Month: October 1996

How can we polarize a molecule?

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How can we polarize a molecule? — AD, Manaus City, Amazonia, Brazil

Some molecules, including water, are naturally polarized. This means that they have a positively charged end and a negatively charged end. But even normally non-polar molecules such as carbon dioxide can be polarized by exposing them to strong electric fields. Electric fields exert forces on electric charges and cause the electric charges in a molecule to rearrange—the positive charges in the molecule shift in one direction and the negative charges in that molecule shift in the other. As a result of this applied electric field, the molecule acquires a polar character—a negatively charged end and positively charge end. However, this polar character disappears as soon as the electric field is removed.

How do radio waves transport energy?

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How do radio waves transport energy? — AD, Manaus City, Amazonia, Brazil

Radio waves consist of nothing more than electric and magnetic fields that are perpetually recreating one another as they travel through space at the speed of light. An electric field is a phenomenon that exerts forces on electric charges and a magnetic field is a phenomenon that exerts forces on magnetic poles. Both electric and magnetic fields contain energy because they are capable of doing work on and thus transferring energy to electric charges or magnetic poles that they encounter. In a radio wave, this energy or capacity to do work moves along with the fields at the speed of light. The radio transmitter uses electric power to create the radio wave and the radio wave delivers that power to the receiver. While most modern receivers use local electric power to amplify the information arriving in the radio wave, simple “crystal radios” are able to reproduce sound using on the power that is arriving in the radio wave itself.

Why does my voice sound different to me when I listen to a recording of myself?

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Why does my voice sound different to me when I listen to a recording of myself?

When you hear yourself speak directly, much of the sound that reaches your ears travels to them through the bones and tissues of your head. This type of sound conduction tends to emphasize the low frequencies in your voice so that your voice sounds lower to you than it does to other people. When you listen to a recording of your voice, you are hearing your voice as other people hear it, without the modifying effects of bone and tissue conduction. Everyone else listening to the tape thinks that your voice sounds normal but you think it sounds higher than normal.

How efficient are solar energy cells and windmills in producing energy for every…

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How efficient are solar energy cells and windmills in producing energy for everyday use? — JJ, San Antonio, TX

There are several ways to measure their efficiencies. One way is to compare the energy these devices extract from sunlight or from the wind to the electric energy they produce. By that measure, solar cells are roughly 15% efficient and windmills are roughly 50% efficient. However, you’re probably most interested in their cost efficiency—in how much power these devices can produce for a given operating cost. By that measure, both devices are somewhat more expensive to build and operate than conventional fossil-fuel power plants. As a result, the United States continues to rely on fossil-fuel plants because they cost less for each kilowatt-hour of electric energy produced. Nonetheless, solar cells are gradually becoming cheaper and they may become cost effective in the next decade or two. Windmills are already cost effective in some countries that rely entirely on imported fossil fuels. Denmark, for example, uses windmills extensively for electric power. While windmill power plants do exist in the United States, they are largely the results of regulation rather than market forces. But that, too, may change in the next decade or two.

How does a tsunami form and how far does it go when it hits land?

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How does a tsunami form and how far does it go when it hits land? — JM, Berkley, MI

A tsunami is simply a giant surface wave on water. Surface waves have several important characteristics, one of which is wavelength—that is, the distance between one crest and the next. The longer its wavelength, the faster a surface wave moves and also the deeper it extends below the surface of the water. In general a surface wave extends downward about one wavelength, so that if the crests are 100 meters apart, the wave is about 100 meters deep.

The wavelength of a tsunami is enormous—hundreds or even thousands of meters. As a result, a tsunami travels hundreds of kilometers per hour and extends downward deep into the ocean. Because it disturbs so much water, it carries a great deal of energy and it delivers this energy to the shore when it hits. Tsunamis are normally created by earthquakes or volcanic eruptions that sudden shift the supporting surfaces of a large amount of water. The water experiences a sudden impulse when the land or seabed shifts and a wave is emitted. You can launch a similar wave simply by shaking the end of a basin of water. But when a large region of land or seabed moves, the wave that’s launched has a very long wavelength and tremendous energy. This tsunami heads off with enormous speed until it encounters the gradual shallowing of a seashore. There it becomes deformed because the lack of water in front of it causes its crest to become incomplete. Eventually the tsunami breaks in churning surf. The height of this breaking wave crest and the distance it travels onto shore before it stops depends on the total energy of the tsunami, but heights of 10 or 20 meters are not uncommon. Such waves can travel hundreds of meters up a beach or oceanfront if the slope is sufficiently gradual.

Why does white noise cancel out the wide range of frequencies in the real world?…

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Why does white noise cancel out the wide range of frequencies in the real world? What range of frequencies does this technology affect? Can you block out the low thud of a neighbor walking in the unit above yours? — EH, Chelmsford, MA

In the context of sound, a source of white noise emits random, non-repetitive sound waves that have equal acoustic powers at all frequencies. That means that the source emits the same amount of energy each second at each frequency, over the entire audible spectrum. What white noise does is to numb your hearing by creating a featureless, uniform background noise at every frequency you can hear. Since your sensitivity to sound volume is logarithmic, meaning that the acoustic power in a sound has to double before you notice that it’s substantially louder, this uniform background makes it extremely difficult for you to hear small sounds. Regardless of a small sound’s frequencies, the white noise is already exposing your ears to those frequencies and the small sound only makes a small change in the volumes of these frequencies. For an analogy, think about how much more you would notice a small blinking red light in the dark than in bright white sunlight. Similarly, white noise creates the acoustic equivalent of white illumination, making it hard for you to notice small noises that would be very easy to hear against complete silence. If the sounds your neighbor makes are small enough, this numbing effect should make them much less noticeable.

There are also much more sophisticated devices that really cancel noise out. However, these look like earphones and must be worn directly on your ears. These devices use microphones to measure the pressure fluctuations in the sounds and then cause the earphones to create exactly the opposite pressure fluctuations. With these noise cancellation devices properly adjusted, the air pressure fluctuations that are sound never reach your ears at all—they are simply cancelled away to nothing before they arrive.

How do glaciers flow downhill?

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How do glaciers flow downhill? — LO and NB, Bothell, WA

Ice is a rather soft material and its crystals can deform permanently when exposed to sufficient stress. If you squeeze ice hard enough, its crystals will gradually change shape in much the same way that a copper penny will change shape if you squeeze it in a press. Since the pressures at the base of a glacier are enormous, the ice crystals there gradually deform to relieve the stress they’re experiencing. This slow deformation allows the whole structure of the glacier to move gradually downhill. If ice crystals were harder, like those in most rocks, glaciers wouldn’t flow. But they are very soft and so the glacier slowly flows downhill.

How does a three-way light bulb work? – AER

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How does a three-way light bulb work? – AER

A three-way light bulb has two filaments inside it. One filament is smaller than the other, consuming less electricity and emitting less light. At the low light setting, only the smaller filament has current running through it and the bulb emits a dim light. At the medium light setting, only the larger filament has current running through it and the bulb emits a medium light. At the high light setting, both filaments have currents running through them and the bulb emits a bright light. To control the two filaments, the bulb has three electrical connections. The two filaments share one of the connection and each has one additional connection of its own. A complicated switch in the lamp determines whether to deliver current to one filament or the other or both. In each case, current flows toward the filament through one connection and returns from the filament through the other connection.

I’ve heard two explanations for how flight is achieved: (A) through lift generat…

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I’ve heard two explanations for how flight is achieved: (A) through lift generated by differential pressure (Bernoulli’s effect) and (B) through elastic collisions between air molecules and the underside of a wing. Which is correct? How does the fact that planes can fly upside down enter into this picture?

Explanation A is entirely correct and explanation B is partly correct. If you extended explanation B to include all collisions between air molecules and the entire wing, then it would also be correct. Explanation A is the continuous fluid picture of flight and the revised explanation B is the granular fluid picture of flight. To the extent that gases are incompressible fluids (as required for Bernoulli’s equation to be completely valid), these two explanations are essentially equivalent.

The lift experienced by a plane’s wing depends on its shape and on its tilt or “angle of attack” into the wind. In general, wings are airfoils—curved shapes that are designed to obtain significant lift forces while experiencing minimal drag forces. Most airplane wings are more highly curved on their tops than on their bottoms and obtain upward lift forces as a result. These lift forces occur because the stable airflow that forms around such a wing involves faster-moving and thus lower-pressure air above the wing than beneath it. However, some airplane wings are symmetric—they have equal curvatures on top and on bottom. These symmetric wings compensate for their symmetry by attacking the air at an angle. When they are tipped so that their leading edges are higher than their trailing edges, these wings also experience upward lift forces. The air again flows more rapidly over than under the wings and the pressure is lower above the wings than beneath them. Even an inverted non-symmetric wing can adjust its angle of attack to obtain an upward lift force, which is how a plane can fly upside down.

In all of these cases, the forces are really exerted on the plane’s wings by the impacts of countless air molecules. These air molecules hit harder and more often beneath the wings than above them and thus exert a net upward force on the plane. The fact that some wings have more surface area on their highly curved tops doesn’t lead to larger downward forces because many of the collision forces exerted by molecules on the top surface of the wing cancel one another, in the same way that forces exerted on opposite sides of a sheet of paper cancel one another.

Why are there dimples on golf balls? – DM

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Why are there dimples on golf balls? – DM

If there were no turbulence around a golf ball as it moved through the air, there would be regions of slow-moving high-pressure air in front of it and behind it, and regions of fast-moving low-pressure air around its sides. Because of their symmetry, these pressures wouldn’t exert any overall force on the golf ball and it would fly through the air without experiencing any air resistance. But there is turbulence behind a moving golf ball and this turbulence spoils the high-pressure region behind the ball. Since there is less high-pressure behind the golf ball to push it forward, the ball experiences a backward force—the slowing force of pressure drag. The size of this pressure drag force is roughly proportional to the size of the turbulent wake.

The size of the turbulent wake depends on the airflow behind the ball. On a smooth ball, air flowing into the rising pressure behind the ball experiences friction with the ball’s surface and loses energy. This surface air soon reverses its direction of flow, triggering a large turbulent wake. A golf ball’s dimples complicate the airflow very near the ball’s surface so that new, rapidly moving air is able to flow in close to the ball’s rear surface, where it can delay the onset of the flow reversal. The turbulent wake that eventually forms is relatively small, so that the golf ball experiences less pressure drag than a smooth ball. That’s why a golf ball can travel so far before slowing down.