Authors: Robert M. Hazen
The simplest electric motor incorporates a permanent magnet and an electromagnet. Magnetic forces drive the rotary motion
Electrical generators are the exact opposite of electric motors: they convert rotary motion into electrical energy. The basic generator, first put to practical use by Thomas Edison, is little more than a loop of wire spinning in a magnetic field. Because of the rotation, the field seen by the loop is constantly changing, so a current flows in the wire, first one way and then the other. This alternating (AC) current comes out of the generator on wires and can be used to run electrical circuits. Almost all electricity used in the United States is produced in this way.
Anything that can turn an axle can power a generator. Flowing water, pressurized steam, wind, or a gasoline engine can drive a rotating turbine that houses coils of copper wire. In a large generating plant, powerful electromagnets surround the wire loops. As a rotating loop of copper wire cuts through the magnetic field lines, electrons are pushed back and forth—60 times per second in the United States—to produce the 60-cycle alternating current that lights cities and runs air conditioners.
The aspect of electricity and magnetism that we encounter most often in everyday life is the electrical circuit. A circuit is a continuous path of material through which electrical charge can flow. The most common circuits are made from copper wires.
Any flow of electrical charges is called an electric current. Although both positive and negative charges can constitute a current, in everyday situations we give this name to the movement of electrons. A toaster and a lightbulb, for example, both get their energy from the movement of electrons through the copper wires of your home.
Electric current is usually discussed in terms of a unit called
the ampere, or amp. The amp measures how many charges go by a particular point in the wire (you can imagine a microscope traffic counter sitting in the wire and pushing a button every time an electron goes by, then adding things up every second). Typical household currents run from one amp (in a 100-watt lightbulb) to 50 amps (in an electric stove with all burners going and the oven on full blast).
Every circuit must also have a source—a device that supplies the energy to push electrons through the wires—which can be either a battery or a generator. In a battery, stored chemical energy is expended to provide the kinetic energy needed for electrons to move through the circuit. Current from a battery always flows in one direction, and is called DC (direct current). Current from a generator, on the other hand, alternates in direction and is called AC (alternating current).
The “pressure” with which electrons are pushed through a wire—the electrical potential—is measured in a unit called the volt. The higher the potential—the more volts—the more electrons can be pushed through a given wire. Some typical voltages encountered in everyday situations: flashlight batteries—1.5 V; car batteries—12 V; household current—115 V; high-voltage transmission lines—500,000 V.
Since all of the four experimental laws we have discussed so far were discovered by other people, you may be wondering why they are called Maxwell’s equations. There are three reasons: (1) he was the first to see that the equations formed a coherent system; (2) he added a small piece to the third law (he proved that
there was a kind of electrical current that no one had thought about up to that point); and (3) most important, he realized that the four equations predicted the existence of a new kind of energy wave—one that we now call electromagnetic radiation.
The third and fourth equations show that every field, magnetic or electric, induces a corresponding electric or magnetic field. Back and forth, ad infinitum, the fields create and modify each other. This sort of eternal oscillation, Maxwell realized, creates a wave that moves through space. Like ripples from a pebble thrown in a pond, these energy waves radiate out from their source.
All waves can be described by three closely linked characteristics: speed, wavelength, and frequency. Each wave consists of a series of crests and troughs. Wavelength is the distance between adjacent crests, speed is measured by the movement of the crests, and frequency is a measure of how many crests pass a given point in a second. The most common unit for measuring frequency is the hertz (named after the German radio pioneer Heinrich Hertz [1857–94]). One hertz (1Hz) corresponds to one crest going by a point each second. Look at the plate on any appliance in your home—it will say 60Hz, another reminder that household electrical current changes direction 120 times each second.
When Maxwell saw that his equations predicted the existence of waves, the first thing he did was calculate how fast those waves would move. He found that the speed of the mysterious waves depended on things like the force that one electrical charge or magnet exerts on another. These numbers had been measured in laboratories, so Maxwell was able to predict the velocity with high accuracy. His result: the waves move at 186,000 miles per second. This, of course, is what he knew, and we know, as the speed of light. Light itself is the mysterious electromagnetic wave.
The speed of light is so important that physicists denote it by a special letter—
. It is the only speed that is actually built in to the laws of nature. It figures prominently in many fundamental theories, like the theory of relativity and its famous equation,
E = mc
. It also denotes the speed of other types of radiation like X-rays and radio waves.
According to Maxwell’s calculation, his waves were actually composed of electrical and magnetic fields alternately creating each other as they move through space. The frequency of an electromagnetic wave is simply the frequency of the oscillating field that caused it. If you wave a charged comb in the air once a second, for example, you create an electromagnetic wave with a frequency of 1 hertz and a wavelength of 186,000 miles. Atoms can vibrate a trillion times a second, giving waves about a hundredth of an inch in length. It takes a lot more energy to wiggle an electron trillions of times per second than just once, so higher-frequency waves are also higher-energy waves. The important point, however, is that Maxwell’s equations predict that electromagnetic waves should exist at all frequencies and all wavelengths, not just for the narrow band we call visible light.
Think about waves on the ocean. They range from mini-ripples to swells a few yards long to tidal effects that span the oceans. These ocean waves are all intrinsically the same, differing only in the size, frequency, and energy contained in the moving water. If you travel in an ocean liner you notice only a narrow range of these waves—the ones that make the ship rise and fall. Other waves are all around, but you don’t sense them.
The same thing is true of electromagnetic radiation. Our eyes, like an ocean liner, sense a very narrow range of wavelengths—
those around a few thousand atom diameters (about a ten thousandth of an inch)—but longer and shorter waves are all around us. The complete set of these waves is called the electromagnetic spectrum. All types of waves in this spectrum travel at 186,000 miles per second, and all are produced by moving electromagnetic fields.
With his discovery Maxwell not only solved the mystery of the nature of light, but pointed to extraordinary practical consequences. As soon as he realized that visible light is only a narrow band of electromagnetic radiation, he postulated the existence of other waves of both longer and shorter wavelengths. These other waves included what we now call radio, microwave, infrared, ultraviolet, X-rays, and gamma rays.
There is no theoretical limit to the wavelength of electromagnetic radiation; frequencies from zero to infinity are possible. In practice, however, we can only detect a limited range of waves, from radio waves a few thousand miles long to gamma rays with wavelengths smaller than atomic nucleii. Scientists and engineers have divided the spectrum into several regions, somewhat arbitrarily, based on how the radiation is produced and how it is detected.
Radio waves, microwaves, visible light, and X-rays are all parts of the electromagnetic spectrum. Electromagnetic waves, which surround us all the time, are produced any time an electric charge accelerates. Frequencies range from thousands (kilohertz) or millions (megahertz) of cycles per second to many trillions of cycles per second
Radio waves encompass all electromagnetic radiation with wavelengths of a few yards to thousands of miles, the longest waves that we can easily produce and detect. Radio waves are very useful because they travel through air without being absorbed, they are easily generated and detected, and the longer wavelengths bend around Earth’s curvature. Radio waves are the ideal medium for global communication. When you watch TV or listen to your car radio, you are using signals that have been transmitted by radio waves.
Both radio and television signals begin in tall antennas, where electrons are accelerated back and forth to create electromagnetic waves. All stations have a basic “carrier” frequency—the frequency of the wave that you read on the radio dial. The way that music or conversation is impressed on the carrier depends on the type of signal being sent. FM stations vary the frequency slightly (frequency modulation) while AM stations vary the signal strength (amplitude modulation). The difference between AM and FM is analogous to sending signals with a flashlight. If you send a signal by alternately dimming and brightening the flashlight, you are acting like an AM station. If, on the other hand, you send a signal by changing the color of the emitted light, you are like an FM station.
AM radio waves are about 1,000 feet in wavelength—long enough to bend around Earth’s curvature. A strong station can be heard for hundreds of miles, especially at night when interference from other electromagnetic radiation is minimal. FM stations use radio waves only a few feet in wavelength. These waves do not bend around Earth, so FM stations must rely on line-of-sight transmission. This is why your favorite FM stations fade out when you drive more than about 50 miles from town.
The radio part of the electromagnetic spectrum is wide, but it can only accommodate a finite number of separate channels. In addition to thousands of radio and television stations, there are hundreds of thousands of marine, aviation, amateur, and public safety broadcasters. The vast number of radio transmitters now in use would hopelessly clutter the airways, leading to electromagnetic chaos, without strict international controls over the allocation of broadcast frequencies, licensing, and operation of all radio stations. One of the principal responsibilities of the
International Telecommunications Union and regional groups like the U.S. Federal Communications Commission is to allocate the long-wavelength end of the electromagnetic spectrum so that no two stations have frequencies that overlap.
Microwaves are electromagnetic waves about a tenth of an inch to a foot long. Longer microwaves, which have many features in common with radio waves, pass freely through air and can carry information. Unlike radio transmission, however, microwaves can be focused into a beam and therefore are often sent in highly directional signals, relayed with security from one cluster of hornlike antennas to another across the countryside. Furthermore, micro waves can be fine-tuned to yield a hundred times more useful frequencies than radio.
Line-of-sight transmission is essential for microwaves, so many microwave transmitters are prominently situated on tall towers or hilltops. More recently, microwaves have been used to communicate between Earth’s surface and satellites, which then beam the signal back to a different point on the earth. Many of the long-distance phone calls made in the United States are now routed through satellites via microwaves, as is satellite television. The TV dishes you see in backyards and on rooftops and in hotel parking lots are all designed and carefully positioned to receive micro wave signals sent down from satellites in fixed orbit. Commercial cellular phones operate in the same way, providing a link between a central transmitter and the mobile phone. In order to avoid cluttering the available channels, electronic systems break something like a large city into small units (“cells”), each with its own channel, and pass you along from one cell to the next, using whatever channel is available in each new cell.