The Physics of Superheroes: Spectacular Second Edition (46 page)

Fig. 40.
Iron Man, in his first appearance in
Tales of Suspense # 39,
fights his way out of a Vietnamese prison camp using a top-hat transistor and a “magnetic turbo-insulator.”

There’s only one aspect of the scene summarized above that is physically correct, and that involves the “top-hat transistor.” There is no such thing as a “magnetic turbo-i nsulator”; this is just technobabble. The “turbo” modifier is just to make these insulators sound cool. There are magnets that are nonmetallic—that is, they are electrical insulators, but still generate a large magnetic field—and devices called “top-hat transistors” do indeed exist. They are so named because they look like small cylinders, about the size of pencil erasers (this was back in the early 1960s, long before the microminiaturization of transistors enabled millions of such devices to be fabricated on a chip measuring only a few millimeters on each side), with a small disc at their base at which the electrodes extended, and this makes them look a little like the top-hat playing piece in a Monopoly game set. The panel showing Tony Stark employing such a device to amplify the current to his “magnetic turbo-i nsulator” is physically plausible. But the second-to-last panel, in which he then employs said device to deflect the grenades and bazooka shots using “reverse magnetism” is not.

While every electron, proton, and neutron inside every atom has an intrinsic magnetic field, the natural tendency of magnets to line up north pole to south pole has the effect of canceling out the magnetism of most atoms. Any magnetic field that Iron Man would create using a powerful electromagnet in the palm of his glove would only be effective if: (1) the grenades being tossed at him were for some reason already magnetized and (2) they were all perfectly thrown so that their north poles were all pointing in the same direction and (3) the magnetic field created by Iron Man’s hand was also oriented so that the north pole was directed toward the incoming grenades and not the south pole, which would have the effect of accelerating the weapons toward him. It is unlikely that Tony Stark could always count on his opponents to cooperate with suitably magnetically oriented weapons.

Iron Man’s reverse magnetism ray actually has a better chance of working on nonmagnetic objects! Recall our discussion in Chapter 19 concerning Magneto and the phenomenon of diamagnetic levitation. Unlike metals such as iron or cobalt, for which the internal atomic magnetic fields align in the same direction, many materials, including water, are diamagnetic. In this case, when they’re in an external magnetic field, the atomic magnets orient themselves to oppose the applied field. Thus, the very process of trying to magnetize the object leads to a repulsive force. Iron Man’s reverse magnetism could repel objects, but only if they were diamagnetic, and it would not work on many metallic objects that are either ferromagnetic or paramagnetic (which align with an applied field). Magneto creates these large magnetic fields through his mutant power, but Iron Man must do it the old-fashioned way, using electromagnets (similar to the one constructed by Superboy in Chapter 18). As Iron Man does not carry around with him an electrical dynamo like the one used by Superboy, a few shots of this reverse magnetism ray would drain his batteries faster than a fight with the Hulk. Furthermore, the recoil of such weapons is considerable. When supplying a large force against a target, they will induce an equal and opposite force against the gun and the shooter holding it. Tony Stark was clever to build his repulsor rays into his gloves. By locking the servo-motors that enable his armored arms to move, his iron suit provides a large and rigid inertial mass to take up the recoil whenever he fires this glove-based weapon.

While “reverse magnetism” may not be physically practical, hand-held pulsed-energy weapons have begun to make the transition from comic-book fantasies to military research facilities. Certainly these weapons cannot be the same “magnetic repulsors” as Iron Man uses, for the reasons argued above. The energy needed to generate a magnetic field large enough to deflect an object using only diamagnetic repulsion is so large that it would be more effective to employ conventional weaponry. Nevertheless, “pulsed” energy systems are under active development by the military. By generating a large voltage inside the weapon that can be rapidly discharged in a thousandth of a second, the power (change in energy per time) can be quite high. This electromagnetic pulse, if directed at a target, would deposit this energy in a localized region faster than the heat could be safely dissipated away. High-intensity laser beams delivered in extremely brief pulses are used in physics laboratories to nearly instantaneously melt a small region of a solid’s surface, and in principle the same process could be employed in an offensive capability. The big drawback is once again the energy requirements of such a weapon. If one must carry a miniature power plant around in order to fire such a pulsed energy weapon, the element of surprise in any combat situation would be lost.

What is a transistor? What is this electronic device that, at least according to Stan Lee, is endowed with miraculous abilities that enable Iron Man to successfully fend off the Mandarin, the Crimson Dynamo, and Titanium Man? A short answer is that transistors are valves that regulate the flow of electrical current through a circuit. Such answers are easy to remember, but they tell us nothing about how transistors actually function. The first question we should address is: What exactly is a semiconductor, which is neither a metal nor an insulator? We hear a great deal about how we are living in the “Silicon Age,” but what is so special about silicon? In the next few pages I will try to condense more than fifty years of solid-state physics as I answer these questions.

Silicon is an atom, a basic element of nature, just like carbon, oxygen, or gold. A silicon atom’s nucleus has fourteen positively charged protons and (usually) fourteen electrically neutral neutrons, and to maintain charge neutrality there are fourteen negatively charged electrons surrounding the nucleus. These electrons reside in the “quantum- mechanical orbits” that, as discussed in chapters 21 and 22, arise from the wavelike nature of all matter. The possible “electron orbits” are specific for each element, and determine the allowed electron energies.

Quantum mechanics enables us to calculate, via the Schrödinger equation, the allowed “orbits” of the electrons in an atom, and knowing how many different possible orbits an electron can have in an atom is like knowing the number and arrangement of chairs in a classroom (stay with me here; this classroom metaphor is going to be useful in explaining metals, insulators, and semiconductors). The chairs only represent possible or virtual classes; it is not until the students enter and take their seats that the class is real. If only one student comes in and takes a seat, this is like having only one electron in a possible quantum- mechanical orbit. We would call this class hydrogen, in analogy with the atom that has only one electron in its neutral, stable form. If there were two students sitting in the class, we would have helium, fourteen students would make up silicon, and so on. The first students to enter the class take the seats at the front of the room, close to the blackboard, in our hypothetical example. The last students to enter take seats near the back of the auditorium, far from the blackboard (where the positively charged nucleus will be). This arrangement, with the closest seats filled by students, describes the lowest-energy configuration. For a carbon atom with six electrons, the closest orbits are occupied. If the carbon atom gains some energy, say, from absorbing light, some of its electrons will then occupy higher-energy orbits (seats farther from the front).

Whether a material is a metal, a semiconductor, or an insulator depends on the energy separation between the highest level filled with an electron and the nearest available unoccupied level. In the classroom analogy, the solid can be thought of as a very large auditorium with many rows of seats, provided by the constituent atoms that make up the material. There will be an empty balcony that contains an equal number of seats. If the electrons sitting in the lower-energy orchestra seats
⁷⁷
are to conduct electricity when a voltage is applied across the solid, then they gain extra energy. But they can move only to a higher energy if there is an empty state for the electron to be promoted into (recall the discussion of quantized energy levels in Chapter 21). The electrical properties of any solid are determined by the number of electrons residing in the lower orchestra seats and the energy separation between the lower occupied seats and the next empty ones in the balcony.

The difference between insulators and metals is clear in this analogy. An insulator is a solid in which every single seat in the orchestra is filled, while a metal is a material for which only half of the seats in the lower level are occupied. In a metal, there are a large number of empty seats in the orchestra available to an electron, and the application of a voltage, whether big or small, can accelerate the electrons to higher energy states (which correspond to carrying an electrical current). Metals are good electrical conductors because their lowest occupied auditorium seats for electrons are only half-filled. For the insulator, every seat is occupied, and absent promotion to the balcony, no current will result when a voltage is set up across the material. If I raise the temperature of an insulator, providing external excess energy in the form of heat, some of the electrons can rise into the previously empty balcony. In the balcony there are many empty seats for the electron to carry a current, but this will last only as long as the temperature is elevated. If the temperature is lowered, the electrons in the balcony will descend and return to their low-energy seats in the orchestra.

If the insulator absorbs energy in the form of light, it can immediately promote an electron to the balcony. When the electron returns to its seat in the orchestra, it has to conserve energy and thus gives off the same amount of energy that it previously absorbed. It will either do this by giving off light of the same energy as initially absorbed, or the electron can induce atomic vibrations (heat). This is why shining light on an object warms it up—the electrons absorb the energy of the light, but then can return the absorbed energy in the form of heat. If the energy of the light is insufficient to promote an electron from the highest filled orchestra seat to the lowest empty balcony seat, the light is not absorbed. In this case the lower-energy light is ignored by the electrons in the solid, and passes right through it. Insulators such as window glass are transparent because the separation between the filled orchestra and empty balcony for this material is in the ultraviolet portion of the spectrum, so visible light with a lower energy passes right through. On the other hand, metals always have available empty seats to absorb light even in the half filled orchestra. No matter how small the light’s energy, an electron in a metal can absorb this energy and then return it upon going back to its lower-energy seat. This is why metals are shiny—and good reflectors. They always give off light energy equal to what is absorbed, and there is no lower limit to the energy of light they can take in.

A semiconductor is just an insulator with a relatively small energy gap (compared with the energy of visible light) separating the filled lower band from the next empty band. For such an energy separation, a certain fraction of electrons will have enough thermal energy at room temperature to be promoted to the balcony. When electrons are excited to the upper deck, the material now has two ways to conduct electricity. For every electron promoted to the higher energy band that is able to conduct electricity, an empty state is left behind. The empty chairs in the previously filled orchestra can be considered as “positive electrons” or “holes,” and can also carry electrical current. If an electron adjacent to an empty seat slips into this chair, then the empty spot has migrated one position over. In this way we can consider the hole to move in response to an external voltage, and also carry current. Of course, the original electrons will eventually fall back down into the orchestra, filling the empty seats they left behind (though not necessarily their original seats). When certain semiconductors absorb light, there are enough excited electrons in the upper band and holes in the lower band to convert the material from an insulator to a good electrical conductor. As soon as the light is turned off, the electrons and holes recombine, and the material becomes an insulator again. These semiconductors are called “photoconductors” and are used as light sensors, as their ability to carry an electrical current changes dramatically when exposed to light. Certain smoke detectors, television remote controls, and automatic door openers in supermarkets make use of photoconductors for their operation.

Semiconductor devices are typically constructed out of silicon because it has an energy gap conveniently just below the range of visible light. Furthermore, it is a plentiful element (most sand is composed of silicon dioxide) that is relatively easy to purify and manipulate. There are times when the physical constraints of the size of the energy gap in silicon limits a device’s performance, and in this case, other semiconducting materials can be used, such as germanium or gallium arsenide. Iron Man’s, and the military’s, night-vision capabilities make use of a semiconductor’s photoconducting properties and a small energy gap that is in the infrared portion of the electromagnetic spectrum.