Author: Lou Bloomfield

I recently visited an audio store where I saw electrostatic speakers. These spea…

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I recently visited an audio store where I saw electrostatic speakers. These speakers have no moving parts like conventional speakers and are more expensive. How do they produce sound? — BC, Ottawa, Canada

Electrostatic speakers uses the forces between electric charges (so called “electrostatic forces”) to move a thin metal diaphragm back and forth rapidly. The motions of this diaphragm compress and rarefy the air in front of it, producing sound. On each side of the diaphragm is a rigid metallic grill that can hold electric charges. When the speaker is silent, the diaphragm has a large positive electric charge on it and both the metal grills have large negative charges on them (it could be the other way around, depending the speaker’s exact design). The diaphragm is then attracted equally toward both grills and the electrostatic forces cancel perfectly. The diaphragm doesn’t undergo any acceleration. To make the speaker produce sound, the electric charges on the two grills are changed so that the electrostatic forces on the diaphragm don’t cancel. Instead, the diaphragm is pulled strongly toward whichever grill has more negative charge on it (or less positive charge). The charges on the grills fluctuate as the music plays and the diaphragm accelerates back and forth between the grills. It pushes on the air as it does and produces sound. You’ll notice that the diaphragm is a moving part, so the claim that the speaker has “no moving parts” is misleading. The speaker cone of a conventional speaker only moves back and forth, too, so it has an equal claim to having “no moving parts.” The relative expense of an electrostatic speaker comes from the requirement of careful construction and the need for a high voltage adapter to match an amplifier to the speaker.

Don’t microwaves change the molecular structure and composition of food, by ejec…

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Don’t microwaves change the molecular structure and composition of food, by ejecting some electrons from atoms and forming cancer-causing free radicals? If I should stand away from a microwave to avoid possible leakage, why would I eat microwaved food?

Microwaves don’t affect the molecular structure of the food, except through the thermal effects we associate with normal cooking (e.g., denaturing of proteins with heat and caramelizing of sugars). That’s because, like all electromagnetic waves, microwaves are emitted and absorbed as particles called “photons.” The energy in a microwave photon is so tiny that it can’t cause any chemical rearrangement in a molecule. Instead, it can only add a tiny amount of heat to a water molecule. During the microwave cooking process, microwave photons stream into the food and heat it up. But millions of them would have to work together in order to cause non-thermal chemical changes in the food molecules and they don’t normally do that. The photons can only work together if there is a conducting material, such as a metal wire, inside the oven. In that case, the photons can accelerate mobile electric charges along the conducting paths and create sparks. Such sparks can cause chemical damage, but nothing worse than the chemical damage caused by scorching food with a flame or broiler. Even if your microwave is full of sparks for some reason, I doubt that the food will be any worse for you than it would be if you cooked it over an open flame or barbecue.

Do whales drink salted water?

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Do whales drink salted water? — GR, Montreal, Quebec

No. Marine mammals rely on water obtained from their food. Because they don’t sweat, they only lose water through their urine, which they concentrate to minimize the loss of water. What little water these animals do need comes from eating foods that are already relatively low in salt. Most of the lower sea animals, including fish, have active systems—ones that consume ordered energy—for eliminating salt so that when a sea mammal eats one of the lower animals, it inherits that animal’s relatively salt-free water. Moreover, metabolizing fats and carbohydrates produces water as a byproduct.

What are gas permeable contact lenses made from and what do they use to pigment …

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What are gas permeable contact lenses made from and what do they use to pigment them? — TG, Tulsa, OK

A gas permeable contact lens is one that allows oxygen to diffuse through it to the cornea of the wear’s eye. While conventional hard lenses were made almost entirely of a plastic known as poly(methyl methacrylate) or PMMA, commonly known as Plexiglas or Lucite, gas permeable hard or semirigid lenses are copolymers containing both methacrylate and siloxane molecular units. The polymers used in soft lenses are made only of siloxane molecular units and are commonly known as silicon rubbers. The molecules in silicon rubbers are mobile at remarkably low temperatures, giving silicon rubber its flexibility. In fact, these molecules are so mobile that they must be linked together or “vulcanized” to keep them from flowing as a liquid at room temperature. Even when they have been linked together, portions of these molecules are very mobile, so that gas atoms and molecules can diffuse easily through them. I’m not sure what chemicals are used to color contact lenses, but I expect that the dye molecules are permanently linked to the polymer molecules to keep them in place.

How does the auto-focusing system on a camera work?

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How does the auto-focusing system on a camera work? — RM, Lititz, PA

There are several different systems for autofocusing. I think that the three most popular systems are optical contrast, rangefinder overlap, and acoustic distancing. The optical contrast scheme places a sophisticated light sensitive surface in the focal plane of the camera’s lens. This sensor recognizes when sharp focus is achieved by looking for the moment of maximum contrast in the image. When the lens is out of focus, the image is fuzzy and has little contrast. But when the lens is focused properly, the image is sharp and the sensor detects the strong spatial variations in darkness and brightness. The camera automatically scans the focus of its lens until it detects maximum image contrast.

The rangefinder overlap system observes the scene in front of the camera through two auxiliary lenses that are separated by a few inches. It uses mirrors to overlap the images from these two lenses and can determine the distance to the objects in the picture by the angles of the mirrors. The camera uses this distance measurement to set the focus of its main lens.

The acoustic distancing system bounces sound waves from the objects in front of the camera to determine how far away they are. The camera then adjusts its main lens for that distance. While this acoustic scheme has the advantage of working even in complete darkness, it’s confused by clear surfaces—if you take a picture through a window, it will focus on the window. The optical schemes will focus on the objects rather than the window, but they will only work when there is light coming from the objects. That’s why many autofocus cameras that use optical autofocus schemes have built in lights to illuminate the objects during the autofocusing process.

What is light?

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What is light? — KB, Winnipeg, MB

Light consists of electromagnetic waves. An electromagnetic wave is a self-sustaining disturbance in the electric and magnetic fields that can exist even in empty space. You have probably seen two electrically charged objects push or pull on one another, such as when a sock clings to a shirt as you pull the two from the clothes dryer. You have probably also seen two magnetically poled objects push or pull on one another, such as when a magnet pulls itself toward a refrigerator door. These electric and magnetic forces are mediated by electric and magnetic fields respectively and, while those fields certainly exist in the space between the sock and shirt or between the magnet and refrigerator, they can also exist all by themselves. In an electromagnetic wave, the electric field creates the magnetic field and the magnetic field creates the electric field so that these two fields go on creating one another indefinitely as the wave travels through space at an enormous speed—the speed of light. Electromagnetic waves are distinguished by their frequencies or wavelengths, characteristics that are familiar to anyone who has watched water waves approaching the beach. But only a certain group of electromagnetic waves are visible to our eyes—those with frequencies between about 4.0*1014 cycles per second and 7.5*1014 cycles per second (wavelengths between about 750 nanometers and 400 nanometers). Outside of this range are infrared light at the low frequency end and ultraviolet light at the high frequency end.

Why does water boil at lower or higher temperatures under varying atmospheric pr…

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Why does water boil at lower or higher temperatures under varying atmospheric pressures? Do changing vapor pressures above a liquid play a role in changing boiling points of liquids? — KC, East Greenwich, RI

A liquid boils when its vapor pressure reaches atmospheric pressure. While a liquid will evaporate at temperatures below the boiling temperature, that evaporation only occurs from the surface of the liquid. That’s because atmospheric pressure crushes any bubbles that try to form within the body of the liquid. Every once in a while, a few molecules of the liquid break free inside the liquid and form a bubble of gas. The pressure inside such a bubble is the vapor pressure of the liquid at its present temperature. If the liquid’s temperature is below its boiling temperature, atmospheric pressure is greater than the pressure inside one of these spontaneous vapor bubbles and it crushes the bubble. But once the temperature of the liquid reaches the boiling temperature, the bubbles will have enough pressure to remain stable against atmospheric pressure. Each bubble that forms begins to float upward toward the top of the liquid and more molecules evaporate into it as it rises, so that it grows larger and larger.

If you lower atmospheric pressure, the liquid will boil at a lower temperature because the vapor pressure reaches atmospheric pressure more easily. If you raise atmospheric pressure, the liquid will boil at a higher temperature because the vapor pressure must rise higher before it reaches atmospheric pressure.

How does a gravity powered water pump work?

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How does a gravity powered water pump work? — JA, Hiawassee, GA

I believe that the pump you’re interested in is one that uses the energy released when water flows downhill to lift a small fraction of that water upward. While there are many possible designs for such a pump, the classic version used a phenomenon called “water hammer” to lift water upward. In this technique, a column of water is allowed to accelerate downhill through a pipe until it’s flowing at a good speed through the pipe. The pump then closes a valve at the lower end of the pipe, so that the water has to stop abruptly. Since water accelerates in response to imbalances in pressure, the stopping process involves an enormous pressure surge at the lower end of the moving water column. A one-way valve at the lower end of the pipe opens during this pressure surge and allows a small fraction of the water to escape from the pipe. The escaping water rises upward through a second pipe for delivery to a home or business. According to a reader, the escaping water actually enters a head tank that is normally filled with air and thus compresses that air. The compressed air is then used to push water through the pump’s outlet and provide the pumping action. This pumping scheme is apparently called a “hydraulic ram.”

The only trick to operating such a pump is opening and closing the valve at the lower end of the first pipe. This valve must open long enough that the water in the pipe reaches a good speed and then it must close very suddenly to provide the pressure surge that lifts the small amount of water upward for delivery.

Does water drain in the opposite direction in the southern hemisphere? – TL

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Does water drain in the opposite direction in the southern hemisphere? – TL

In principle, yes, but in practice, no. To explain why, I’ll begin with the origins of directional circulations on earth. Because the earth is turning, motions along its surface are complicated. The ground at the equator is actually heading eastward at more than 1000 miles per hour. The ground north or south of the equator is also heading eastward, but not as quickly. The ground’s eastward speed gradually diminishes until, at the north and south poles, there is no eastward motion at all. As a result of this non-uniform eastward motion of the ground, objects that travel in straight lines because of their inertia end up drifting eastward or westward relative to the ground. For example, if you took an object at the equator and threw it directly northward, it would drift eastward relative to the more slowly moving ground. If someone else threw an object southward from the north pole, that object would drift westward relative to the more rapidly moving ground. In the northern hemisphere, objects approaching a center tend to deflect away from that center to form a counter-clockwise circle around it. This process is reversed in the southern hemisphere so that objects approaching a center there tend to form a clockwise circle around it. Thus hurricanes are counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere.

When water drains from a basin in the northern hemisphere, it flows toward a center and should have a tendency to deflect into a counter-clockwise swirl. However, the effect is very weak in a small washbasin. The direction in which the water swirls as it drains is determined by other effects such as how the water was sloshing before you opened the drain or how symmetric the basin is. For this earth’s rotation-driven swirling effect (the Coriolis effect) to dictate the direction of a circulation the objects involved must move long distances over the earth’s surface. Even tornadoes don’t always rotate in the expected direction; they’re just not big enough to be spun consistently by the Coriolis effect.

How does heat conduct through different materials? – B

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How does heat conduct through different materials? – B

In electric insulators, heat is carried by motions of the atoms themselves. You can think of this heat transfer as a bucket-brigade process—one atom jiggles its neighbor, which in turn jiggles its neighbor, and so on. If one end of an insulator is hotter than the other, this jiggling effect will gradually transfer thermal energy from the hotter end (more vigorous jiggling) to the colder end (less vigorous jiggling). Imperfections and weaknesses in most electric insulators make them relatively poor conductors of heat, although there are a few exceptional materials such as diamond that use the bucket-brigade mechanism very effectively and are excellent thermal conductors. In electric conductors, mobile electrons help out by carrying thermal energy from one atom to another over long distances. Even in a material that doesn’t make good use of the bucket-brigade mechanism, the mobile electrons provide substantial thermal conductivity. Thus good electric conductors, such as copper, silver, and aluminum, are also good thermal conductors.