hundreds of other experiments. Driven by a desire to understand fascinating natural phenomena, these researchers never dreamed that electricity might someday transform society. Today we would say they were doing basic research.

The results of their experiments were summarized in laws, and these laws were brought together by the Scottish physicist James Clerk Maxwell in 1861. Maxwell’s four equations, which in their abbreviated mathematical form have become a popular adornment of physics department sweatshirts, play the same role in electromagnetism that Newton’s laws do for motion and gravity: they summarize everything there is to know on the subject.

When you run a stack of papers through a photocopy machine, individual sheets may stick together. The force that holds the sheets together is said to be the electrical force, and objects that respond to the electrical force are said to have an electrical charge. Some simple experiments show that there are in fact two kinds of electrical charge. If you run two plastic combs through your hair, the force between them will be repulsive: they will be pushed apart. If you take one of those combs and bring it near a piece of glass that has been rubbed with fur, however, the objects experience an attractive force: they will be pulled together. There are two kinds of electrical forces, so it is reasonable to suppose they are generated by two kinds of electrical charge, which for historical reasons are called positive and negative.

Electrical charge is carried by subatomic particles—the building blocks of atoms (Chapter 4). The atom’s massive central nucleus is positively charged, while lighter, negatively charged electrons orbit the nucleus. An object is electrically charged if its atoms possess either an excess of electrons (in which case it has a negative charge) or a deficit of electrons (in which case it has a positive charge). In most situations the ponderous nuclei of atoms move very slowly, while electrons move easily. Thus, large objects usually acquire an electric charge by having electrons removed or added to their bulk. When you comb your hair, for example, electrons are stripped from your hair and pulled into the comb. As a result, the comb acquires a negative charge. This is why it will pick up bits of dust and paper (try this experiment yourself next time you comb your hair on a dry day). It also explains why the comb will then attract your hair and why the individual strands of hair repel each other—why they can “stand up.” Vigorous combing
can even make your hair stand on end as the deficit of electrons (and consequent positive charge) increases.

The French physicist Charles Coulomb (1736–1806) first wrote down the law that describes forces between electric charges:

and

This law says that if two objects have an excess of electrons (and therefore have negative charges), they will repel each other, but if one has an excess and one a deficit, they will attract. It also says that the form of the equation that describes the electrical force is strikingly similar to the one that describes gravity.

Coulomb’s law describes the force between electrical charges that do not move—what is called static electricity. Electrostatic forces dominate the world as we know it. Plus attracts minus in chemical bonds, and thus holds materials together. Every object you see is made from atoms, themselves collections of negative electrons attracted to positive nuclei. Just as the gravitational force keeps the Earth and the other planets in orbit around the sun, electrostatic attraction keeps negative electrons in orbit around the positive nucleus of an atom.

The repulsion of electrons by electrons, on the other hand, keeps one object from passing through another. You can’t put your hand through this book, for example, because electrons in atoms in your hand are repelled by electrons in atoms in the
book. You don’t fall through the floor, because electrons in your shoes repel electrons in the floor. Every time you touch or feel something, you are making use of the electrostatic force.

Photocopies are products of electrostatic forces at work. A polished plate of selenium metal can hold an electrical charge for extended periods of time. When exposed to light, however, the charge leaks off. The key to xerography is to project a pattern of light and dark (such as a printed page) onto the charged plate. A similar pattern, consisting of charged and uncharged regions, is then created on the plate as charge leaves the lighted areas. Electrostatic forces cause a special black plastic powder to cling only to the charged areas of the selenium plate. The powder is transferred to paper, then melted in place, producing a copy of the original document.

Human beings have known about magnets and magnetism for thousands of years. Naturally occurring magnets, called lode-stones, were scientific curiosities in the ancient world, and slivers of lodestone that lined up in a north-south direction were the first compasses. That a magnetic force exists can be verified by anyone who uses magnets to hang notes and miscellany on the refrigerator.

All known magnets share one feature: each magnet, whether it be the size of an atom, a compass needle, or the planet Earth itself, has two poles. Each pole is usually labeled north or south, depending on which end of the Earth they would point to if they were allowed to act as a compass. Magnetic poles have properties reminiscent of electric charge. Poles with the same character always repel, while opposite poles always attract (north attracts south, but repels north).

There is, however, an important difference between electrical charges and magnetic poles—a difference enshrined in Max well’s second equation. No matter how hard you try, the law says, you can never create an isolated magnetic pole. Unlike electrical charges (which can exist as independent positive or negative particles), magnetic poles always come in pairs. If you cut a 2-inch-long bar magnet in half, you don’t get one north end and one south end. You get two 1-inch-long bar magnets, each with its own north and south pole. Cut those pieces in half and you just get more magnets. Even the individual atoms are tiny “dipole” (two-pole) magnets. Thus, Maxwell’s second equation states:

Maxwell’s second equation says nothing about how magnetic fields come to be. Static electricity and magnetism seem to be very different things, and there is no obvious connection between a photocopy machine and refrigerator decorations. The nature of magnetism, and the connection between it and electricity, is the subject addressed in Maxwell’s third and fourth equations.

The relationship between electricity and magnetism can be stated succinctly: every time an electric charge moves, a magnetic field is created; and every time a magnetic field varies, an electric field is created. Electricity and magnetism are two inseparable aspects of one phenomenon: you cannot have one without the other.

If we could see or feel electric and magnetic fields, their close ties would be obvious because we’d always see them together. But in day-to-day life we are not usually aware of electrical
effects when we use magnets, nor do we sense magnetic fields when we use electricity. We have to use instruments to tell us about the connection between the two.

The story of the discovery of this connection is a curious one. The Danish physicist Hans Oersted (1777–1851) was giving a physics lecture when he noticed that flipping a switch to start the flow of an electric current caused a nearby compass needle to twitch. Further experiments convinced him that a magnetic field is present whenever electrical charge flows through a wire. This finding (usually expressed in a suitable mathematical form) is Maxwell’s third equation:

One common application of this law of nature is a device called the electromagnet. The simplest electromagnet is a loop of wire carrying an electric current. Because the current produces a magnetic field, the loop acts as a magnet. Unlike the permanent magnets that you use to hold things on your refrigerator, however, an electromagnet can be turned off and on by opening and closing the switch that controls the current.

A single loop produces a magnetic field with a north and a south pole. In fact, you can think of a current-carrying loop as equivalent to a small bar magnet with its north pole coinciding with the north end of the magnetic field created by the current. The only difference is that the polarity of the loop’s field can be reversed by reversing the direction of the current. Electromagnets are found in many devices and machines, from ordinary doorbells to the large magnets that lift cars around in auto junkyards.

The other side of the electric/magnetic coin concerns the ability of magnetic fields to produce electrical forces. If the magnetic field in the region of a loop of wire is changed (by moving a magnet near the wire, for example), electrons will flow in the wire, even though there seems to be nothing in the wire to make them accelerate. This phenomenon, called electromagnetic induction, is described in the last of the Maxwell equations:

Electric current passing through a loop of wire creates a simple electromagnet, an essential component of every electric motor
.

Physicists Oersted, Henry, Faraday, and Maxwell did not know that their work would someday lead to large-scale generation and use of electricity. They could not have foreseen our twenty-first-century technological society, which nevertheless is almost entirely based on their discoveries. Electric motors and generators are simply practical applications of Maxwell’s third and fourth equations.

Your home contains dozens of electric motors. Fans, hair dryers, razors, mixers, can openers, and virtually all major appliances incorporate at least one. All of these motors convert electricity into magnetic fields, which in turn cause useful rotary motion. What happens when you flip the switch?

The simplest electric motors combine a permanent magnet with an electromagnet. Stationary electrical contacts pass current through rotating loops of wire, thus turning each loop into the equivalent of a small bar magnet. The north and south poles of the electromagnet are oriented so that each is attracted to the appropriate pole of the permanent magnet. The result: the loop starts to rotate as like poles repel and opposite poles attract. As soon as the rotating loop completes half a turn the current switches direction, causing the poles of the electromagnet to flip. Each pole of the rotating electromagnet now finds itself attracted to the next pole of the permanent magnet, so the loop continues to rotate.

Most motors are more complex than the simple one described above. Typically, a motor incorporates multiple sets of permanent magnets or several synchronized electromagnets. Different arrangements of the basic components lead to motors that turn with a constant speed or a high torque or by small steps. In every case, however, the basic principle is the same: electricity is converted into magnetic fields.