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Page 105 of 160 (1595 Questions and Answers)

1041. Is it possible to construct "home-made" thermal windows (double pan) so condensation can be avoided? I work in stained glass and want to make an energy efficient window. — JAA, York, PA
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Yes, you should be able to make your own thermal windows. The value of having two vertical panes of glass that are separated by a narrow gap is that heat has trouble flowing across gap. While air is a poor conductor of heat, it carries heat reasonably well via convection. But with only a narrow gap of air between two vertical glass panes, convection doesn't work well. Air heated by its contact with the warmer pane tends to flow directly upward, rather than toward the cooler pane. Similarly, air cooled by its contact with the cooler pane tends to flow directly downward, rather than toward the warmer pane.

But as you've anticipated, you may have trouble with condensation on the inside surface of the cooler pan. Your best bet at avoiding this problem is to completely seal the space between the two panes and to fill it with very dry air or even bottled nitrogen gas—which can be obtained cheaply from a local gas supply company. You'd have to blow the dry air or nitrogen in through one hole and allow the trapped air to flow out through another hole. After the trapped air has been replaced several times with dry gas and you're sure there is little moisture left between the panes, you can stop replacing the air and seal both holes. But with stained glass, you have many potential gaps through which moisture can enter the trapped air, so achieving a seal could be very difficult. In that case, you might just put a desiccant at one edge of the window. Drierite is an inexpensive material that resembles little white pebbles and that can absorb quite a bit of moisture. If you put some Drierite between the two panes before you did your best to seal the space between them, I would expect the Drierite to remove enough moisture from the trapped air to avoid condensation problems. After a few years, enough moisture may have leaked in through cracks to cause trouble, in which case you would simply replace the Drierite. One useful type of Drierite is blue when fresh and turns pink when it has absorbed its fill of moisture.


1042. How does a halogen lamp get so hot?
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Like all incandescent bulbs, a halogen lamp creates its light as visible thermal radiation from an extremely hot tungsten wire. In fact, the wire in a halogen lamp is allowed to get even hotter than the one in a normal bulb. But while the glass envelope of a normal bulb gets only moderately hot during use, the glass envelope of a halogen bulb gets extremely hot. That's because the halogen bulb is using a chemical trick to keep tungsten atoms from getting away from the filament. Each time one of those tungsten atoms tries to leave, it's picked up by halogen molecules inside the glass envelope and returned to the filament. These halogen molecules can even pick the tungsten atoms up off the glass envelope and return them to the filament, but only if the glass envelope is allowed to get extremely hot. That's why the glass envelope of the halogen bulb is allowed to run so hot—if it weren't, it would accumulate the tungsten atoms permanently and it would darken. And since the tungsten atoms wouldn't be returned the filament, the filament wouldn't last as long.

1043. How do you make an energy converter to convert water into energy? — SB
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I'm afraid that there is no simple way to convert water into energy. People have been trying to use fusion to extract the nuclear energy stored in the hydrogen nuclei in water. But while billions of dollars have been spent on research, there is no viable scheme for this process for controlled fusion in sight. The stars are powered by hydrogen fusion, but people on the earth aren't likely to be using it as a source for peaceful energy any time soon.

1044. How does dry ice work to freeze things? — JH
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Solid carbon dioxide or "dry ice" sublimes into gaseous carbon dioxide at a temperature well below 0° C. Since it takes energy to separate the molecules of carbon dioxide from one another, the dry ice absorbs heat as it sublimes and takes that heat out of any warmer objects nearby. Those nearby objects become colder and colder as the heat leaves them and eventually they begin to freeze.

1045. How does fog form? — KB
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The interface between a droplet of water and the air around it is a busy place. Water molecules are constantly leaving the droplet to become water vapor in the air and water molecules in the air are constantly returning to the droplet as liquid water. What determines whether the droplet grows or shrinks is the difference between these two rates. If more water molecules return to the droplet than leave, the droplet will grow. If more water molecules leave the droplet than return, the droplet will shrink. How often water molecules leave the droplet depends on the droplet's temperature. How often water molecules return to the droplet depends on the moisture content of the air.

This dynamic balance of growth and shrinkage occurs right in the middle of the air all the time. Tiny water droplets form by accident, even in reasonably dry air, but in most cases they quickly shrink back to nothing because the leaving rate is higher than the returning rate. However, when air that contains lots of moisture experiences a decrease in temperature, the returning rate can exceed the leaving rate. When that happens, the tiny droplets that appear by accident don't immediately disappear. Instead, they grow larger and larger. Depending on the altitude, we call the white mist that results clouds or fog.


1046. Our problem concerns temperature. At different temperatures, solubility of compounds varies. If we extract water from a pond at two degrees Celsius and then test it at room temperature, our reading isn't going to be accurate. On the other hand, it isn't practical for us to perform out tests outside. The substances we are testing are nitrites, nitrates, ammonia, pH, hardness, oxygen level, phosphates, temperature, and ORP. — J&E, Missouri
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If you collect pond water at 2° C and then bring it into a room at 20° C, there will be a few subtle changes in the water's contents. While the amounts of various dissolved materials can't change unless atoms move in or out of the water, how they interact with one does change somewhat with temperature. I would be very surprised if anything that's dissolved in that pond water comes out of solution when you warm it to room temperature, so if all you want to do is to determine the concentrations of various dissolved materials, go ahead and do it at room temperature. You might have to be careful with dissolved gases, because it's relatively easy for gas molecules to enter or leave the pond water without your noticing that it's happening, but the nitrites, nitrates, hardness, and phosphates aren't going anywhere. Ammonia can leave as a gas, so you should be a little careful with it. I don't know enough about ORP (oxidization reduction potential) to say anything about it. But you'll have to be very careful with oxygen concentration because you can modify this just by pouring the water through air and making bubbles.

However, to be sure that the contents of the pond water are interacting with one another just as they were in the pond, you should cool the water back down to 2° C before making any measurements. This is particularly important for pH measurements, since water's pH decreases slightly with increasing temperature.


1047. What is pH and why is it so important to my garden pond and spa? — NW, California
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pH is a measure of the concentration of dissolved hydrogen ions in water. When a hydrogen atom loses an electron and becomes a hydrogen ion—a proton—it can dissolve nicely in water. Actually, this proton sticks itself to the oxygen atom of a water molecule, producing a hydronium ion (H3O+) that is then carried around by shells of water molecules. The higher the concentration of hydrogen (or hydronium) ions in water, the lower the water's pH. More specifically, pH is negative the log (base 10) of the molar hydrogen ion concentration. That means that water with a pH of 6 has ten times as many hydrogen ions per liter as water with a pH of 7.

Pure water naturally contains some hydrogen ions, formed by water molecules that have spontaneously dissociated into hydrogen ions (H+) and hydroxide ions (OH-). Pure water has enough of these hydrogen ions in it to give it a pH of 7. But if you dissolve acidic materials in the water, materials that tend to produce hydrogen ions, the pH of the water will drop. If you dissolve basic materials in the water, materials that tend to bind with hydrogen ions and reduce their concentration, the pH of the water will rise. Water with too many or too few hydrogen ions tends to be chemically aggressive and we do best in water that has a pH near 7.


1048. What is a magnet?
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A magnet is an object that has magnetic poles and therefore exerts forces or torques (twists) on other magnets. There are two types of these magnetic poles—called, for historical reasons, north and south. Like poles repel (north repels north and south repels south) while opposite poles attract (north attracts south). Since isolated north and south magnetic poles have never been found in nature, magnets always have equal amounts of north and south magnetic poles, making them magnetically neutral overall. In a permanent magnet, the magnetism originates in the electrons from which the magnet is formed. Electrons are intrinsically magnetic, each with its own north and south magnetic poles, and they give the permanent magnet its overall north and south poles.

1049. Is it possible to charge batteries using static electricity? Can lightning or atmospheric charges be stored in a capacitor and then released into a cell for charging? — JM, Lafayette, NT
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Yes, static electricity has energy associated with it and that energy can be used to charge batteries, at least in principle. Static electricity is literally stationary separated electric charges—essentially separated charges stored on capacitor-like surfaces. As you suggest, it may be easiest to transfer these separated charges into a real capacitor and then to use this charged capacitor to recharge an electrochemical cell. Whether such a procedure can be carried out efficiently and in a cost-effective manner isn't clear to me. The charges involved in lightning have so much energy per charge—so much voltage—that they're hard to use for anything. Even the charges that you accumulate when you rub your feet on a wool carpet on a cold, dry winter day acquire an enormous amount of energy per charge. To charge most batteries, you need lots of low energy charges, not the small numbers of high-energy charges that are typical of static electricity. Using this tiny current of high-energy charges to charge a battery is equivalent to trying to fill a swimming pool with water from a high-pressure car-washing nozzle—too little water under too much pressure. You can do it, but there are better ways.

1050. What are some unusual conductors of electricity?
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How about graphite and cadmium sulfide? Graphite, such as that in the lead of a pencil, conducts electricity even though it's not formally a metal. If you draw a dark line on a sheet of paper, that line can act as a wire for sensitive electric circuits. Cadmium sulfide is a photoconductor—a material that is electrically insulating in the dark but that conducts electricity when exposed to light. Photoconductors of this sort are used in some light sensors, as well as in xerographic copiers and laser printers.

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