Month: May 1997

How is the high explosive used in a fission bomb detonated so precisely together…

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How is the high explosive used in a fission bomb detonated so precisely together? – F, United Kingdom

Most modern nuclear weapons produce a super-critical mass of fissionable nuclear fuel by crushing a sphere of that material with high explosives. As the material’s size shrinks, its density increases and it passes rapidly through critical mass to achieve a highly super-critical mass. Nuclear chain reactions then grow exponentially in the material and huge amounts of energy are released. However, the process of crushing a solid sphere of metal to several times its normal density requires sophisticated high explosives triggered at precisely the right moments. The triggering is done with very high-speed electronic devices and explosive detonators that respond almost instantly to high voltage pulses. Perhaps the most critical components in this system are high speed, high voltage switches known as krytron tubes. Because these devices have limited uses outside of nuclear weapons, their export is tightly controlled and it’s a big news story whenever someone is caught trying to smuggle them outside the United States.

How do you determine the critical mass of a particular radioactive element or is…

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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.

What about the effects of microwaves on the cellular structure of the item in th…

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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.

Is there a standard time that one should wait before eating food that has been h…

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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.

You stated that thermodynamics overwhelms just about everything sooner or later….

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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.

I understand that an ear thermometer measures a person’s temperature by studying…

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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.

How does an ear thermometer work so quickly?

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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.

What are the relative efficiencies of the fission and fusion reactions in thermo…

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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.

Is there any easy way to mold plastics?

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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.