Month: October 1996

What is the difference between the magnetic and electric ballasts used in fluore…

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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.

Can the light from a fluorescent lamp be collimated into a beam of parallel rays…

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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.

Does light have mass? If so, then how can it travel at the speed of light? Doesn…

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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.

Who invented the microwave oven and how did he think of it?

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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.

How does Styrofoam work?

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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.

Why do people put salt on icy sidewalks in the winter?

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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.

What is the principle behind adding salt to water to keep the boiling temperatur…

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What is the principle behind adding salt to water to keep the boiling temperature lower? Do other substances have the same effect?

Actually, it’s the other way around! Adding salt or sugar (or anything else that dissolves in water and that doesn’t boil easily itself) to water actually raises the water’s boiling temperature! That’s because the salt or sugar molecules interfere with the evaporation of water molecules and boiling is just a special type of evaporation.

Boiling occurs when the evaporation of water molecules becomes so rapid that bubbles of evaporating water molecules form inside the body of the water itself and are able to grow larger and larger, despite the crushing pressure of the surrounding atmosphere. Below water’s boiling temperature, any bubble of water vapor that forms inside the body of the water will be smashed almost instantly. But at water’s boiling temperature, the pressure of water vapor inside each bubble is high enough to keep the bubble from being crushed. However, adding sugar or salt to the water makes it harder for water molecules to enter one of these water vapor bubbles because the water molecules in the water cling to the salt or sugar molecules and thus don’t evaporate as often. With fewer water molecules entering a water vapor bubble, that bubble can’t sustain itself and is crushed. Only when you heat the salty or sugary water above the boiling temperature of pure water is there enough evaporation into each water vapor bubble to support it against atmospheric pressure.

How do helicopters fly with such small wings without them breaking off?

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How do helicopters fly with such small wings without them breaking off?

As you suggest, the blades of a helicopter are really rotating wings. But unlike the wings of a normal airplane, the helicopter blades are always moving through the air, even when the helicopter’s body is not. That’s why a helicopter can obtain an upward “lift” force from the air while it’s hovering motionless—the wings keep moving and obtaining that lift force. A second difference between a helicopter’s rotating blades and the wings of a normal aircraft is that a helicopter’s blades are under enormous tension. Were it not for this tension, the end of each blade would naturally travel in a straight line at constant speed, a behavior that we associate with inertia—objects that are free of outside forces travel at constant velocity (they follow straightline paths at constant speeds). To make the end of its blade travel in a circle (which is certainly not a straight line), the helicopter must pull the end of the blade toward the pivot about which the blade is turning. Thus as the blades turn, each blade experiences an enormous tension pulls the parts of the blade toward the pivot. This tension is what stiffens the blade, just as tension stiffens the strings of a guitar or a violin. Just as it’s hard to break a guitar string by bending it, it’s hard to break a helicopter blade by bending it. However, both guitar strings and helicopter blades will snap if they’re exposed to more tension than they can tolerate. The manufacturers of the blades work hard to make each blade strong enough to withstand the enormous tension it experiences in use. As long as the blades can tolerate this tension, they won’t break and will have no trouble supporting the body of the helicopter.

How do light emitting diodes work and what is responsible for their different co…

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How do light emitting diodes work and what is responsible for their different colors?

Light emitting diodes are diodes that have been specially designed to emit light rather than heat during their operations. Whenever current is flowing through a diode, electrons are moving from the n-type semiconductor on one side of the diode’s p-n junction to the p-type semiconductor on the other side of the junction. Once an electron (which is negatively charged) arrives in the p-type semiconductor, it’s attracted toward an electron hole (which is positively charged) and the two move together. The electron soon fills the hole and it releases a small amount of energy when it does. In a normal diode, electrons lose energy at a rate of 0.6 joules of energy per coulomb of charge as they recombine with the electron holes. That means that the current flowing through the normal diode loses 0.6 volts as it flows through the diode. The missing energy becomes thermal energy or heat.

But in a light emitting diode (an LED), each electron that arrives in the p-type semiconductor after crossing the p-n junction recombines with an electron hole in a remarkable way. It gives up its extra energy as light! Each time an electron and an electron hole recombine, they emit one particle of light, a photon, and the frequency, wavelength, and color of that light depends on the amount of energy given up by the electron as it falls into the electron hole. The semiconductor material from which an LED is made has a characteristic called its band gap. This band gap measures the energy needed to pull an electron away from an electron hole in the material. If this band gap is small, the LED will emit infrared light. If this band gap is larger, the LED will emit red, orange, yellow, green, or even blue light (the farther to the right in that list, the more energy is required). Because each electron loses more energy in recombining with an electron hole in an LED than it would in a normal diode, the current flowing through an LED loses more voltage (typically 2 volts for red LEDs and as much as 4 volts for blue LEDs) than does the current flowing through a regular diode (typically 0.6 volts).

Physicists, chemists, materials scientists, and engineers have been working for years to perfect the materials used in LEDs, making them more and more efficient at turning the electrons’ energies into light. Until recently, there were no suitable materials from which to build blue LEDs, but recent developments of large band gap semiconductors have made blue LEDs possible. In fact, even blue laser diodes are now being made. A laser diode is a specially designed LED in which all of the photons are copies of one another rather than being emitted independently by the individual electrons as they drop into their respective electron holes.

One final note: it’s now possible to obtain a “white” LED! This device is actually a blue LED, combined with a fluorescent phosphor that converts the blue light into white light.

How are some light emitting diodes able to emit more than one color? Can light e…

Lou Bloomfield Leave a comment

How are some light emitting diodes able to emit more than one color? Can light emitting diodes emit different amounts of light or can they only be on or off?

Light emitting diodes (LEDs) that emit more than one color are actually two different LEDs connected to a single circuit in opposite directions. When current flows in one direction around that circuit, one of the LEDs emits light. When the current reverses directions, the other LED emits light. And when the current reverses directions rapidly, both LEDs emit light alternately. If one LED emits red light and the other green light, then the overall device will appear yellow or orange when they are both operating alternately in rapid sequence. The amount of light that an LED emits depends on the current flowing through it—the more electrons that are falling into holes in the p-type semiconductor, the more light that’s being emitted. However, many devices that use LEDs just turn them on or off because that’s easier than controlling the current flowing through them. Some day, flat panel displays may use three colors of LEDs—red, green, and blue—in order to present full color images like those on a current television screen. For that scheme to work, the LEDs must be able to emit different brightnesses, so the current flowing through each one must be adjustable.