. Can you tell me the difference in lifting power of helium versus hydrogen? — FL, Napa, CA
A balloon experiences an upward buoyant force that's equal in amount to the weight of the air it displaces. If that balloon is filled with helium or hydrogen, both of which have very low densities, then this upward buoyant force may be more than the balloon's weight and the balloon may accelerate upward. Helium weighs a little more per cubic foot or cubic meter than hydrogen does, so replacing the helium with hydrogen will make it easier to float the balloon. A cubic foot of hydrogen weighs 0.0056 pounds less than a cubic foot of helium and a cubic meter of hydrogen weighs 89 grams less than a cubic meter of helium. Any weight saving made by replacing helium with hydrogen in your balloon can be viewed as extra lifting power. As you can see, the effect is small and hydrogen is a whole lot more dangerous than helium.
. 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.
. 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.
. 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."
. Can you suggest an experiment to prove that a helium balloon floats because helium is lighter than oxygen? — CR
If you have a balance scale, you can do a series of comparisons. First compare a cup of water to a cup of salad oil, using the balance, to show that the salad oil is less dense than the water. Then show that the salad oil floats on water. Then compare an air-filled balloon to an identical helium balloon, using the balance, to show that the helium is less dense than air. Then show that the helium floats on air. It's just like the salad oil on water, but now it's the helium on air. You can't simply pour the helium on the air to show that it floats, because they'll mix. So you leave the helium wrapped up in a rubber balloon and then let it float on air. It floats just fine!
. I've heard that, technically speaking, our atmosphere is a fluid. Can you discuss this?
Since both gases and liquids are fluids, the earth's atmosphere is certainly a fluid. Any material that flows in response to sheer stress (tearing) is considered a fluid. The earth's atmosphere flows in responses to sheer stress—for example when you drive your car past another car, the air in between experiences this tearing and it flows in a complicated fashion. Winds are another important example of fluid flow in the earth's atmosphere.
. When raisins are added to a solution containing water, baking soda, and vinegar, why do the raisins dance? — RE, Troy, IL
Baking soda and vinegar react in water to release carbon dioxide molecules. If the chemicals are sufficiently dilute in the water, the carbon dioxide molecules may remain dissolved in the water almost indefinitely. But when the water has impurities in it, the carbon dioxide molecules tend to come out of solution as gas bubbles at those impurities. The impurities allow the molecules to form tiny gas bubbles—a process called nucleation. In the present case, the raisins serve as the impurities that nucleate gas bubbles. As the gas bubbles grow on the surfaces of the raisins, the raisins experience upward buoyant forces from the surrounding water. The bubbles float upward, carrying the raisins with them and causing the raisins "to dance."
. How would you figure out how much pressure a 100 lb. woman's high heel would produce as she walks? — JB, Boulder, Colorado
If the woman were standing still, with about half her weight on the heel of her right shoe, she would be exerting a force of 50 pounds on the floor under that heel. Since a spiked heel is about 0.33 inches on a side, its surface area is about 0.1 square inches (0.33 inches times 0.33 inches). Since a force of 50 pounds is applied to an area of 0.1 square inches, the pressure on the floor is 50 pounds divided by 0.1 square inches or 500 pounds per square inch. That's about 30 times as much pressure as the atmosphere exerts on objects at sea level.
But when the woman is walking, she often lands hard on that heel, so that it supports her entire weight and then some. The extra force comes about because she is accelerating—when she lands, she is heading downward and the floor must push upward extra hard on her to stop her downward motion. If we suppose that the total downward force she exerts on the heel reaches a peak of 200 pounds—not at all unreasonable—the pressure the shoe exerts on the floor reaches 2000 pounds per square inch. No wonder spiked heels damage floors and present a serious hazard to nearby toes!
. Could the traffic flow on freeways be modeled as a one-dimensional gas? You can see waves of motion among the cars and these waves travel faster as the cars pack more tightly. — RH, Escondido, California
There are many similarities between the cars traveling on a freeway and the molecules in a gas. As you point out, disturbances at one point in the traffic cause ripples of motion to spread backward through the cars—similar to what happens in a gas. However, normal gas molecules only interact with one another when they actually touch, while cars interact at much larger distances—unlike gas molecules, cars don't do so well when they collide with one another. To avoid collisions, the drivers watch what's happening far ahead of them and react accordingly. In that sense, traffic's behavior resembles that of a non-neutral plasma—a gas of charged particles that all have the same electric charge and therefore repel one another even at large distances. If you were to send such a plasma through a narrow pipe, its particles would jostle back and forth as they tried to stay as far as possible from one another. Ripples of motion would pass through the plasma and this motion would be very similar to that of cars on a freeway.
. What are the effects of water pressure on fish, submarines and divers?
All three of these objects contain solids, liquids, and gases, so I'll begin by describing how pressure affects those three states of matter. Solids and liquids are essentially incompressible, meaning that as the pressure on a solid or a liquid increases, its volume doesn't change very much. Without extraordinary tools, you simply can't squeeze a liter of water or liter-sized block of copper into a half-liter container. Gases, on the other hand, are relatively compressible. With increasing pressure on it, a certain quantity of gas (as measured by weight) will occupy less and less volume. For example, you can squeeze a closet full of air into a scuba tank.
Applying these observations to the three objects, it's clear that the solid and liquid portions of these objects aren't affected very much by the pressure, but the gaseous portions are. In a fish or diver, the gas-filled parts (the swim bladder in a fish and the lungs in a diver) become smaller as the fish or diver go deeper in the water and are exposed to more pressure. In a submarine, the hull of the submarine must support the pressure outside so that the pressure of the air inside the submarine doesn't increase. If the pressure did reach the air inside the submarine, that air would occupy less and less volume and the submarine would crush. That's why the hull of a submarine must be so strong—it must hide the tremendous water pressure outside the hull from the air inside the hull.
Apart from these mechanical effects on the three objects, there is one other interesting effect to consider. Increasing pressure makes gases more soluble in liquids. Thus at greater depths and pressures, the fish and diver can have more gases dissolved in their blood and tissues. Decompression illness, commonly called "the bends", occurs when the pressure on a diver is suddenly reduced by a rapid ascent from great depth. Gases that were soluble in that diver's tissue at the initial high pressure suddenly become less soluble in that diver's tissue at the final low pressure. If the gas comes out of solution inside the diver's tissue, it causes damage and pain.