Author: Lou Bloomfield

Is there rain above the clouds?

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Is there rain above the clouds? — JM, Arlington Heights, Illinois

No. If you are above the clouds, then the sky above you is free from droplets of condensed moisture. While that doesn’t mean that there is no water overhead, that water must be entirely in the form of gaseous water molecules. Since rain forms when droplets of condensed moisture grow large enough to descend rapidly through the air, the absence of any condensed droplets makes it impossible for full raindrops to form. In short, no clouds overhead, no rain.

Is there a formula or equation for figuring out the pressure of air at a certain…

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Is there a formula or equation for figuring out the pressure of air at a certain altitude? — DLH, Conifer CO

Unfortunately, the answer is no. The atmosphere is too complicated to be described by a simple formula or equation, although you can always fit a formulaic curve to measured pressure values if you make that formula flexible enough. The complications arise largely because of thermodynamic issues: air expands as it moves upward in the atmosphere and this expansion causes the air to cool. As a result of this cooling, the air in the atmosphere doesn’t have a uniform temperature and, without a uniform temperature, the air’s pressure is difficult to predict. Radiative heating of the greenhouse gases and phase changes in the air moisture content further complicate the atmosphere’s temperature profile and consequently its pressure profile. If you want to know the air pressure at specific altitude, you do best to look it up in a table.

I know that the medium of electromagnetic waves is a photon. What is a photon? …

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I know that the medium of electromagnetic waves is a photon. What is a photon? What is it made of? — ON, Istanbul, Turkey

First, an electromagnetic wave consists of an electric and a magnetic field. These two fields create one another as they change with time and they travel together through empty space. An electromagnetic wave of this sort carries energy with it because electric and magnetic fields both contain energy. That much was well understood by the end of the 19th century, but something new was discovered at the beginning of the 20th century: an electromagnetic wave cannot carry an arbitrary amount of energy. Instead, it can carry one or more units of energy, units that are commonly called “quanta.” An electromagnetic wave that carries only one quanta of energy is called a “photon.”

The amount of energy that a photon carries depends on the frequency of that photon—the higher the frequency, the more energy. Photons of visible light carry enough energy to induce various changes in atoms and molecules, which is why they provide our eyes with such useful information about the objects around us—we see how this visible light is interacting with the world around us.

In science, we learned that a color’s energy depends on its wavelength

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In science, we learned that a color’s energy depends on its wavelength—that violet light with its short wavelength has more energy than red light with its long wavelength. But in art, we learned that red, orange, and yellow are warm and blue and violet are cool. Is that because of how the people feel about the colors, like fire is red and water is blue? — ON, Istanbul, Turkey

Both of your observations are correct: short wavelength light, such as violet, carries more energy per particle (per “photon”) than long wavelength light, such as red, and red light does appear “warmer” than blue light. But the latter observation is one of feelings and psychology, rather than of physics. It is ironic that colors we associate with cold and low thermal energies are actually associated with higher energy light particles than are colors we associate with heat and high thermal energies.

How does a VCR Plus system work? Are codes built in for every possibility of cha…

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How does a VCR Plus system work? Are codes built in for every possibility of channel and time or does it calculate somehow? I know that if you enter a random number (including single digits) that some program is scheduled. — LK, Huntington, West Virginia

The VCR Plus codes contain just enough information to tell the VCR what time and day a program starts, what channel that program is on, and how long it will last. What is remarkable about these codes is not that they exist, but that many of them are so short. A long number that contained the complete date, the entire channel number, and the length of the program in minutes would obvious fulfill the requirements, but the actually numbers are never that long. While I don’t know the precise encoding scheme, the date is clearly compressed—a daily or weekly program is represented by a very small code—and so is the record time for programs with a common duration. The VCR Plus codes get significantly longer when they must represent one-time only shows and shows with complicated durations. Even then, the date is truncated so that there are no current codes to represent a show five years in the future.

How does an automatic transmission in a car work?

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How does an automatic transmission in a car work? — ORL, Trondheim, Norway

An automatic transmission contains two major components: a fluid coupling that controls the transfer of torque from the engine to the rest of the transmission and a gearbox that controls the mechanical advantage between the engine and the wheels. The fluid coupling resembles two fans with a liquid circulating between them. The engine turns one fan, technically known as an “impeller,” and this impeller pushes transmission fluid toward the second impeller. As the liquid flows through the second impeller, it exerts a twist (a “torque”) on the impeller. If the car is moving or is allowed to move, this torque will cause the impeller to turn and, with it, the wheels of the car. If, however, the car is stopped and the brake is on, the transmission fluid will flow through the second impeller without effect. Overall, the fluid coupling allows the efficient transfer of power from the engine to the wheels without any direct mechanical linkage that would cause trouble when the car comes to a stop.

Between the second impeller and the wheels is a gearbox. The second impeller of the fluid coupling causes several of the gears in this box to turn and they, in turn, cause other gears to turn. Eventually, this system of gears causes the wheels of the car to turn. Along with these gears are several friction plates that can be brought into contact with one another by the transmission to change the relative rotation rates between the second impeller and the car’s wheels. These changes in relative rotation rate give the car the variable mechanical advantage it needs to be able to both climb steep hills and drive fast on flat roadways.

Finally, some cars combine parts of the gear box with the fluid coupling in what is called a “torque converter.” Here the two impellers in the fluid coupling have different shapes so that they naturally turn at different rates. This asymmetric arrangement eliminates the need for some gears in the gearbox itself.

I work finding sites for cellular & PCS wireless telephone antennae. I would lik…

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I work finding sites for cellular & PCS wireless telephone antennae. I would like to know how radio waves work and how they are able to carry voice and data information. What are these waves and do they exist naturally or do we set them up using electric charges? — PAB, Madison, WI

Radio waves are a class of electromagnetic waves, specifically the lowest frequency, longest wavelength electromagnetic waves. Actually, the electromagnetic waves used in cellular & PCS transmissions are technically known as microwaves because they have wavelengths of less than 1 meter, but there are no important differences between radio waves and microwaves.

Like all electromagnetic waves, radio waves and microwaves consist of coupled electric and magnetic fields that sustain one another in stable structures that move rapidly through empty space. Because an electromagnetic wave’s electric field changes with time, it is able to create the wave’s magnetic field and, because its magnetic field changes with time, that magnetic field is able to create the wave’s electric field. Since they consist only of electric and magnetic fields, these waves cannot stay still—they must move (although you can trap them between mirrors so that they appear to stand in one place as they bounce back and forth). While they contain no true mass, they do contain energy and an electromagnetic wave carries energy from one place to another.

Electromagnetic waves are created whenever electrically charged particles change speed or direction; whenever they accelerate. Since there are accelerating electric charges everywhere—thermal energy keeps them moving about—there are also electromagnetic waves everywhere. But the radio waves used in communications systems are generated deliberately by moving electric charges back and forth. When charges are sent up and down a radio antenna, these charges are accelerating and they form complicated electric and magnetic fields that include electromagnetic waves. Once launched, those electromagnetic waves propagate through space at approximately the speed of light.

To send information with radio waves, a transmitter makes modifications in one or more the wave’s characteristics. In an amplitude modulation scheme (AM), the transmitter changes the strength or “amplitude” of the wave to convey information—like sending radio smoke signals. In the frequency modulation scheme (FM), the transmitter changes the frequency of the wave to convey information—like whistling a tune with a complicated melody.

How does a fan compare to a propeller? Why does a fan blow air while a propeller…

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How does a fan compare to a propeller? Why does a fan blow air while a propeller has “lift”? — DB, Austin, TX

A fan and a propeller are actually the same thing. Both are rotating wings that push the air in one direction and experience a reaction force in the opposite direction as a result. Each experiences a “lift” force, typically called “thrust,” in the direction opposite the airflow. If you put a strong fan on a low-friction cart or a good skateboard, it will accelerate forward as it pushes the air backward. Similarly, if you prevent a propeller plane from moving, its spinning blades will act as powerful fans.

How does a rice cooker know when to turn off?

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How does a rice cooker know when to turn off? — JS, Tokyo, Japan

The rice cooker turns off when there is no longer enough liquid water on its heating element to keep that element’s temperature at the boiling temperature of water (212° F or 100° C). As long as the element is covered with liquid water, it is hard for that element’s temperature to rise above water’s boiling temperature. That’s because as the water boils, all of the thermal energy produced in the heating element is converted very efficiently into chemical potential energy in the resulting steam. In short, boiling water remains at 212° F even as you add lots of thermal energy into it.

But as soon as the liquid water is gone (and, fortuitously, the rice is fully cooked), there is nothing left to keep the heating element’s temperature from rising. As more electric energy enters the element and becomes thermal energy, the element gets hotter and hotter. A thermostat, probably a bimetallic strip like that used in most toasters, senses the sudden temperature rise. It releases a switch that turns off the electric power to the rice cooker.

How can I clean a dirty CD which has a very difficult to remove stain? Which mat…

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How can I clean a dirty CD which has a very difficult to remove stain? Which materials are best for cleaning? — AM, Mexico

Most CD’s are made from polycarbonate plastic (though other plastics with the same index of refraction are occasionally used). Polycarbonate is a pretty tough material, so it should survive most common stain or gum removing solvents. Try your favorite solvent on an unimportant CD first; such as one of the free discs that come occasionally in the mail. However, if the stain molecules have diffused into the plastic and have become trapped within the tangle of plastic molecules, you’re probably out of luck. Removing such a stain will require wearing away some of the plastic. Since the disc’s surface finish must remain smooth and the thickness of the disc shouldn’t change much, serious resurfacing is likely to make the disc unplayable. Also, stay away from the printed side of the disc—it has only a thin layer of varnish protecting the delicate aluminum layer from injury. Solvents can wreck this side of the disc. Finally, if the stain is a white mark (or a scratch), you may be able to render the disc clear again by filling the tiny air gaps that make it white with another plastic. I’ll bet that a clear furniture polish or liquid wax will soak into the white spot, replace the air, and render the disc clear and playable.