. 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.
. If living organisms maintain their order by exporting disorder to their environment, do they create more disorder than the order they maintain? — CC
Living organisms create more disorder in their surroundings than they create order in themselves. Overall the disorder of the combined system—organisms and environment—increases. This result is an unavoidable consequence of the second law of thermodynamics, which notes that the entropy (disorder) of an isolated system can never decrease. While it is possible in principle for a living organism to export disorder so efficiently that the overall disorder remains unchanged, that perfection is never achieved. Instead, living organisms export far more disorder than is required for them to maintain order in themselves. As a result, living organisms are net producers of disorder.
In that respect, people are much more vigorous producers of disorder than most other living organisms. People seek order not only in their bodies, but also in the objects around them and they achieve this ordering by consuming order in their environment—fossil fuels, minerals, pure water—at a furious pace and producing disorder in its place—burned gases, garbage, polluted water. Fortunately, sunlight is a tremendous source of order for our earth and it undoes some of the disordering caused by living organisms. However, we are consuming much of the order that sunlight stored on earth over millions of years in only a few generations. At this pace, we're destined to have troubles with the disorder we're creating. Many of the environmental issues that face us today can be viewed from this order/disorder perspective: we have to learn how to create less disorder.
. What happens to ice when it is left in the freezer—does it evaporate? I have noticed that over time the ice cubes shrink? — J & K
When you leave ice in a frostless refrigerator, it gradually sublimes and shrinks away to nothing. Sublimation is equivalent to evaporation, except it involves a solid converting directly into a gas. The surface of an ice cube is a busy place, with water molecules landing and taking off all the time. If more water molecules land than leave, the ice cube will grow in size. If more water molecules leave than land, the ice cube will shrink. The water molecule landing rate is determined by how much moisture there is in the air. In a frostless refrigerator, the air is extremely dry, meaning that it contains very few water molecules. Thus the landing rate in a frostless refrigerator is very low and the ice cubes shrink. If you watch the ice cubes in an older style refrigerator, you will find that they grow over time because the air in that refrigerator is moist and the landing rate is high. Incidentally, this sublimation of water molecules from ice is why snow disappears gradually even when the weather remains cold and is also how freeze drying of food is done.
. How does a single lens reflex camera work?
When rays of light from a distant object reach the camera's lens, those rays are spreading apart or "diverging." You can understand this by following the rays of light from one spot on the object, say the tip of a person's nose. The rays of light reflected from the nose spread outward in all directions and only a small portion of them passes into the camera's lens. These light rays are diverging from one another as they travel.
The camera's lens is a converging lens, meaning that it bends the paths of these light rays so that they diverge less after passing through it. In fact, the lens bends the rays so much that they begin to come together or "converge" after the lens and all the rays of light from the person's nose merge to a single point in space somewhere beyond the lens. Exactly how far from the lens the rays come together depends on the structure of the lens and on the distance between it and the person's nose. When you focus the lens, you're moving the lens so that the rays come together at just the right place to illuminate a single spot on a piece of photographic film. When the distance between the lens and film is just right, all the light from each point on the person comes together at a corresponding point on the film. The lens is then forming a real image of the person on the film and the film records this pattern of light to make a photograph.
In a single lens reflex camera, light passing through the lens doesn't always fall on the film. Most of the time, this light is redirected by a mirror that follows the lens so that the real image forms on a special glass sheet near the top of the camera. When you look through the viewfinder of the camera, you are actually using a magnifying glass to inspecting this real image, making the camera effectively a telescope. You (or the camera, if it is automatic) then focus the lens to form a sharp real image on the glass sheet before taking the picture. Since this glass sheet is the same optical distance from the lens as the film is, focusing on the glass is equivalent to focusing on the film. When you take the picture, the redirecting mirror quickly flips out of the way and a shutter opens to allow light from the lens to fall directly onto the camera's photographic film. For a brief moment, light from the person passes through the lens and onto the film, forming a real image that is permanently recorded on the film. Then the shutter closes and the mirror swings back to its normal position.
. What are positive and negative g's?
Let me start with the concept of inertia. Like all objects in this universe, we naturally tend to keep doing what we're doing—if we are stationary, we tend to remain stationary, and if we are moving, we tend to keep moving in a straight line at a steady pace. In fact, the only way that your speed and/or direction of travel (in short, your velocity) can change is if something pushes on you. When that happens, you accelerate (which is to say your velocity changes).
Whenever you accelerate, the various parts of your body can no longer follow their inertia; they must accelerate, too. This acceleration requires forces within your body and you can feel these forces. In fact, they make it feel as though a new type of gravity were acting on the parts of your body. You can't distinguish true gravity from the experience of acceleration because they feel exactly the same. The strength of this gravity-like experience depends on how fast you accelerate and it points in the direction opposite your acceleration. If you accelerate upward, as you do when an elevator first starts moving upward, this gravity-like sensation points downward and you feel extra heavy (the experience of "positive g's") If you accelerate downward, as you do when a rising elevator comes to a stop, this gravity-like sensation points upward and you feel unusually light (the experience of "negative g's") Since there is no fundamental limit to how rapidly one can accelerate, these positive and negative g's can become extremely strong and can easily feel stronger than the true force of gravity. However, when these gravity-like sensations become a few times stronger than gravity itself, they become difficult to tolerate. That's why elevators start and stop gradually and why the turns on roller coasters aren't too sharp.
. How are incandescent light bulbs made? — SU
The glass enclosures are made from a ribbon of hot glass that's first thickened and then blown into molds to form the bulb shapes. These enclosures are then cooled, cut from the ribbon, and their insides are coated with the diffusing material that gives the finished bulb its soft white appearance.
The filament is formed by drawing tungsten metal into a very fine wire. This wire, typically only 42 microns (0.0017 inches) in diameter is first wound into a coil and then this coil is itself wound into a coil. The mandrels used in these two coiling processes are trapped in the coils and must be dissolved away with acids after the filament has been annealed.
The finished filament is clamped or welded to the power leads, which have already been embedded in a glass supporting structure. This glass support is inserted into a bulb and the two glass parts are fused together. A tube in the glass support allows the manufacturer to pump the air out of the bulb and then reintroduce various inert gases. When virtually all of the oxygen has been eliminated from the bulb, the tube is cut off and the opening is sealed. Once the base of the bulb has been attached, the bulb is ready for use.
. Why are sparks generated when iron is brought in contact with a spinning grinding wheel? — JF, Rochester, NY
When the iron touches the spinning wheel, the two experience sliding or "dynamic" friction—the iron acts to slow the wheel while the wheel acts to move the iron. Because you hold the iron in place, it doesn't move but its surface begins to experience severe wear—the iron is skidding across the surface of the wheel. Sharp projections from the wheel are tearing particles away from the iron and throwing them in the direction of the wheel surface's motion. Because the two surfaces, iron and wheel, are pushing on one another and they are moving relative to one another in the directions of their forces, they are doing physical work on one another—meaning that they are exchanging energy. This energy is actually being converted from the wheel's rotational energy into thermal energy in the iron and in the wheel, both of which become hot. You can feel similar heating by rubbing you hands against one another vigorously. The wheel's surface begins to glow red-hot and the particles that fly off the iron emerge so hot that they burn in the air. The sparks you see are the iron particles burning up. Depending on what type of iron or steel you use, you'll see different spark patterns. An expert can actually identify an alloy by this pattern.
. Can a photocell ever absorb too much light? If it can, does it explode or simply stop absorbing light? — MR
Since not all of the light power absorbed by a photocell is converted into electric power, a photocell that's exposed to too much light will overheat. High temperatures are disastrous for all semiconductor devices, including computer chips and photocells. If a semiconductor device overheats slightly, the excessive thermal energy will change the electronic properties of the semiconductor layers so that these layers won't behave as they were chemically prepared to do. In an overheated photocell, charge will be allowed to flow backward so that the photocell will become less energy efficient. But if a semiconductor device overheats seriously, the semiconductor layers will change permanently—atoms, molecules, and entire structures will migrate and rearrange, and the device will never work properly again.
By itself, an overheated photocell won't fail dramatically; it will just stop working. If you've overheated it severely, it will remain broken from then on. But if the photocell is part of a larger collection of power generating elements that continues to produce power, that photocell may suddenly consume all of the power from the other elements. In that case, the photocell may explode as its temperature skyrockets.
. How does a photocell absorb light and turn it into power? — MR
A photocell is actually a large diode—a one-way device for electric current. Like most diodes, the photocell consists of two different layers of chemically altered or "doped" semiconductors, the anode layer and the cathode layer, and the junction between these two layers has the peculiar property that it normally allows electrons to cross it in only one direction. There is what's called a "depletion region" at the junction, a very thin insulating layer with two electrically charged surfaces—the surface on the cathode side is positively charged and the surface on the anode side is negatively charged.
When an electron, which is negatively charged, approaches the depletion region from the anode side, it first encounters the depletion region's negatively charged surface and is repelled. But when the electron approaches from the cathode side, it first encounters the depletion region's positively charged surface and is attracted. If it has enough energy when it approaches the depletion region from the cathode side, the electron can cross the depletion region to reach the anode layer. Thus electrons can move relatively easily from the photocell's cathode layer to its anode layer but they can't go back.
When a photocell is exposed to light, some of the light particles (photons) are absorbed in the diode's cathode layer. When such an absorption occurs, the photon's energy may be transferred to an electron in the cathode, giving that electron the energy it needs to cross the depletion region and reach the anode. But once the electron has arrived at the anode it can't return to the cathode directly across the depletion region. Instead, it must flow through an external circuit in order to return to the cathode. As that electron flows through the external circuit, it can give up some of its energy, obtained from the light photon, to devices in that circuit. In that manner, light energy has provided energy to an electrically powered device.
. If I measure current from a photocell, am I indirectly measuring power as well? — MR
As long as current is free to flow from one end of the photocell to the other, the amount of current flowing through that circuit is almost exactly proportional to the number of light particles (photons) striking the photocell each second. Since the rate at which photons strike a photocell is generally proportional to the light power striking that photocell, you can use a measurement of current to make a measurement of light power. While there are a few subtle details that you must be careful about, particularly changes in the light spectrum and unanticipated impediments to the free flow of current through the circuit, this relationship between the current and the light power is very useful. For example, most camera light meters use photocells to determine exposures.