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

Lou Bloomfield Leave a comment

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?

Lou Bloomfield Leave a comment

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.

What function does the Degauss button actually perform on computer monitors and …

Lou Bloomfield Leave a comment

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.

What do engineers have to consider about waves when they are building bridges?

Lou Bloomfield Leave a comment

What do engineers have to consider about waves when they are building bridges? — K

There are two answers to this question because there are two possible interpretations of the word “waves.” If you mean waves in the water beneath the bridge, then naturally the engineers must plan for the forces exerted on the bridge by the moving water that flows around its surfaces. But a more interesting wave issue is waves in the bridge itself. The bridge’s surface can experience waves, just as a taut rope or a long beam can have waves running through it. For example, when a heavy object drops on the surface of the bridge, a ripple heads outward along the bridge surface and doesn’t stop completely until it reaches the ends of the bridge. In fact, the wave will reflect from various portions of the bridge and its effects may not disappear for many seconds after the incident that started the waves.

Most of the time, these waves aren’t important and can be ignored. But occasionally some special event will cause enormous waves to begin traveling through a bridge. The classic example was the Tacoma Narrows Bridge in Washington State that collapsed in 1940 when wind-driven waves in its surface ripped it apart. The entire collapse was captured on film and is a fascinating to watch. When a large group of soldiers crosses a footbridge, they are often instructed to break step so that their rhythmic cadence doesn’t excite intense waves that might damage the bridge. In general, modern bridges are engineered to dampen these waves—wasting their energy through friction or friction-like effects so that they die away quickly. While it might be fun to watch waves traveling along the surface of a bridge from a safe vantage point, you probably wouldn’t want to be on a bridge when it was experiencing strong ones.

If you were out in space and could see every individual person clearly, would it…

Lou Bloomfield Leave a comment

If you were out in space and could see every individual person clearly, would it look like they were walking at a slant? — KD, McMinnville, OR

To the astronauts orbiting the earth, up and down have very little meaning. Because they are falling all the time, these astronauts have no feeling of weight and can’t tell up from down without looking. If an astronaut were to look at a person walking on the ground below, that person might easily appear at a strange angle, depending on the astronaut’s orientation and point of view.

Why do you hear different music coming from a compact disc when the laser of the…

Lou Bloomfield Leave a comment

Why do you hear different music coming from a compact disc when the laser of the CD player is just going around the same part of the CD over and over again? — KD, McMinnville, OR

The CD player’s laser doesn’t really go over the same part of the CD over and over again. As the disc turns, the laser slowly moves outward from the middle of the disc toward its edge. The laser beam is focused to an extremely small spot inside the disc and it is carefully following a tight spiral ridge in the aluminum layer inside. This ridge runs continuously from the center of the disc to its edge. With each revolution of the disc, the laser works its way outward by one more turn of the spiral. The ridge has interruptions in it every so often and it is this pattern of interruptions that contains the information needed to reproduce sound.

How does a parabolic microphone work?

Lou Bloomfield Leave a comment

How does a parabolic microphone work? — KL, Regina, Saskatchewan

A parabolic microphone is effectively a mirror telescope for sound. When sound waves strike the dense, rigid surface of the parabolic dish, they partially reflect. This reflection occurs because sound travels much faster in a rigid solid than in the air and changes in the speed of a wave cause part of it to reflect. In this case, the reflection redirects the sound waves inward because the reflecting surface is curved and the sound waves form a real image of the distant source that produced them. While you can’t see this real image with your eyes, you can hear it with your ears. If you were to mount a large parabolic dish so that it faced horizontally and then moved your ear around in the focal plane of the dish, you would hear sounds coming from various objects far away from the dish. The same effect occurs for light when it bounces off a curved mirror—a real mirror telescope. A TV satellite dish is the same thing, but this time for microwaves! In all three cases, the real images that form are upside down. To make a parabolic microphone, you normally put a conventional microphone in the central focus of a parabolic surface so that the microphone receives all the sound coming from objects directly in front of the parabola. To listen to different objects, you simply steer the parabola from one to the other. This is exactly what a TV satellite dish does when it wants to “listen” to a different satellite—it steers from one to the other.

I heard of a laser induced fluorescence instrument that is used in aiding cancer…

Lou Bloomfield Leave a comment

I heard of a laser induced fluorescence instrument that is used in aiding cancer diagnosis. Could you tell me how this instrument works?

You are probably referring to a device developed at the BC Cancer Research Center in Vancouver, British Columbia and now available commercially from Xillix Technologies. A scientist from that research center gave me the following description of their technique.

The instrument is based on the discovery that most tissues when illuminated by blue or UV light emit a natural fluorescence spectral signature known as autofluorescence. This fluorescence signature is the sum of the emission of the various biochemical fluorphores present in the tissue. If the tissue chemical or physical structure changes, then the spectral signature changes. By exploiting differences in the spectral signature between cancerous and healthy tissue one can create an imaging device that can “see” the difference in the color of the autofluorescence of the tissue and detect changes that may indicate the presence of cancer. The sensors used to see the low levels of fluorescence light employ similar technology to military night vision devices. Once areas of change are located and confirmed by analysis of a biopsy sample treatment can begin. This technique is primarily useful for early stage cancers that are not visually apparent to a physician.

How can I build an AM radio?

Lou Bloomfield Leave a comment

How can I build an AM radio?

That’s a very open ended question so I’ll describe the simplest AM radio I can think of—a crystal radio. A crystal radio already addresses most of the issues of AM radio and more sophisticated AM radios just improve on its performance.

You need only four basic components for a crystal radio: an antenna, a tank circuit, a diode, and a high-impedance earphone.

The antenna is a long wire that projects upward into the electromagnetic fields of the passing radio wave so that electric charges begin to move up and down its length. The ideal length for this wire is a quarter of the wavelength of the wave you’re trying to receive, but since that’s hundreds of meters for a typical AM station, you’ll have to settle for a shorter than ideal antenna.

The tank circuit is a coil of wire that’s connected at each end to the two ends of a capacitor. In a typical crystal radio, one of these items—either the coil or the capacitor—is adjustable and forms the tuning element that allows you to select a particular AM station. The tank circuit is a resonant device—electric charges and current flow back and forth through it rhythmically at a specific frequency. If that resonant frequency is adjusted so that it coincides with the transmission frequency of an AM radio station, the small currents flowing in the antenna that’s connected to the tank circuit will excite large movements of charge and current in the tank circuit.

The diode is also connected to the tank circuit. Its job is to extract some of the charge that oscillates back and forth in the tank circuit and to send that charge to the earphone. By allowing current to flow only in one direction, the diode samples the overall amount of charge moving in the tank circuit. What it passes to the earphone is a measure of how strong the radio wave is, which is actually the form in which the AM radio station is transmitting sound information.

The high-impedance earphone uses the diode’s tiny charge deliveries to reproduce sound. The diaphragm inside the earphone moves back and forth as the amount of charge passing through the diode fluctuates up and down. Each time the radio wave increases in strength, the diaphragm moves in one direction. Each time the radio wave decreases in strength, the diaphragm moves in the other direction. Thus as the radio station varies the strength of its radio wave, the earphone’s diaphragm moves back and forth and it reproduces the sound.

I heard on a news report that there is a paint that will generate heat from a 12…

Lou Bloomfield Leave a comment

I heard on a news report that there is a paint that will generate heat from a 12-volt battery. What can you tell me about this subject? — JF

Generating heat from a battery is relatively easy. All you need is a material that conducts electricity only moderately well and you’re in business. If you allow current to flow through that material from the battery’s positive terminal to its negative terminal, the current will lose energy as it struggles to get through the material and the current’s lost energy will become thermal energy in the material. The only difficult part of this task is in choosing the right material so that it doesn’t produce too much or too little heat. In short, the electric resistance of the finished material has to be in the right range. For a solid system that you can cut and tailor, that’s not much of a problem. But for a paint, it could be tricky. To make an inexpensive paint, it would probably need to use carbon powder as the electric conductor. A thin layer of carbon granules held in place by a plastic of some sort would probably provide a suitable conducting surface that would become warm when you allowed current to flow through it from a battery. There are copper and silver conducting paints that might also work, but these are rather expensive and I’m not sure how they behave at elevated temperatures.