Month: December 1996

How do these digital thermometers work? I read somewhere that they contain alcoh…

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How do these digital thermometers work? I read somewhere that they contain alcohol instead of mercury. — KM, Lincoln, NE

Most modern liquid-in-glass thermometers do contain alcohol rather than mercury, but these aren’t the digital thermometers you are referring to. The alcohol thermometers are the ones with the red line that moves upward in a glass tube as the temperature increases. I believe that the digital thermometers you’re interested in are the ones with numbers that change colors as the temperature changes. For example, when its 72° F, the number “72” is brightly colored while the other numbers are essentially black. Those thermometers use liquid crystals to measure temperature. More specifically, they use chiral nematic liquid crystals—long asymmetric molecules that arrange themselves in orderly spirals in the liquid. When light strikes these spiral structures, some of it reflects. But the reflection is strongest when the light’s wavelength is an integer or half integer multiple of the spiral’s pitch—the distance between adjacent turns of the spiral. Since light’s wavelength is related to its color, the light reflected by these liquid crystals is colored. Because the pitch of a chiral nematic liquid crystal changes with temperature, so does its color. Slightly different liquid crystals are inserted behind each number on the thermometer so that each number becomes colored at a different temperature.

How do the thermometers with digital (electronic) numbers work?

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How do the thermometers with digital (electronic) numbers work? — KM, Lincoln, NE

The electronic fever thermometers that you can buy in a grocery store use a thermistor to measure temperature. A thermistor is a semiconductor device that acts as a temperature-sensitive electric resistor. At very low temperatures, a thermistor is essentially an insulator—it has no mobile electric charges and thus can’t carry electricity. But as its temperature increases, thermal energy rearranges the charges in the thermistor and it has more and more mobile electric charges. Its ability to conduct electricity increases with temperature fairly dramatically—it gradually becomes an electric conductor. The thermistor used in a fever thermometer is designed to undergo this rapid change in electric resistance at temperatures near 98° F. A simple computer inside the thermometer measures the thermistor’s electric resistance and determines the thermistor’s temperature. It then uses a liquid crystal-based display to show you what that temperature is.

Airplanes can land at many different altitudes above sea level. How does the alt…

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Airplanes can land at many different altitudes above sea level. How does the altimeter work at these different landing altitudes to show zero when the plane has finally landed? — BC, Canada

Their altimeters don’t read zero once they have landed—they read the altitude of the airport! Each airport’s altitude is reported on the navigational maps that pilots use. As the pilot approaches the runway, the pilot watches the altimeter and expects it to reach the airport’s altitude about the time that the plane touches the runway. Before the next take off, the pilot adjusts the altimeter using the airport’s official altitude as a calibration point for the altimeter. Some modern planes also used radar equipment to determine the distance to the ground beneath the plane. These devices do read zero at landing. The satellite-based Global Positioning System (GPS) also provides altitude information to pilots. Since this system reports altitude above sea level, it gives the altitude of the airport at landing, not zero.

In high school physics, we learned that matter and energy can neither be created…

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In high school physics, we learned that matter and energy can neither be created nor destroyed. Is that true in quantum mechanics? What is quantum mechanics and how did the field come about? — JE, College Station, TX

While modern physics continues to maintain that matter and energy can’t be created or destroyed, the picture is a little more complicated than it was before the discovery of relativity and quantum mechanics. First, relativity ties matter and energy together so that matter can become energy and energy can become matter in certain circumstances. As a result, it’s only the sum of matter and energy that can’t be created or destroyed. Second, there are situations in which that sum of matter and energy can change temporarily in an isolated system. Quantum mechanics and its famous “uncertainty principle” permit brief but important violations of the conservation of mass/energy. The shorter a particular violation, the worse it may be. These violations are never directly observable because all observations are done on long time scales. But there are indirect indications of these temporary violations and they’re critical to much of modern high energy and particle physics.

Quantum mechanics developed at the beginning of this century to explain several strange experimental observations, particularly the photoelectric effect and the black-body radiation spectrum. Einstein received his Nobel Prize for explaining the photoelectric effect in terms of quantum mechanics, not for any of his work on relativity.

I recently received a “strong magnetic cup” as a gift. According to the claims…

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I recently received a “strong magnetic cup” as a gift. According to the claims of the maker, water kept in this cup for a minute can lower blood pressure and reduce weight, etc. Please explain how this works. — AL, Pharr, TX

I’m afraid that it works only by psychological effect, if at all. Water itself is non-magnetic and experiences no significant change when exposed to a magnetic field. Although the magnetic field of the cup has an ever so slight effect on the atomic and molecular structure of the water, this effect vanishes when the water leaves the cup. Water from the cup is just plain old water. There are many people in this world who take advantage of the public’s relative inability to distinguish science from pseudoscience. One of the reasons that I enjoy answering questions here is to help people make that distinction. Magnets aren’t magic—they are understandable devices and their effects on everything around them are also understandable.

How do you create sculptures out of glass? – RD

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How do you create sculptures out of glass? – RD

While I know how to work with glass in principle, I’m certainly not able to make sculptures. Although anyone can shape glass, doing so with artistry and precision requires great skill. In effect, glass is a frozen liquid. Its microscopic structure is very similar to that of a liquid and it softens with temperature rather than melting abruptly. If you heat a piece of glass carefully with a propane torch, it will begin to flow as a thick liquid (like cold honey). In that state, it can be reshaped rather easily. But making it take the shapes you want is a whole other story and something I know little about. I have bent lots of glass tubes in my day, but I often kink the tubing or smash it flat by accident. A skilled glassblower can do seemingly impossible things with glass. I should also note that glass can be cut or shaped by a water-cooled abrasive wheel. Again, anyone can slice and dice glass but it takes great skill to do something attractive. I usually chip the glass pieces that I try to cut.

How do you calculate the path light takes after going through a lens and how do …

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How do you calculate the path light takes after going through a lens and how do you measure the curvature of the lens? — AS, Champaign, IL

The surfaces of most lenses are shaped like the surfaces of spheres. Such “spherical” lenses can be characterized by a single distance: the focal length. For converging lenses, those with convex or outward-bulging surfaces, light from a distant object such as the sun will converge together after passing through the lens and will form an image of the object at a distance of the focal length from the center of the lens. You can find this “real” image by holding a sheet of white paper beyond the lens and looking for a clear pattern of light corresponding to the object. If the object is closer to the lens, the image will form a bit farther from the lens. The relationship between the distance to the object (the object distance or OD), the focal length of the lens (F), and the distance to the image (the image distance or OD) is given by a simple formula: 1/F = 1/OD + 1/ID.

This lens formula works for diverging lenses, too, but those lenses have negative focal lengths and produce their images on the object side of the lens. You can only view these “virtual” images by looking at them through the lens itself.

The easiest way of determining a lens’s focal length is by measuring the distance between the lens and the real image it forms of a distant object. However, you can measure the curvatures of the lens’s surfaces and calculate its focal length. Special gauges exist that touch the lens at several points, usually a circle and a central point, and determine how curved its surface is.

Would extreme temperatures affect the strength of a magnet?

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Would extreme temperatures affect the strength of a magnet? — PL, Columbus, OH

Yes! High temperatures disorder materials and destroy magnetic order. Permanent magnets can be demagnetized by heating them, often to surprisingly modest temperatures. Many household magnets can be spoiled by putting them in a hot oven. Even electromagnets will lose most of their strength at very high temperatures because they rely on iron and iron undergoes several phase transitions at high temperatures that destroy its magnetic order. You can show that iron loses its magnetism at high temperatures by heating a steel nail red hot with a propane torch and then trying to pick it up with a magnet. Be careful not to burn yourself. The hot nail won’t stick to the magnet because it won’t have any magnetic order. Once the nail cools, its magnetic order will reappear.

How can you make electricity with magnets? – AL

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How can you make electricity with magnets? – AL

You can make electricity by moving a magnet past a wire. The magnet has a magnetic field around it—something that exerts forces on magnetic poles. If you move the magnet and its magnetic field, you create an electric field—something that exerts forces on electric charges. That’s because whenever a magnetic field changes with time, it creates an electric field. This electric field will push on the mobile electrons in a wire. So when you move a magnet past a wire, you are producing a changing magnetic field in the wire. This changing magnetic field produces an electric field and the electric field makes the electrons in the wire accelerate. The moving electrons are electricity. Generators move magnets past wires (or wires past magnets) to produce electricity.

What is a short in the electrical system of a car? What causes shorts?

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What is a short in the electrical system of a car? What causes shorts? — BM, Puyallup, WA

A short circuit is a conducting path that allows electric current to flow from its source (typically the positive terminal of a battery) to its destination (typically the negative terminal of that battery) without passing through the equipment that the current is supposed to operate. The conducting path is thus a short cut for the current that allows it to complete its circuit too quickly, hence the name “short circuit.” In virtually all automobiles, the whole body of the car is connected to the negative terminal of the battery so that any accidental conducting path from the battery’s positive terminal to the body of the car is a short circuit. Since a short circuit doesn’t include a device that’s designed to consume electric power, the wires of the short circuit must consume that electric power. They often become hot and may cause a fire.