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Page 117 of 160 (1595 Questions and Answers)

1161. What function does the Degauss button actually perform on computer monitors and why is it not available for televisions? — JF, San Francisco, California
Both color monitors and color televisions create their color images by combining the three primary colors of light—red, green, and blue. Each display has an intricate pattern of red, green, and blue phosphor dots or stripes on the inside surface of its picture tube and it produces full color images by adjusting the brightness balance of these tiny glowing spots. Beams of electrons are directed at these phosphors from the back of the picture tube and their impacts with the phosphors cause the phosphors to fluoresce—emit light.

Because the picture tube can't direct its electron beams accurately enough to hit specific red, green, or blue phosphor regions, it needs help from a shadow mask that's located a short distance before the phosphor layer. This thin metal grillwork shades the light-producing phosphors from the wrong electrons. The picture tube has three separate beams of electrons, one for each primary color, and the grillwork ensures that electrons in the red beam are only able to strike phosphors that produce red light. The same goes for the blue beam and the green beam.

The grillwork must stay in perfect registry with the pattern of phosphors on the inside of the picture tube, even as their temperatures change. That's why this grillwork is made of Invar, a special steel alloy that doesn't change size when its temperature changes. Unfortunately, Invar can be magnetized and its magnetic fields can then steer the electrons so that they strike the wrong phosphors. If you were to hold a strong magnet near the face of a computer monitor, you would probably magnetize the Invar shadow mask and spoil the color balance of the images on the monitor.

To demagnetize the Invar, you must expose it to a magnetic field that fluctuates back and forth and gradually diminishes to zero. The Invar's magnetization would also fluctuate back and forth and would dwindle to nothing by the time the demagnetizing field had vanished. Traditionally, this demagnetizing was done with a large wire coil that was powered by alternating current so that its magnetic field fluctuated back and forth. This coil was gradually moved away from the picture tube so that the influence of its magnetic field slowly diminished to zero, leaving the Invar completely demagnetized. In good computer monitors, this coil and an automatic power source for it are built in. When you push the degauss button, you see a burst of colors as the demagnetizing coil's fluctuating magnetic field erases the magnetization of the shadow mask and also steers the electrons wildly.

Apparently, degaussing circuitry has been built into all color televisions sets for the past 20 or 30 years. When you turn on your television, a demagnetizing coil activates briefly and removes minor magnetization from the television's invar mask.

1162. Is there any easy way to mold plastics?
The easiest way to mold plastics is to form them directly inside a mold. Most plastics are made by attaching small molecules to one another in a process called polymerization. You begin with one or more small molecules or "monomers" and cause them to link together into in a "polymer." You can initiate this polymerization with chemical catalysts, light, or even heat. There are many plastic-forming systems that you can buy commercially. You simply mix a few chemicals together, pour the mixture into a mold and wait. Once the polymerization has finished, you have a molded piece of plastic.

If you don't want to do the polymerization yourself, you can start with a finished plastic and melt it. Most plastics that haven't been vulcanized into one giant molecule (as is done in rubber tires) will melt at high enough temperatures (although some burn or decompose before they melt). These molten plastics can be stretched, squeezed, or poured into molds to make just about any shape you like.

1163. What are the relative efficiencies of the fission and fusion reactions in thermonuclear weapons? Is every last grain of fissile and fusible matter converted to energy or is there a loss somewhere?
While both fission and fusion convert substantial fractions of the mass in a thermonuclear weapon into energy, most of the bomb's initial matter remains matter, not energy. When a uranium nucleus fissions to become smaller nuclei, about 0.1% of the uranium nucleus's mass becomes energy. When two deuterium nuclei—the heavy isotope of hydrogen—fuse together to become helium, about 0.3% of the deuterium nuclei's masses become energy. Despite these seemingly small percentages, this scale of matter to energy conversion dwarfs that of chemical explosives, which convert only parts per billion of their masses into energy.

While fusion is somewhat more energy efficient than fission, that's not the whole reason why hydrogen bombs (thermonuclear bombs) are more powerful than uranium bombs (fission bombs). The main reason is that thermonuclear bombs can be much larger than fission bombs because there is no upper limit to the amount of hydrogen you can assemble in a small region of space. In contrast, if you assemble too much fissile uranium in a small region of space, a chain reaction will begin and the material will overheat and explode. At the height of the cold war, the Soviet Union built gigantic thermonuclear weapons with explosive yields as large as 100 megatons of TNT.

1164. How does an ear thermometer work so quickly? — SN, West Covina, California
An ear thermometer examines the spectrum of thermal radiation emitted by the inner surfaces of a person's ear. All objects emit thermal electromagnetic radiation and that radiation is characteristic of their temperatures—the hotter an object is, the brighter its thermal radiation and the more that radiation shifts toward shorter wavelengths. The thermal radiation from a person's ear is in the invisible infrared portion of the light spectrum, which is why you can't see people glowing. But the ear thermometer can see this infrared light and it uses the light to determine the ear's temperature. The thermometer's thermal radiation sensor is very fast, which accounts for the speed of the measurement.

1165. I understand that an ear thermometer measures a person's temperature by studying the thermal radiation emitted by their ear. What is the farthest range that a person can emit thermal radiation that can still be received? Does this range depend on how hot the inner person is? — M
The thermal radiation that a person emits is mostly infrared light and, like all light, it can travel forever if nothing gets in its way. In principle, if you can observe something through a telescope, you can also measure its temperature. For example, astronomers can measure the temperature of a distant star by studying the star's spectrum of thermal radiation.

However, there are several complications when using this technique to measure a person's temperature. First, anything that lies between the person and you, and that absorbs or emit thermal radiation, will affect your measurement. That's because some of the thermal radiation that appears to be coming from the person may be coming from those in between things. Fortunately, air is moderately transparent to thermal radiation but many other things aren't. In fact, to get an accurate reading of person's temperature, you'd have to cool the telescope and the light detector so that they don't add their own thermal radiation to what you observe. You'd also have to use a mirror telescope because glass optics absorb infrared light.

Second, the temperature that you observe will be that of the person's skin and not their inner core temperature. That's because the person's skin absorbs any infrared light from inside the person and it emits its own infrared light to the world around the person. You can't observe infrared light from inside the person because the person's skin blocks your view. All you see is their skin temperature.

1166. You stated that thermodynamics overwhelms just about everything sooner or later. Could you explain why? — MT, San Antonio, Texas
Thermodynamics is a statistical science that deals with systems that are so complicated or vast that they can't be followed in complete detail. It makes predictions of behavior based on probability theory and while some of its laws predict probable outcomes rather than certain outcomes, a sufficiently probably event is effectively a certain event. For example, I can say with near certainty that if you play the lottery 50 times, you won't win the jackpot 50 times. I can't be truly certain of that fact, but the likelihood of my prediction being correct is pretty good.

In a sense, probability is destiny. Thermodynamics observes that vast systems tend to evolve toward the mostly likely configurations. To understand this process, consider what happens when you mix hot and cold water. The most likely final configuration for the mixed water is for it to reach a uniform temperature about half way in between the two original temperatures. While it's possible for the water to end up extremely hot in one place and extremely cold in another, that outcome is extremely unlikely. It's so unlikely that it never happens.

So in what sense does thermodynamics overwhelm things? The world is filled with relatively ordered arrangements and these ordered arrangements are unlikely by themselves (how they came to be ordered in the first place is another matter for another questions). If you take a crystal vase and drop it on the floor, it's going to evolve toward a more likely arrangement of atoms and dropping it a second time isn't going to return it toward its original unlikely state. In short, ordered systems naturally drift toward disorder when given a chance. How quickly they drift depends on their situation. A coffee cup will remain a nicely ordered object for thousands or millions of years if you don't disturb it. But in a hot environment, or one that is chemically aggressive, it may not last very long.

One last thought: how do living organisms maintain their order in the face of this tendency to disorder? They do it by consuming order and exporting disorder—they eat ordered foods and release disordered wastes to their surroundings.

1167. Does microwave cooking affect the nutritional value of food?
No more so than conventional heating does. Overheating some nutrients can damage them, so that microwave cooking does affect food's nutritional value. But microwave cooking is far less likely to cause serious molecular damage to food than flame broiling or frying.

1168. Is there a standard time that one should wait before eating food that has been heated in a microwave oven? - M
Apart from the usual precautions with hot food, there is nothing unsafe about food cooked in a microwave oven. You can eat it the instant the microwave oven turns off. The microwaves in the oven are absorbed so quickly that they vanish almost immediately after the oven stops producing them. By the time you get the oven door open, there is nothing hazardous left inside the cooking chamber or in the food. However, a microwave oven tends to heat foods unevenly, particularly if they were initially frozen. Thus you should be careful to stir the food or test its temperature at various places so that you don't burn yourself. You should be particularly wary of solid foods, such as raisin biscuits, that are generally dry but have moist, microwave-absorbing objects inside them. Those moist objects can become dangerously hot and have been known to cause life-threatening burns in people who tried to swallow them without letting them cool off.

That said, a reader notes that the uneven cooking in a microwave oven can lead to bacterial safety problems—if parts of the food aren't heated sufficiently to kill dangerous bacteria, then you could be exposing yourself to those bacteria. He suggests using the microwave oven for reheating only. He also notes that the lack of surface heating leaves the food relatively tasteless, as compared to more conventional cooking.

1169. What about the effects of microwaves on the cellular structure of the item in the oven? I've heard that cells are ruptured violently by microwave radiation and that the ingestion of such materials affects the immune system. - AB
Just about any cooking damages the cells of the food being cooked, so microwave cooking is nothing unusual. Since our digestive systems destroy cells in the food we eat, cellular damage in cooking is inconsequential. As for the rumors about the unhealthiness of food cooked in a microwave oven, these are simply myths promulgated by people who don't understand what microwaves are and fear them irrationally. The world was awash in microwaves from natural sources long before the developments of electricity and microwave ovens.

1170. How do you determine the critical mass of a particular radioactive element or isotope? - F, United Kingdom
This questions asks how you can predict the amount of a fissionable nuclear fuel you must assemble in order for that fuel to experience self-sustaining nuclear fission chain reactions. A self-sustaining nuclear chain reaction can only occur when each fission within that material causes an average of one subsequent fission. The size, shape, and density of the nuclear fuel are important to the chain reaction because they determine how much opportunity fragments from one fission event will have at inducing subsequent events elsewhere within the fuel. A properly shaped piece of fuel that is just large enough and dense enough to experience a self-sustaining nuclear chain reaction is said to be at critical mass. Below the critical mass, the chain reaction won't be able to sustain itself and will gradually dwindle away. Above the critical mass, the chain reaction will grow stronger exponentially. Since crossing the threshold from below critical mass to above critical mass has dramatic consequences, it can be quite important to know the point at which it occurs.

The basic calculation of critical mass is straightforward in principle, but it requires a thorough understanding of the nuclear fuel. Because you need to know how likely one nuclear fission is to cause a subsequent nuclear fission, you must know both the types of fragments you can expect from the first nuclear fission and the likelihood that each fragment will induce a subsequent fission in another atomic nucleus before that it escapes from the nuclear fuel. Because the range of possible fragments, their kinetic energies, and their paths through the nuclear fuel are so vast, an accurate calculation of critical mass is extremely complicated. As an indication of the difficulty, note that fission fragments may bounce off nuclei without inducing fission, so that you must consider bent paths as well as straight ones. Not surprisingly, the calculation of critical mass is too difficult to do exactly, even with the help of computers. In fact, one of the reasons that Germany didn't develop nuclear weapons during World War II was that its scientists miscalculated the critical mass of a fission bomb based on enriched uranium and thought that they would need many tons of enriched uranium rather than the true critical mass of about 52 kilograms. Certain that a critical mass of enriched uranium was unattainable, they didn't pursue the project.
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