. How much natural pressure is around us when we are on the ground? Does this pressure decrease in higher places? Why don't people in aircraft explode because the pressure is lower?
Near sea level, the air around us has a pressure of about 100,000 newtons per square meter or 15 pounds per square inch. That means that each square meter of surface on your body is exposed to an inward force of 100,000 newtons or that each square inch of your body is exposed to an inward force of 15 pounds. Your body is thus exposed to enormous inward forces. However, you don't notice these forces because your body is composed of solids and liquids that resist compression ferociously. To see that this is so, try to squeeze a sealed bottle of soda or to squash a coin by stepping on it. It's very hard to shrink the volume of a solid or liquid by squeezing it.
The origin of the large pressure around us is the weight of the atmosphere overhead. The air near you is supporting the weight of several miles or kilometers of air overhead and the weight of this air is squeezing the air down here. When you ascend a mountain, the amount of air overhead decreases and so does the pressure of the air around you. Your body becomes less tightly squeezed by the air around it. However, you don't explode because releasing the pressure on you doesn't change your volume very much. Solids and liquids don't expand very much when the pressure on them is released.
. What does the SPF on sun screens mean? - RC
Sunscreens contain pigments that absorb invisible ultraviolet radiation. While they appear clear and transmit visible light so that you can't see them when they're on your skin, sunscreens are almost opaque to ultraviolet light. A sunscreen's SPF is related to the fraction of ultraviolet light that it absorbs. An SPF of 15 means that a normal layer of this sunscreen on your skin transmits only 1 part in 15 of the ultraviolet light that reaches it from the sun. An SPF of 40 means that a layer of this sunscreen transmits only 1 part in 40 of the ultraviolet light. The true transmission of the sunscreen depends somewhat on how you apply it and how much you apply, so these SPF ratings are only approximate. A sunscreen contains a mixture of dye molecules that transmit visible light but absorb ultraviolet light (and convert its the light's energy into thermal energy). Most if not all of these dye molecules are artificial organic compounds that have been carefully selected to be non-toxic and non-irritating. The first popular sunscreen contained a compound called PABA that caused skin reactions in many people, but more recent dye choices are less likely to cause skin trouble.
. If airplane cabins are pressurized to provide adequate oxygen for the passengers to breathe, what provides this compressed air? - EL
The air that you breathe inside an airplane is actually pumped into the cabin through the jet engines. The first component of a jet engine is a compressor that takes the low-density air outside and boosts its pressure and density. While most of this air then continues through the engine to the combustion chamber, part of it is diverted to the cabin. But before it can be released into the cabin, the air must be chilled by an air conditioner. That's because compressing air adds energy to it and raises its temperature. The compressed air leaving the jet engine's compressor is hot, even though no combustion has taken place yet. So the air is first cooled and then sent into the cabin.
. What is a shockwave and a sonic boom? - EL
A plane that is flying faster than the speed of sound is outrunning its own sound. As a result, its sound spreads out behind it as a conical structure, with the plane located at the apex of that cone. This cone moves along with the plane. Since the planes sound is all contained inside the cone, you can't hear the plane until the cone passes by you. When the edge of the cone does pass you, you hear a great deal of sound all at once. In fact, there is a pressure jump right at the surface of the cone (sound and pressure are closely related) and this cone itself is a shockwave. As the shockwave (or cone surface) passes you, you hear a loud booming sound, a "sonic boom". Note that the sonic boom occurs when the shockwave passes your ears, not when the plane "breaks the sound barrier". When you hear the sonic boom depends on where you are relative to the moving plane, so different people hear it at different times.
. What is the "sound barrier"? - EL
The "sound barrier" is more a psychological barrier than a real impediment. In the early days of high-speed flight, there was concern that a plane flying at or beyond the speed of sound in air would encounter unanticipated phenomena that would rip it apart. However, when Chuck Yeager finally did exceed the speed of sound for the first time in 1947, he found the transition from subsonic to supersonic uneventful. The only way that he could tell he was traveling faster than the speed of sound was with the help of his instruments.
. Does the volume in the cooking chamber of a microwave oven affect the rate at which it cooks the food? In other words, which cooks faster, a small microwave oven or a large one? - RP
The size of a microwave oven's cooking chamber should have little or no effect on how quickly it cooks food. The oven's magnetron tube delivers a certain amount of microwave power to the cooking chamber and virtually all of that power will eventually be absorbed in the food. It may take a few moments longer for a large cooking chamber to fill with microwaves when you first start the oven, but soon the food inside it will be exposed to the same intensity of microwaves as food cooking inside a smaller microwave oven with a similar magnetron power.
On the other hand, the magnetron's power does affect cooking speed so that an oven with a more powerful magnetron will cook food faster than one with a less powerful magnetron. The speed of cooking in a microwave oven also depends on how much food it contains because the food shares the microwave power. In general, doubling the amount of food in the microwave doubles the cooking time.
. How do light sticks work? - AE
When you bend a plastic light stick, you break a small glass ampoule and allow two chemicals that are contained inside the stick to mix. One of these chemicals is a powerful oxidizing agent and the other is a chemical that when oxidized ("burned") is left in an electronically excited state. In other words, the chemical reaction between the molecules of the two chemicals creates a new molecule that has excess energy in it. The molecule releases this energy as a particle of light, a photon. Although I am not certain exactly which chemicals are used in a modern light stick, I believe that one is hydrogen peroxide (the oxidizer) and the other is luminol (the chemical that is oxidized). Upon oxidization, luminol emits a photon of blue or ultraviolet light. The green light that you see emerging from a typical light stick is actually a second photon that is emitted by a fluorescent dye contained in the light stick. This dye absorbs the blue or ultraviolet photon emitted by the luminol and then reemits a new photon with somewhat less energy and a green color.
. Can a compound have triple bonds? If so, please give an example. — BA, IL
Yes, some compounds contain triple bonds. Acetylene is the simplest such molecule, with two carbon atoms connected by a triple bond. Each carbon atom has one hydrogen atom attached to it, so the entire molecule is a four-atom chain: hydrogen-carbon-carbon-hydrogen. The triple bond between carbon atoms is extremely strong—the atoms are sharing 6 electrons between them.
. What is red mercury, where does it come from, and where is it used?
Mercuric sulfide, a red mineral known as cinnabar, is the world's principal source of mercury and has also been used frequently as a vermilion paint pigment. It has been mined in Almaden, Spain for several thousand years and occurs in a number of other countries, including the United States.
. How does a light bulb work? — DH, Casselberry, FL (and also KH)
In a common incandescent light bulb, an electric current flows through a double-spiral coil of very thin tungsten wire. As the electric charges in the current flow through this tungsten filament, they collide periodically with the tungsten atoms and transfer energy to those tungsten atoms. The current gives up its energy to the tungsten filament and the filament's temperature rises to about 2500° C. While all objects emit thermal radiation, very hot objects emit some of the thermal radiation as visible light. A 2500° C object emits about 12% of its heat as visible light and this is the light that you see coming from the bulb. Most of the remaining heat emerges from the bulb as invisible infrared light or "heat" light. The glass enclosure shields the filament from oxygen because tungsten burns in air.