. Why can't you make nuclear weapons with any old element?
Only a few elements/isotopes are fissionable, meaning that only a few elements/isotopes have nuclei that shatter when struck by a neutron. Moreover, only a few of this fissionable nuclei release more neutrons than they take to fission. Of naturally occurring isotopes, only Uranium 235 is suitable for nuclear weapons. Plutonium 239 is also suitable, but it must be made artificially in a nuclear reactor.
. You said that in the Three Mile Island Incident, it overheated due to the lack of cold water. How did that happen? Isn't that a huge oversight?
The loss of cooling water was unexpected and was caused by a pump failure. The broken pump was actually part of the power-generating loop, not the reactor core-cooling loop. When everything was working properly, water flowing through a loop that included the reactor core transferred heat to water flowing through the generating plant loop. But when the generating plant loop shut down, the reactor core loop had nowhere to deposit its heat and the water in it boiled. Backup cooling water evidently did not exist, did not work, or was not sufficient to keep the reactor core from over heating. I don't know whether it was poor design or poor maintenance that caused this disaster.
. How do the "user-friendly" MRI machines work vs. the old catacomb type? (the opened vs. closed types)
The shape of the MRI machine is dictated primarily by the strong magnetic field it uses to record information about protons in a person's tissues. This field needs to be very uniform over a large region of space and the simplest way of producing such a uniform field is with a huge coil of current-carrying wire. The person would go inside the coil, in the uniform field and other parts of the MRI machine would record the information. While the coil could be dressed up to look more like a tubular hole than a coil of wire, it was still very confining. Newer MRI machines use two smaller coils of current carrying wire, one above the other, to create a uniform field for imaging. This arrangement is trickier because the two coils must be shaped very carefully to ensure that the field is appropriately uniform. Moreover, most MRI machines use superconducting wires in these coils to achieve very high magnetic fields. Since superconducting coils must be cooled to very low temperatures, they require liquid helium coolants and sophisticated thermal insulation. While the single coil magnets required only a single refrigerator and insulating chamber, those with two coil magnets required two refrigerators and insulating chambers. That increases the expense of the magnet and its operation, but produces a more open imaging region.
. If bones "stop" electrons, then why do we see a skeletal image on an X-ray? Would we get a negative image?
The bones cast shadows on the film; wherever there is bone, few X-rays strike the film. When the film is developed, it turns black wherever X-rays hit it. Thus the areas that were shadowed by the bone appear white.
. On an X-ray result picture, why is the film in the background blue? Is this the only way it will show up? If so why?
The X-ray image itself is formed by tiny black silver particles, just as in a normal black and white photographic negative. If those particles were supported by a clear plastic sheet, then the X-ray should appear either clear or black and have no color. The blue you are referring to must be caused either by a colored pigment in the plastic X-ray film sheet or by a colored light used to illuminate the X-ray. I suspect the later. Fluorescent lamps tend to be bluish and the ones used to view X-rays are probably particular blue. It probably increases the apparent contrast in the image so that small variations in density become visible.
. What exactly does the bone do with the X-rays that the skin doesn't?
The skin's atoms are too small to experience the photoelectric effect with X-rays. Most X-rays go right through skin and soft tissue. However calcium atoms are large enough to experience the photoelectric effect and thus absorb many of the X-rays. Bones cast a shadow on film, which is how an image of your bones is formed.
. How do black holes work?
As you assemble more and more mass together in a small volume, the gravity there becomes stronger and stronger. At first, it becomes more and more difficult to throw a ball upward hard enough to make it sail away from the mass into space. Eventually, you need a cannon to get the ball to leave. And by the time you get enough mass together, the gravity becomes so strong that light itself begins to have trouble escaping. Light falls in gravity, just like anything else. But it travels so fast that you barely notice it falling. However when the gravity becomes strong enough, light falls enough to cause some weird effects. A black hole forms when the gravity is so strong the even light is unable to escape from the mass.
. How do compasses work?
A compass contains a magnetized needle, with a north pole at one end and a south pole at the other. Since opposite magnetic poles attract one another, the north pole of the compass is attracted toward any south poles it can find and the south pole of the compass is attracted toward any north poles it can find. The earth happens to have a strong south magnetic pole near its north geographical pole and a north magnetic pole near its south pole. As a result, compass needles turn (the experience torques) until their north magnetic pole ends are pointed northward (toward the south magnetic pole located there).
. How do electronic water softeners, where a coil of wire is wrapped around the incoming water pipe, work?
I've never heard of such a water softener, but I can voice some skepticism about it anyway. Hard water is water that contains substantial amounts of dissolved calcium, magnesium, and iron. These elements form multiply charged ions in solution and these multiply charged ions tend to bind with soap and detergent molecules to form an insoluble scum. To soften the water, you must remove those ions. A conventional water softener does this by replacing them with sodium ions. The active part of a conventional water softener is an ion exchange resin that releases sodium ions as it binds up the calcium, magnesium, and iron ions. Eventually the resin runs out of sodium and it must be regenerated by flushing it with strong salt water. This regenerating process flushes the calcium, magnesium, and iron ions out of the resin and puts the sodium ions back into it. As for the electronic water softener, where does it put the calcium, magnesium, and iron ions and what does it replace them with? It can't make those ions disappear and, if it were to extract them without replacing them, it would leave the water electrically charged. So I'm skeptical that any device that doesn't chemically treat the water directly can soften the water.
. I read in an article about batteries about a Reverse Coulomb Counter. What is it?
Although I've never heard of such a device myself, I can guess what it means. A coulomb is a standard unit of electric charge. Since a battery is a pump for electric charge, measuring the number of coulombs that have flowed through a battery is a way to determine what fraction of that battery's storage capacity has been used. (It's analogous to measuring how many grams of sand have flowed through the neck of an egg timer or how many liters of water have flowed out of a water tower.) When a battery is being recharged, measuring the number of coulombs that have flowed in the reverse direction through the battery is a way to determine how much recharging has occurred. Thus, I suspect that a "reverse coulomb counter" is a device that monitors the flow of charge backward through a battery as it is being recharged. This backward flow of charge should be almost exactly proportional to the extent of recharging.