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Page 46 of 160 (1595 Questions and Answers)

451. What is plastic explosives made of?
I don't know for sure, but I suspect that they are plasticized materials (polymer molecules and softening chemicals) in which either the polymer molecule or the plasticizer or both are explosives. Actually, I just looked it up and found that it is based on RDX (a nitrated form of hexamethylenetetramine). The RDX is mixed with oils, waxes, and plasticizers to make a stiff putty. That being the case, it isn't really based on polymer molecules so that the name "plastic" refers more to its ability to assume different shapes at will.

452. Why do some glues dry faster than others?
Some glues literally "dry," since they contain a plasticizer chemical that evaporates to leave a firmer plastic. Other glues polymerize directly during the gluing process. For the glues that dry by evaporating plasticizer, the choice of plasticizer is critical. Water leaves relatively slowly compared to volatile organic solvents such as toluene or acetone. That is why water-based white glue dries more slowly than organic-based plastic cement. But the glues that polymerize during the gluing process (they "cure" rather than "dry") have a broad range of speeds. Some of those glues polymerize very rapidly (e.g. superglues and 3-minute epoxies) and some go much slower (normal epoxies). In general, slower glues produce stronger materials because they contain long polymer molecules. The fast curing glues form too many short polymer molecules and are not as tough.

453. Why is it so expensive to recycle plastic?
Different plastics are handled differently for recycling. Thermosets, such as rubber in tires, cannot be melted and cannot be recycled. Only thermoplastics can be melted for true reuse. There are 6 common thermoplastics that are recycled. These are numbered 1 through 6 on their bottoms. Objects made from one of these plastics can be collected together, melted, and then reformed into new useful objects. Unfortunately, the melted and reformed plastic isn't as pure as the original. The plastics manufacturers would rather clean up petroleum into petrochemicals and then make pure plastics than start with plastic objects, clean them, and reuse them. Because the recycler can't control what was in the plastic objects, these objects cannot be used for critical applications such as food containers or plumbing. Thus most recycled plastic is used for less profitable applications. If the recycler could be absolutely sure that the plastic hadn't been contaminated, some of it could be reused very easily. Plastic milk jugs could be reformed into plastic milk jugs over and over again.

454. By firing neutrons into a nucleus to change an atom, can you make gold using other cheaper metals?
Yes. However, trying to build gold with nuclear reactions is an expensive way to make the precious metal. Furthermore, you would probably end up with radioactive gold because at least some of the nuclei you made would have the wrong numbers of neutrons in them and would be unstable.

455. Can (or has) the nucleus be seen through microscopes?
Not exactly. A microscope "sees" an object by sending waves at that object and then looking at the waves it reflects or transmits. For example, a common light microscope sends light waves at an object and allows you to observe the transmitted or reflected light.

Unfortunately, light waves can't resolve details smaller than about 1/2 their wavelength. With a light microscope, the smallest objects you can make out are about 1/4 of a micron wide. To see still smaller objects, you must use something with a shorter wavelength than light. Because of quantum physics, even seemingly particulate objects such as electrons have a wave character and a wavelength, and fast moving electrons have much shorter wavelengths than light. Electron microscopes can resolve details down to about 1/2 the electron wavelength (in principle) and that brings their resolution down toward the level of individual atoms.

But to see a nucleus, which is much smaller than an atom, you need particles with even smaller wavelengths than are available in electron microscopes. The electrons in particle accelerators have such small wavelengths that they can resolve features as tiny as nuclei. However, the particles making up nuclei are always moving so that the "images" formed by accelerators are blurry. Nonetheless, it's possible to learn much about the structure of nuclei from these accelerator experiments. In fact, people now look at features even smaller than nuclei. They are presently looking at the individual nucleons (protons and neutrons) that make up nuclei and even at the quarks that make up those nucleons.

456. How do you do work on an atom so that the nuclear force can overcome the repulsive force.
Building large nuclei is a curiosity in modern science, not a sensible scheme for synthesizing elements. Most of the heavy elements in our world were made during a supernova explosion sometime before the formation of our present solar system about 5 billion years ago. During the supernova explosion, the temperatures became so high that nuclei of all sorts crashed into one another violently and many heavy nuclei were created. The work needed to build these giant nuclei came from the heat of this horrendous explosion.

457. How do you keep the nuclear bomb stable until you're ready to use it? (For example, on the way to Hiroshima)
The nuclear material will only explode if it is assembled to the point of critical mass. If that assembly is done slowly, the material will overheat and melt, perhaps causing a minor explosion but creating more of a radiation hazard than a nuclear detonation. Only if the assembly is done rapidly to well over the critical mass will the bomb explode. To keep a nuclear weapon "safe", the bomb makers ensure that the assembly cannot occur prematurely. They probably remove the triggers for the high explosives or block the paths through which the nuclear material must move. In most cases, even an accidental triggering of the high explosives use in assembly wouldn't cause the bomb to explode because all of the high explosives must be triggered at the same time for the assembly to work properly. If only part of the explosives ignited, the bomb would fizzle (very loudly).

458. If we were ever to have a nuclear war, would we have to live underground?
The long-term effects of nuclear war would come primarily from the release of radioactive isotopes into our environment. Large nuclei, such as that of uranium 235, have many more neutrons than protons. These neutrons "dilute" the repelling protons and made these large nuclei less unstable. But once a large nucleus shatters into fragments of medium size, these fragments acquire electrons and become "normal" atoms with medium sized nuclei. Unfortunately, these medium sized nuclei need fewer neutrons than they wind up with and they are generally unstable. While they resemble normal atoms chemically, they contain unstable cores and eventually decay. The decays release energy and this energy can do chemical damage to surrounding material. If the atom has been incorporated into a biological system (e.g. a person), it can do chemical damage to that biological system, perhaps causing cancer or genetic damage. To avoid this insidious damage, people would have to stay away from the fallout chemicals. That would be a difficult task, even underground.

459. What is the difference between the nuclear bomb and the H-bomb?
The fission bomb (uranium or plutonium bomb) derives its energy from the shattering of large nuclei; those in uranium or plutonium. The H-bomb (hydrogen, thermonuclear, or fusion bomb) derives most of its energy from the fusion or coalescence of small nuclei; those in hydrogen. The H-bomb releases more energy per kilogram than the fission bombs and can be made larger than the fission bombs. However, triggering a hydrogen bomb requires the enormous temperatures of a fission bomb.

460. When a plane drops a nuclear bomb, what sets the detonation process into effect?
The altitude at which the bomb explodes affects its results. Near or on the ground, the blast would cause incredible local damage, but less long-range damage. Above the ground, the blast would cause less local damage, but more long-range damage. So the bomb-makers build altitude sensing equipment into the bomb; probably a pressure sensing or radar-based altimeter. When the bomb has determined that it is at the right height, it triggers. High explosives assemble the critical mass as quickly as possible (typically by crushing the central sphere with carefully shaped high explosive charges). Once the fissionable material exceeds its critical mass, the chain reaction starts and the bomb explodes.
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