What makes an airplane fly?

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What makes an airplane fly? — BO, Pemberton, MN

As an airplane’s wing moves through the air, the airstream approaching the wing separates into a flow over the top of the wing and a flow under the bottom of the wing. The wing is shaped and tilted so that the flow over the wing follows a longer path to arrive at the sharp trailing edge of the wing than the flow under the wing must follow. Because it has a shorter distance to travel, the flow under the wing initially arrives at the trailing edge of the wing first and flows up and around that trailing edge to meet the flow over the wing. This type of flow has a kink in it at the wing’s trail edge and is unstable. A few moments after the wing begins moving through the air, the kink at the trailing edge blows away from the wing altogether. This kink leaves as a vortex—a whirling cyclone of air—and as it does, it causes the flow over the wing to speed up so that the two airflows join together cleanly at the wing’s trailing edge. To increase its speed, the flow over the wing converts some of its pressure energy into kinetic energy. Because the flow over the wing has used up some of its pressure energy, and thus experienced a drop in pressure, there is an unbalanced pressure across the wing: the pressure beneath the wing is greater than the pressure above the wing. This imbalance in pressure leads to an overall upward force on the wing and this upward force is what supports the plane’s weight so that it remains suspended in the air. Overall, the airstream is deflected downward as the result of this complicated flow pattern around the wing and the air pushes the wing upward in response. A nice image of the airstream leaving a plane’s wings can be seen at the Canon website, http://www.usa.canon.com/explorers/flight.html.

What is the difference between crystal and glass?

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What is the difference between crystal and glass?

The “crystal” that’s used in fine glassware is actually a glass, but it is chemically different from the glass that’s used in more common glassware. Both materials are formed by melting together a mixture of silicon dioxide (also called quartz or silica) and other chemicals and both are glasses, meaning that their atoms are arranged haphazardly and not in the crystalline lattices of such materials as salt or sugar. The chemicals that are added to silicon dioxide to make normal glassware—sodium oxide and calcium oxide—make the glass easier to melt and work with at the expense of strength and increased damping. That’s why normal glassware is relatively soft and emits a dull sound when you rap it; it experiences lots of internal friction. The chemicals added to silicon dioxide to make “crystal” glassware include lead oxide, which makes the glass easier to melt and soft enough to cut and shape easily. However, lead “crystal” glassware has less internal damping than ordinary glassware and emits a ringing tone when you rap it because it experiences very little internal friction.

Why is incandescent lighting better in residential construction than metal halid…

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Why is incandescent lighting better in residential construction than metal halide, high-pressure sodium, or mercury vapor lighting systems? — JC, Halifax, Nova Scotia

While incandescent lighting isn’t nearly as energy efficient as those other light systems, it produces a more eye pleasing light than some of the alternatives. Our eyes are optimized for sunlight, so that we find the spectrum of light from hot objects particularly pleasant. The heart of an incandescent bulb is a hot tungsten filament. High-pressure arc lamps such as sodium vapor or mercury vapor lamps (metal halide lamps are just somewhat color-corrected high pressure mercury vapor lamps) produce a much less even spectrum of light. High-pressure sodium vapor lamps are wonderfully energy efficient, but their light is orange or pink. High-pressure mercury vapor lamps are also quite energy efficient, but their light is somewhat bluish. Even metal halide lamps aren’t quite white. The other problem with high-pressure arc lamps is that they take time to warm up and then can’t be restarted until they cool off. They’re best in applications that don’t require them to be turned on or off frequently.

A much better choice, both in terms of energy efficiency and light color, is a fluorescent or compact fluorescent lamp. Such lamps typically use less than 25% of the energy required for comparable incandescent lighting, provide excellent color rendering that can be chosen to match that of incandescent lighting, and they last much longer than incandescent bulbs. Even though compact fluorescent lamps are more expensive than incandescent bulbs up front, they last so much longer and save so much energy that each one typically saves you about $45 over its working life.

How do neon lights work?

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How do neon lights work? — MT, Cement City, MI

A neon light uses a high voltage transformer to place electric charges on the wires at each end of a neon-filled glass tube. One end of the tube receives positive charges and the other end receives negative charges. Since like charges repel one another, the vast numbers of like charges at each end push apart strongly and some of them leave the wire and enter the neon gas. Once they’re in the gas, these charges are draw quickly toward the opposite charge at the far end of the tube. As they travel through the tube, these moving charges pick up speed and kinetic energy but they occasionally collide with neon atoms as they travel and can transfer some of their kinetic energies to the neon atoms. The neon atoms retain this extra energy only briefly before getting rid of it in the form of visible light—the familiar red glow of a neon lamp. Overall, electric charges stream from one end of the tube to the other, frequently colliding with the neon atoms and causing those atoms to emit red light. If you look closely at a neon lamp, you’ll see that it is the gas itself that’s emitting the red light.

I know that microwaves only heat polar molecules but what about aluminum foil an…

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I know that microwaves only heat polar molecules but what about aluminum foil and graphitic carbon, which are both heated by microwaves even though they have no dipole moments? — EB

Aluminum foil and graphitic carbon are both conductors of electricity. When they’re exposed to microwaves, the electric fields in those microwaves causes currents to flow through them. If the aluminum were thick enough, it would be able to handle the currents without trouble. But aluminum is very thin and the current that flows through it may be more than it can tolerate, particularly if it’s only a narrow strip. It then becomes very hot. The effect is the same as would happen if you plugged the aluminum foil into an electric outlet and sent current through it that way. The same heating occurs in the carbon—the current that flows in it heats it up. In short, relatively poor conductors of electricity become hot in a microwave because they permit currents to flow in response to the microwave electric fields but then can’t tolerate those currents without becoming hot.

Assuming microwave ovens cook on the principle of “moist” heat cookery, what a…

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Assuming microwave ovens cook on the principle of “moist” heat cookery, what are the general effects of microwave cooking on various foods, including effects on chemical structure? — EJ, Sydney, Australia

Microwave ovens cook by depositing thermal energy in the water molecules, which isn’t the same as cooking food in moist hot air. Microwave cooking tends to heat food uniformly throughout where as more conventional “moist” heat cooking still heats food from the outside in. Nonetheless, the chemical effects on food are very similar for both types of cooking. Virtually all of these effects are caused by elevating the temperatures of the food. I’m not an expert on the chemistry of cooking, but elevated temperatures certainly denature proteins and caramelize sugars.

How do radios work?

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How do radios work?

A radio station launches a radio wave by moving electric charges rhythmically up and down their antenna. As this electric charge accelerates back and forth, it produces a changing electric field—a structure in space that pushes on electric charges—and a changing magnetic field—a structure in space that pushes on magnetic poles. Because the electric field changes with time, it creates the magnetic field and because the magnetic field changes with time, it creates the electric field. The two travel off across space as a pair, endlessly recreating one another in an electromagnetic wave that will continue to the ends of the universe. However, when this wave encounters the antenna of your radio, its electric field begins to push electric charges up and down on that antenna. Your radio senses this motion of electric charges and thus detects the passing radio wave.

To convey audio information (sound) to you radio, the radio station makes one of several changes to the radio wave it transmits. In the AM or Amplitude Modulation technique, it adjusts the amount of charge it moves up and down its antenna, and hence the strength of its radio wave, in order to signal which way to move the speaker of your radio. These movements of the speaker are what cause your radio to emit sound. In the FM or Frequency Modulation technique, the radio station adjusts the precise frequency at which it moves charge up and down its antenna. Your radio senses these slight changes in frequency and moves its speaker accordingly.

When you were saying that even humans travel as waves (which I can picture), is …

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When you were saying that even humans travel as waves (which I can picture), is this the theory behind how the people in the show Startrek are “beamed” to certain planets and back to the ship?

The fact that all objects, including people, travel as waves in our universe is probably not what the writers of Startrek had in mind when they “invented” the transporter. In Startrek, the transporter seems to disassemble the people involved at one location and then reconstruct them at another. That disassembly/reassembly process is purely science fiction while the wave propagation of matter is quite real. We never notice this wave propagation for large objects because their wave effects are too small to detect and because watching an object propagate prevents its wave properties from having any significant consequences. Each observation of an object tends to localize it and minimize its wave properties, so that watching an object moves makes the effects of its wave properties minimal.

I know that photons are particles of light

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I know that photons are particles of light—but how are photons related to the “excited” electrons in the atoms of a gas discharge?

An atom in a gas discharge emits light when one of its electrons shifts from an orbital with extra energy into an empty orbital in which it will have less energy. Since an electron can only travel around the atom’s nucleus in an allowed orbit—an orbital—and the energy it has while in that orbital is very specifically defined, such a shift from one orbital to another results in the emission of a photon of light with a very specific energy. Because a photon’s energy is directly proportional to the frequency of the light, and light’s frequency and wavelength are related by the speed of light, the amount of energy the electron gives up in shifting from one orbital to another determines the photon’s energy, frequency, and wavelength.

When you walk on snow when it is cold (-20° C), the snow squeaks; but when i…

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When you walk on snow when it is cold (-20° C), the snow squeaks; but when it is relatively warm (-5° C) the snow doesn’t squeak. Why? — PW, Alberta, CA

Near ice’s melting temperature, the surfaces within warm snow become more and more liquid-like. These liquid-like surfaces not only allow the warm snow to stick together as firm snowballs, but they act as lubricants so that the snow is particularly slippery. At much lower temperatures, the snow’s surfaces are much more solid and they slide uneasily and noisily across one another. The cold snow squeaks because it hasn’t “been oiled.”