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Page 53 of 160 (1595 Questions and Answers)

521. Why do people put salt on icy sidewalks in the winter?
Whenever a molecule dissolves in water, the water molecules bind to that molecule and surround it, forming a shell of water molecules around the impurity. Salt water is filled with these tiny balls of water, each one surrounding a single salt ion (either a sodium positive ion or a chlorine negative ion). These little water balls can't crystallize into ice because ice can't fit a sodium ion or a chlorine ion into its orderly structure. As a result, the presence of salt in the water makes it harder for the water to crystallize into ice. The water has to exclude the salt from the crystals that form as it freezes and this difficult process requires that the salt water be cooled below the freezing temperature of pure water before it will freeze. The more salt the water contains, the lower the temperature at which that salt water will freeze. This effect even works when you just sprinkle salt on ice. As long as the temperature of the ice isn't too cold, the salt will begin to dissolve in the water molecules of the ice and ice's crystalline structure will begin to break down. The result will be a puddle of cold salty water. That's why people use salt to melt the ice on sidewalks. But if the ice is too cold, the salt will remain separate and the ice will stay pure ice. That's why salting only works when the temperature isn't too far below freezing.

522. How does Styrofoam work?
Styrofoam is a rigid foam consisting of gas trapped in the closed bubbles of polystyrene. Polystyrene itself is a clear plastic that's used in many disposable food containers. It's a stiff, amorphous solid at temperatures below 100° C, where amorphous means that it has none of the long-range order associated with crystalline solids. The long, chain-like polystyrene molecules are arranged like a tangled bowl of spaghetti noodles. Amorphous plastics tend to be clear because they're very homogeneous (uniform) internally and let light passes through them without being deflected or reflected. Plastics that are partially crystalline tend to be white. I think that items bearing the #5 recycling label are made of polystyrene.

But when air or another gas is injected into melted polystyrene and the mixture is beaten to a froth, it forms a stiff white solid when it cools. The whiteness comes about because of inhomogenieties—the gas spoils the uniformity of the plastic so that light is deflected and reflected as it passes through the material. The Styrofoam retains the rigidity of the polystyrene plastic below 100° C, so that it's suitable for beverage containers for liquids that are no hotter than boiling water. At one time, one of the gases used to make polystyrene foams was Freon, but I believe that Freon is no longer used for this purpose.

523. Who invented the microwave oven and how did he think of it?
In 1945, American engineer Percy Le Baron Spencer was working with radar equipment at Raytheon and noticed that some candy he had in his pocket had melted. Radar equipment detects objects by bouncing microwaves from them and Spencer realized that it was these microwaves that had heated the candy (as well as his body...oops!). Raytheon soon realized the potential of Spencer's discovery and began to produce the first microwave ovens: Radaranges. These early devices were large and expensive and it wasn't until 1967, when Amana, a subsidiary of Raytheon, produced the first household microwave oven, that microwave ovens became widely available.

524. Does light have mass? If so, then how can it travel at the speed of light? Doesn't the mass of an object (particle) approach infinity as its velocity approaches the speed of light?
Light has precisely zero mass and that makes all the difference. You're right that taking a massive particle up to the speed of light is impossible because doing so would, in a certain sense, give the particle an infinite mass. But the more important issue here is that doing so would require an infinite amount of energy and momentum.

Most physicists use the word mass to mean a particle's mass at rest—its rest mass—and as you bring the particle to higher and higher speeds, its rest mass doesn't change. However, the relationship between the particle's energy and its momentum does change with speed and the particle's momentum begins to increase more rapidly than it should according to the older, pre-relativistic mechanical theories. In an effort to explain this anomalous increase in momentum while retaining the old Newtonian laws of motion, people sometimes assign a fictitious "mass" to the particle; one that equals the rest mass when the particle is stationary but that increases as the particle's speed increases. As a particle approaches the speed of light, its momentum increases without limit and so does its "mass." Not surprisingly, the limitless rises in energy, momentum, and "mass" prevent the massive particle from ever reaching the speed of light.

As for light, it really does have zero mass and therefore can't be described by the Newtonian laws of motion. All light has is its momentum and its energy. In fact, light can't travel slower than the speed of light because that would require it to have a mass! So the world of particles is divided into two groups: massless particles that must travel at the speed of light and massive particles that can never travel at the speed of light.

525. Can the light from a fluorescent lamp be collimated into a beam of parallel rays?
While a converging lens or a concave mirror can always direct light from a bright source in a particular direction, the degree of collimation (the extent to which the rays become parallel) depends on how large the light source is. The smaller the light source, the better the collimation. Spotlights and movie projects use extremely bright, very small light sources to create their highly collimated beams. Since fluorescent lamps tend to be rather large and have modest surface brightnesses, I'm afraid that you would be disappointed with the best beam that you could create from that light. The ultimate collimated light source is a laser beam. In effect, the identical photons of light in a laser beam all originate from the same point in space, so that the collimated beam is as close to perfectly collimated as the nature of light waves will allow.

526. What is the difference between the magnetic and electric ballasts used in fluorescent lights?
Fluorescent lights work by sending an electric current through a vapor of mercury atoms in what is known as an electric discharge. Unfortunately, electric discharges are very unstable—they are hard to start and, once started, tend to draw more and more current until they overheat and damage their containers and power sources. Thus a fluorescent light needs some device to control the flow of current through its discharge. Since normal fluorescent lamps are powered by alternating current—that is, the current passing through the discharge stops briefly and then reverses direction 120 times each second in the United States and 100 times each second in many other countries (60 or 50 full cycles of reversal, over and back, each second respectively)—the current control device only needs to keep the current under control for about 1/120 of a second. After that the current will reverse and everything will start over.

Older style fluorescent lights use a magnetic ballast to control the current. This ballast consists essentially of a coil of wire around a core of iron. As current flows through the wire, it magnetizes the iron. Because energy is required to magnetize the iron, the presence of the iron inside the coil of wire slows down the current when it first appears in the wire by drawing energy out of that current. This effect, typical of devices known to scientists and engineers as "inductors", prevents the current passing through the ballast and then through the discharge from increasing too rapidly once it starts. The magnetic ballast is able to slow the current rise through the fluorescent lamp long enough for the alternating current to begin reversing directions. In fact, as the current in the power line begins to reverse, the ballast begins to get rid of the energy stored in its magnetized core. This energy is used to keep the discharge going longer than it would on its own. The ballast thus smoothes out the discharge so that it stays under control and emits an almost steady amount of light.

Modern electronic ballasts still control the current through the discharge, but they use electronic components to achieve this control. Just as an electronic dimmer switch can control the current through an incandescent light bulb in order to adjust the bulb's brightness, such electronic devices can control the current passing through the discharge in a fluorescent lamp to keep that current from growing dangerously large.

527. Why are there dimples on golf balls? - DM
If there were no turbulence around a golf ball as it moved through the air, there would be regions of slow-moving high-pressure air in front of it and behind it, and regions of fast-moving low-pressure air around its sides. Because of their symmetry, these pressures wouldn't exert any overall force on the golf ball and it would fly through the air without experiencing any air resistance. But there is turbulence behind a moving golf ball and this turbulence spoils the high-pressure region behind the ball. Since there is less high-pressure behind the golf ball to push it forward, the ball experiences a backward force—the slowing force of pressure drag. The size of this pressure drag force is roughly proportional to the size of the turbulent wake.

The size of the turbulent wake depends on the airflow behind the ball. On a smooth ball, air flowing into the rising pressure behind the ball experiences friction with the ball's surface and loses energy. This surface air soon reverses its direction of flow, triggering a large turbulent wake. A golf ball's dimples complicate the airflow very near the ball's surface so that new, rapidly moving air is able to flow in close to the ball's rear surface, where it can delay the onset of the flow reversal. The turbulent wake that eventually forms is relatively small, so that the golf ball experiences less pressure drag than a smooth ball. That's why a golf ball can travel so far before slowing down.

528. I've heard two explanations for how flight is achieved: (A) through lift generated by differential pressure (Bernoulli's effect) and (B) through elastic collisions between air molecules and the underside of a wing. Which is correct? How does the fact that planes can fly upside down enter into this picture?
Explanation A is entirely correct and explanation B is partly correct. If you extended explanation B to include all collisions between air molecules and the entire wing, then it would also be correct. Explanation A is the continuous fluid picture of flight and the revised explanation B is the granular fluid picture of flight. To the extent that gases are incompressible fluids (as required for Bernoulli's equation to be completely valid), these two explanations are essentially equivalent.

The lift experienced by a plane's wing depends on its shape and on its tilt or "angle of attack" into the wind. In general, wings are airfoils—curved shapes that are designed to obtain significant lift forces while experiencing minimal drag forces. Most airplane wings are more highly curved on their tops than on their bottoms and obtain upward lift forces as a result. These lift forces occur because the stable airflow that forms around such a wing involves faster-moving and thus lower-pressure air above the wing than beneath it. However, some airplane wings are symmetric—they have equal curvatures on top and on bottom. These symmetric wings compensate for their symmetry by attacking the air at an angle. When they are tipped so that their leading edges are higher than their trailing edges, these wings also experience upward lift forces. The air again flows more rapidly over than under the wings and the pressure is lower above the wings than beneath them. Even an inverted non-symmetric wing can adjust its angle of attack to obtain an upward lift force, which is how a plane can fly upside down.

In all of these cases, the forces are really exerted on the plane's wings by the impacts of countless air molecules. These air molecules hit harder and more often beneath the wings than above them and thus exert a net upward force on the plane. The fact that some wings have more surface area on their highly curved tops doesn't lead to larger downward forces because many of the collision forces exerted by molecules on the top surface of the wing cancel one another, in the same way that forces exerted on opposite sides of a sheet of paper cancel one another.

529. How does a three-way light bulb work? - AER
A three-way light bulb has two filaments inside it. One filament is smaller than the other, consuming less electricity and emitting less light. At the low light setting, only the smaller filament has current running through it and the bulb emits a dim light. At the medium light setting, only the larger filament has current running through it and the bulb emits a medium light. At the high light setting, both filaments have currents running through them and the bulb emits a bright light. To control the two filaments, the bulb has three electrical connections. The two filaments share one of the connection and each has one additional connection of its own. A complicated switch in the lamp determines whether to deliver current to one filament or the other or both. In each case, current flows toward the filament through one connection and returns from the filament through the other connection.

530. How do glaciers flow downhill? — LO and NB, Bothell, WA
Ice is a rather soft material and its crystals can deform permanently when exposed to sufficient stress. If you squeeze ice hard enough, its crystals will gradually change shape in much the same way that a copper penny will change shape if you squeeze it in a press. Since the pressures at the base of a glacier are enormous, the ice crystals there gradually deform to relieve the stress they're experiencing. This slow deformation allows the whole structure of the glacier to move gradually downhill. If ice crystals were harder, like those in most rocks, glaciers wouldn't flow. But they are very soft and so the glacier slowly flows downhill.
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