Author: Lou Bloomfield

What makes heat rise?

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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.”

Do crystals “grow”?

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Do crystals “grow”? — JJ

Yes, crystals grow. They begin as tiny seed crystals when a few atoms or molecules manage to arrange themselves accidentally in an orderly fashion. From that point on, new atoms or molecules that join the seed continue the orderly pattern and the crystal grows. Some crystals grow from solutions that contain more of particular atoms or molecules than those solutions can handle. Other crystals grow from molten materials that are cooling off and beginning to freeze. Still other crystals form from atoms or molecules that are diffusing randomly through solids, liquids, or even gases and encounter the proper crystal on which to stick.

How does water divining work?

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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.

An architect friend tells our coffee group that liquids (water, in this case) ar…

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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.

How are the nylon ropes of parachutes able to stop the falling parachuter? How m…

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How are the nylon ropes of parachutes able to stop the falling parachuter? How much of a force must they over come, and how might the ropes’ elasticity be affected? — C

When the parachuter opens the parachute and begins to slow down, the parachute’s nylon shrouds briefly exert a large upward force on the parachuter. Over a period of a few seconds, the parachuter slows from a downward speed of about 150 mph to a downward speed of 20 mph and experiences several g’s of upward acceleration. To cause this much upward acceleration, the nylon shrouds must exert an upward force on the parachuter that is several times the parachuter’s weight. The nylon shrouds are quite strong and can easily tolerate this much tension without exceeding their elastic limits. There should be no adverse effects on their elasticities.

How do air currents flow?

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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.

Why does fire burn?

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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.

Are some frequencies of sound more directional than others and, if so, why?

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Are some frequencies of sound more directional than others and, if so, why? — BKZ, Dayton, OH

In open air, sound waves travel in straight lines regardless of frequency or wavelength. But low frequency (long wavelength) sounds don’t fit well in confined spaces and have less directional character to them. That’s why you only need one subwoofer for a sound system—you can’t hear where the lowest frequency sounds are coming from any way. Higher frequency sounds remain relatively directional, even in confined spaces. The same effects apply to electromagnetic waves—in confined spaces, long wavelength radio waves are effectively less directional than short wavelength light waves.

I know that adding salt to water will raise its boiling point, which would seem …

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I know that adding salt to water will raise its boiling point, which would seem to imply that it would take longer to come to a boil. But does it take longer? As a cook I’ve always been told to add a little salt to the water to bring it to a boil faster. It seems to work or is that just the power of suggestion? If it does boil faster, why does it? — ND, Ashland, OR

I think that power of suggestion is at work here. Salt water boils at a higher temperature than pure water. Thus if you set two identical pots of water, one salty and one pure, on burners and heat them at equal rates, the pure water will reach its boiling temperature first.

However, water boils more vigorously when it contains impurities that can nucleate bubbles of water vapor. Just before the water in a pot reaches a full boil, its temperature is often nonuniform and there are some regions that are boiling while others aren’t. The edges and corners of crystals are particularly good at nucleating bubbles, so that tossing salt grains into such nearly boiling water will encourage its hot regions to boil more vigorously, at least until those salt grains dissolve away. The appearance of bubbles makes you think the water is at a full boil when it really isn’t.

Is there any gravitational force between two atoms?

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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.