How Everything Works   Page 97 of 160 (1595 Questions and Answers) Printer Friendly Version

961. How does water divining work? — GD, Mansfield, Australia
I'm afraid that I remain unconvinced that water divining works at all. I believe that the whole issue is psychological—the power of suggestion. A divining rod will twist when something exerts a torque on it but there is no special force between the rod and water that would exert an unusual torque on the rod.

962. How do air currents flow?
Air typically rises near sources of heat and descends elsewhere. Since air doesn't normally accumulate in one place and leave another place empty, it tends to form circulating currents. The air rises near hot objects, flows outward above those objects, cools and descends, and finally flows back toward the hot objects from beneath them. These circulating currents are called convection cycles.

963. What is a barometer, how does it work, and why is it useful in predicting the weather? — HC
A barometer measures air pressure by examining the forces that air exerts on surfaces. The higher the air pressure, the more force air will exert on a certain surface. Most barometers compare the present air pressure with a known pressure by putting those two pressures on opposite sides of a flexible surface. The higher the air pressure, the more that surface will bend away from it.

You can make a simple barometer by inserting a drinking straw in narrow-mouthed jar that's half full of water and by sealing the neck of the jar around the straw (with a rubber stopper, wax, or glue). Make sure that the end of the straw is immersed in the water and that the water level in the straw is above the top of the jar. As the outside air pressure decreases, the trapped air inside the jar will push the water farther up the straw. As the air pressure increases, it will push the water farther down the straw. Try to keep your barometer's temperature constant, because temperature will also affect its water level. You can use your barometer to predict the weather (somewhat) because storms tend to be accompanied by lower air pressures.

964. What makes heat rise? — BN, Burlington, MA
Heat itself doesn't rise—it's a form of energy, not an object. But heated fluids often do rise. That's because raising the temperature of a fluid usually causes that fluid to expand so that its density drops. Whenever a region of less density fluid is surrounded by more dense fluid, the less dense region experiences a net upward force. This result is a consequence of Archimedes' principle that less dense materials float in more dense liquid. With a net force pushing it upward, the heated region floats upward and we say "heat rises."

965. Is there any gravitational force between two atoms? — AW, Karachi, Pakistan
Yes, everything in the universe exerts gravitational forces on everything else in the universe. However, those forces are usually so small that they are undetectable. The gravitational forces between two bowling balls are only barely measurable in a laboratory. The gravitational forces between two atoms are so small as to be hopelessly undetectable.

966. An architect friend tells our coffee group that liquids (water, in this case) are compressible to a slight extent. We tell him hydraulics would be impracticable under his thesis. We would appreciate comments or ammunition. — WAW, Brownsville, TX
I'm afraid that your friend is right—liquids are slightly compressible. A compressible material is one that experiences a decrease in volume when it's exposed to an increase in pressure. Gases are highly compressible—they change volume dramatically with changes in pressure. Liquids are said to be incompressible—they change volume very little with changes in pressure. But very little isn't zero. A liquid is essentially incompressible because its atoms and molecules are touching one another and, since those atoms and molecules have relatively fixed sizes, it's hard to pack them closer together than they already are. But increases in pressure do cause those atoms and molecules to move slightly closer together and the liquid does becomes denser and occupies less volume. The effect is small enough that it has almost no effect on most hydraulic systems—the pressurized fluid loses only parts per million of its volume as you squeeze it with normal pressures. All you really care about in a hydraulic system is that over the range of pressures used, the fluid involved doesn't change volumes much. Thus if you keep the pressure changes small enough, even air can be used in a hydraulic system. For example, pneumatic tube delivery systems are essentially air-operated hydraulic systems. But if the pressure changes are large enough, even liquids and solids can be highly compressible. In fact, plutonium-based nuclear weapons use high explosives to crush spheres of solid plutonium, already one of the densest materials in existence, to several times solid density. You wouldn't think of plutonium as compressible, but under these astronomical pressures it compresses almost like a gas.

967. Why does fire burn? — PJ
Fire is a chemical reaction in which a combustible fuel reacts with oxygen to release large amounts of thermal energy. Many atoms bind very strongly with oxygen atoms and these fuel atoms release energy when they bind with oxygen. Initiating these combustion reactions normally requires some thermal energy to get started. This starting energy is known as activation energy. That's why you have light the fire—you must provide the activation energy. After that, each oxidization reaction produces the activation energy needed to start another oxidization reaction and the fire keeps itself going until it has consumed all of its fuel.

968. What are firewalls? What are they made of? — RB
Firewalls are just insulating, non-flammable walls that prevent the heat from a fire on one side of a firewall from initiating combustion on the other side. As far as I know, most firewalls are made from masonry block. Ceramics such as stone or cement are already fully oxidized and can't burn. Furthermore, most ceramics are poor conductors of heat and many can become almost white hot without melting. As long as a masonry wall is thick enough and sturdy enough, it can tolerate having a fire on one side without conveying that fire's heat to the combustible materials on its other side. If there aren't any holes or flaws in the firewall, it will prevent the spread of fire between adjacent buildings.

969. What is polyester and how does it work insulating clothing? — PGF, Seabrook, SC
Polyesters are a class of polymers, extremely long molecules that are commonly known as plastics. Each polyester molecule is several thousand atoms long, so that a polyester fiber resembles a tiny rope made of microscopic spaghetti strands that are all entangled with one another. Like most electric insulators, polyester plastic is a poor conductor of heat. But in clothing its main insulating effect is to trap air. While air is a terrible conductor of heat, it tends to undergo convection and convection allows it to transport heat pretty effectively. However, when air is trapped by countless tiny fibers, convection is inhibited and the air becomes a great insulator. That's why polyester fibers are such good thermal insulation—they trap air and let the air act as the real insulation.

970. How can one be fire safe while dealing with incandescent and fluorescent light bulbs? — TJ, Woodbridge, VA
Fluorescent tubes produce relatively little heat, so they're relatively fire safe already. However, incandescent light bulbs become very hot and you have to be careful with them to avoid fires. First, make sure that the bulb can get rid of its waste heat. That means that you shouldn't wrap the bulb in insulation because it needs to transfer its waste heat to the air. Second, keep flammable materials away from the bulb, particularly above the bulb since hot air from the bulb rises upward.

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