The Physics of Superheroes: Spectacular Second Edition (36 page)

If an electrical current generates a magnetic field, then could a moving magnetic field induce an electrical current in a nearby wire? The answer, as anyone who has read X-Men comics would know, is yes. In previous battles with the X-Men, and occasionally with puny humans, Magneto employed his mutant talent to transform any metal object into either an offensive weapon or a defensive shield. Magneto’s power is most effective on metals that are already magnetic. There are only three elements (iron, cobalt, and nickel) that are magnetic at room temperature. Magneto can manipulate a steel girder into any form he wishes through the iron it contains, but his power would be limited on a gold wedding band, unless he is willing to expend an extreme effort in polarizing the normally diamagnetic material. But Magneto’s real power lies not in his ability to exert forces on other magnetic materials, but in his control over electrical currents.

For example, the Mutant Master of Magnetism once constructed a computerized control panel that automated the power-dampening fields that neutralized the X-Men’s mutant abilities, keeping them from interfering with his plan of conquest. To prevent the X-Men from deactivating the device, Magneto configured it so that it has no buttons or knobs that can be reprogrammed. Magneto controlled the panel by altering the electrical currents that flowed through its circuits, affecting them through the magnetic fields that they create. Furthermore, by varying the magnetic field over the control panel, Magneto could cause these currents to flow.

How would a varying magnetic field create an electrical current? The answer brings us back to the point that started our discussion of electrical currents and magnetic fields: relative motion.

Just as a magnet can be pushed or pulled when a second magnet is brought near it, an external magnetic field can exert a force on an electrical current. As described in the previous chapter, moving electric charges generate a magnetic field that can attract or repel other magnetic fields, whether created by another electrical current or by a refrigerator magnet. When the charges are not moving, but are sitting in a wire placed in an external magnetic field, there will be no force exerted on them.
⁶⁵
What about if the external bar magnet is moved, while the charges remain sitting still in the wire? Assume that the magnet is moved toward the wire. From the magnet’s point of view, it is not moving at all, but it is the wire that is moving toward it.

Magnetism is, at its heart, all about relative motion. If you were a blindfolded passenger in a car moving at constant speed in a straight line, how could you prove that when you arrived at your destination, it was the car that moved, and not the scenery? If you change your speed or direction, then you will feel a force associated with the acceleration, and this will tip you off that it is you doing the moving. But for uniform motion, you cannot really prove whether you or everything else is moving. All you can say for sure is that you moved relative to your surroundings.

Similarly, when moving a magnet toward a wire, from the magnet’s point of view, it is stationary, and it’s the charges (both the mobile electrons and the fixed positively charged ions) in the wire that are moving toward it. But moving electrical charges create a magnetic field that will interact with the field of the magnet. So, by moving a magnet near a wire, the magnet sees two electrical currents from the positively charged ions and negatively charged electrons. A force is exerted on the charges in the wire and the electrons are free to move in response to this force. In this way, Magneto is able to affect the direction of electrical currents in any device at will, though the precision by which he can guide them depends on how sensitively he can manipulate these magnetic fields.

If relative motion is the only factor that matters when considering whether a magnetic field affects electrical charges, then how about a situation in which the magnet is stationary, but the wire is moved past the magnet? Would that generate a force on the charges? Sure!

If I pull a wire through space, the electrons in the wire are moving, just as surely as if I kept the wire still and applied a voltage across it. In either case, the electrons are moving past a fixed point at a certain speed. Relative to the magnet, it is as if there were an electrical current flowing by it, and we know that electrical currents and magnets interact. In this situation, the charges in the moving wire will feel a force that will induce them to flow. By dragging the wire through the external magnetic field, we convert the physical energy spent moving the wire into a form of electrical energy manifested by the electrical current. For a coil of wire, it does not matter whether the magnet is pulled through the loop or the loop is moved past the magnet. As long as there is a relative motion between the charges in the wire and the magnitude of the magnetic field threading the loop, then a current will be induced, even without an outside voltage. This mechanism may sound a bit far-fetched, but it is in fact how the electricity coming into your house is generated.

A power station, such as the one Electro employs to charge up for a night of crime, operates on the principle that when a magnetic field passing through the plane of a coil of wire is changed, a current is induced in the wire. This is called Faraday’s Law, after Michael Faraday, who was one of the first scientists to introduce the concept of electric and magnetic fields. The direction of this induced current is such that it creates a magnetic field that opposes the changing external magnetic field. This is a consequence of energy conservation, as we’ll explain in a moment. In certain circumstances, this current is termed an “eddy current,” but it occurs whenever the magnetic field passing through a coil is increased or decreased.

Imagine a large magnet bent in the form of a broken ring, so that the north pole faces the south pole, with a coil held in the open gap between its north and south poles. Initially the plane of the coil is at right angles to the magnet’s poles, so the magnetic field passes through the loop. If the coil is now rotated ninety degrees, the plane of the coil faces away from the poles, so the amount of magnetic field passing through the coil is very small. Another right angle turn, and now the coil again faces the poles, and the magnetic field passing through it is large again. A further quarter rotation, and the field through the coil is minimized again, and so on. For every change in the magnetic field passing through the coil, whether an increase or a decrease, a current is induced. The direction of the induced current flips back and forth as the coil rotates. What we have just described is an electric dynamo, and by continuously changing the magnetic field passing through the coil faces, an electric voltage will be generated in the rotating coils. There are tricks for converting an alternating current (referred to as AC) to a direct current (known as DC). There are many practical reasons that we won’t go into for using AC to supply our electrical needs. In the United States the coils are made to rotate sixty times in one second, which is why in the States the AC power has a frequency of 60 Hz (Hz is an abbreviation for the unit of frequency “Hertz” and measures the number of cycles or rotations per second), while in Europe the AC current’s frequency is 50 hz.

A current flows when the magnetic field passing through the rotating coil changes. From a conservation-of-energy standpoint, we realize that it must take energy to rotate the coil, in order to create an electrical current that did not exist before. In
The Dark Knight Strikes Again # 1
—Frank Miller’s dystopian view of the future of the DC universe in which superheroes are forced into servitude and Lex Luthor runs the country—a third of America’s electrical power was supplied by coercing the Flash to continually run on a treadmill. Recall from Chapter 12 that the Flash has managed to find a loophole around the Principle of Conservation of Energy (presumably through his ability to tap into a “speed force”), so in Luthor’s view, one might as well get some economic benefit from this suspension of the rules of physics. In our world, where we have yet to find a single exception to the conservation of energy principle, the energy that turns turbines and generates electricity comes from the same mechanism employed for making tea.

Nearly all commercial power plants generate electricity by boiling water. The resulting steam turns a turbine (a fancy term for a pinwheel) to which the coils of wire in the powerful magnets are connected. As the turbines rotate, so do the coils, and electrical current is produced. To boil the water, one either burns coal, oil, natural gas, or bio-mass. Alternatively, the heat generated from a nuclear reaction can boil water and turn the turbine. But it all just goes toward creating steam in order to turn a pinwheel attached to a coil between the poles of a magnet. The stored chemical energy in the coal, oil, or garbage has the same source as the chemical energy in the food we eat—that is, from plant photosynthesis. The light from the sun is a by-product of the nuclear-fusion reaction running in the solar core. (So, all electrical power plants could be viewed as nuclear plants or solar plants, depending on your political bent.)

The rotation of the blades in wind-generated electrical power results from temperature differences in the atmosphere arising from spatial variations in the sunlight absorbed by the atmosphere or reflected by cloud cover. Obviously, solar cells (to be discussed in Section 3) require sunlight in order to function. Similarly, hydropower, in which the potential energy of water in a dam or waterfall is converted into kinetic energy to turn a turbine, requires solar-driven evaporation, followed by condensation, to replenish the high ground with water. Aside from harnessing the tides, and geothermal power, in which the internal heat of the Earth is used to boil water, all other mechanisms for generating electricity involve the conversion of energy from the sun in one form or another. Clearly, without sunlight, none of us would be here. Perhaps the writers of
Superman
were onto something when they changed the source of Kal-El’s powers from Krypton’s excessive gravity to the light from our sun.

20

ELECTRO AND MAGNETO DO THE WAVE—
ELECTROMAGNETISM AND LIGHT

IN THE MID-18 00S, the expanding American frontier may not have seen many costumed crime-fighters, but there was no shortage of heroes willing to fight for truth, justice, and the Western Way. A good thing, too, as a century later, Western comics would tap into an upsurge in the popularity of cowboys in the 1950s and thereby help keep comic-book publishers solvent during the superhero crunch precipitated by the campaign launched by Dr. Wertham’s
Seduction of the Innocent
.
All-American Comics
, featuring the adventures of Green Lantern and the Justice Society of America, became
All-American Western
—starring the “fighting plainsman” Johnny Thunder (schoolteacher by day, gunfighter by night)—and
All-Star Comics
became
All-Star Western
and detailed the adventures of the Trigger Twins. In the DC universe, the scarred (physically and psychologically) rebel loner Jonah Hex traveled across the western United States, righting wrongs and saving widows. Similarly, Bat Lash and the Vigilante dispatched justice . . . well, vigilante-style. Over in the Marvel universe, Western comics were strictly kid stuff, with the Two-Gun Kid, Kid Colt, Ringo Kid, and the Rawhide Kid working essentially the same beat, moving from town to town every issue (though rarely bumping into one another), facing down rustlers and stagecoach robbers. Back in the real mid-nineteenth-century world, when cowboy lawmen were cleaning up Dodge, physicists were seeking to tame the wild frontiers of electricity and magnetism, in the process laying the foundation for our modern wireless lifestyle.

It was the Scottish physicist James Clerk Maxwell who, in 1862, made the monumental theoretical leap connecting electricity and magnetism and ushered in a new era of scientific advancement. The set of equations elucidated by Coulomb, Gauss, Ampère, and Faraday is nowadays known by the general title of “Maxwell’s equations,” in recognition of his utilizing them to provide a fundamental understanding of electromagnetic radiation. None of these scientists have ever starred in their own comic books, but without these heroes, we’d still be reading by candlelight.

In order to understand how an electric lightbulb works, think back to the water analogy we invoked earlier to explain electric currents: The water pressure of the faucet was analogous to an electrical voltage, while the amount of water per unit time flowing through a hose represented the electrical current. To indicate that the hose was not perfect, and that a finite pressure had to be continuously applied to maintain a steady flow through the hose, we suggested that the hose had both partially blocked regions as well as small pinholes along its length, through which water could escape and avoid participating in the main current flow. For a given hose’s resistance, the greater the water pressure, the larger the water current. Alternatively, for a fixed water pressure, the larger the resistance, the smaller the current. These commonsense principles can be combined into a simple equation
known as Ohm’s law, after Georg Ohm, another pioneering scientist in the early days of electromagnetism, for whom the basic unit of resistance (an Ohm) is named. The longer and skinnier the hose, and the more clogs and holes along its length, the larger its resistance to current flow. A large water pressure at one end of a long narrow, leaky hose will correspond to a bare trickle at the other end, several miles away from the faucet. This is why your jumper cables are relatively short and thick, so that the current supplied from one car battery is not degraded by the time it reaches the second battery.