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Incandescent Lightbulbs
Page 3 of 5 (42 Questions and Answers)

783. How do flashing lights, chasing lights, and any type of Christmas lights work? - N
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Years ago, many strings of Christmas lights consisted of about 20 or 30 light bulbs in series. In this series, electric current passed from one bulb to the next and deposited a small fraction of its energy in each bulb. The result was that each bulb glowed brightly so long as every bulb was working. If a single bulb burned out, the entire string went dark because no current could flow through the open circuit. If you replaced one of the bulbs in a working string with a special blinker bulb, the whole string would blink. The blinker bulb contained a tiny bimetallic switch thermostat that turned it off whenever the temperature rose above a certain point. At first, the bulb would glow and the whole string would glow with it. Then the thermostat would overheat and turn the bulb and string off. Then the thermostat would cool off enough to turn the bulb and string back on. This pattern would repeat endlessly.

But modern electronics has replaced the blinker bulbs with computers and transistor switches. Transistorized switches determine which bulbs or groups of bulbs receive current and glow at any given time and carefully timed switching can make patterns of light that appear to move or "chase." As for the problem with one failed bulb spoiling the string, a reader has informed me that the bulbs are now designed with a fail-safe feature. If a bulb's filament breaks, the sudden surge in voltage across that bulb activates this fail-safe mechanism. Wires inside the bulb connect to allow current to bypass that bulb completely. The remaining bulbs in the string glow a little more brightly than normal and their lives are shortened slightly as a result.


820. How does a heat lamp work and could it be harmful to the eyes of pets from extended exposure? — DM, Osceola, IA
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A heat lamp is much like a normal incandescent lamp, except that the heat lamp's large filament operates at a much lower temperature. Because of this lower temperature, the filament emits relatively little visible light. Instead, it emits mostly invisible infrared light. While you can't see infrared light, you can feel it as heat. Looking at a heat lamp is no more dangerous than looking at the glowing coals in a fireplace. Their thermal radiation heats your skin and the surfaces of your eyes, and is likely to make you uncomfortable enough to turn away before it causes real damage. In contrast, ultraviolet light from a sunlamp can injure your skin and eyes without causing any immediate pain—it's only much later that you feel the sunburn on your skin and corneas. That's why a heat lamp is relatively safe while a sunlamp is not.

903. Which gives off more heat energy, an incandescent light bulb or a fluorescent lamp? Which is more efficient to use in the summer or winter? — TJ, Woodbridge, VA
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An incandescent lamp turns its electric power completely into heat. Even the visible light it gives off is actually thermal radiation. A fluorescent lamp tries not to produce heat—the light it produces is non-thermal (it doesn't involve hot materials). While a fluorescent lamp is only partly successful at not producing heat, it's still several times more energy efficient than an incandescent lamps—fluorescents produces several times as much illumination for the same amount of electric power. This statement is true both in summer and winter, although fluorescent bulbs lose some of their energy efficiencies in very cold or very hot weather. Fluorescent lamps work best at temperatures between about 15° C and 40° C.

918. How does a halogen cooktop unit heat up food? — BS, Logan, UT
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A halogen cooktop unit uses thermal radiation to transfer heat to a pot or pan. All objects emit thermal radiation, but that radiation isn't visible until an object's temperature is at least 500° C. At higher temperatures, a significant fraction of an object's thermal radiation is visible light. In a halogen cooktop unit, an electrically heated tungsten filament is heated to the point where it emits a large amount of thermal radiation. Since the filament is small, it takes only a second or two for the filament to reach full temperature and begin emitting its intense thermal radiation. Any dark object above the unit will absorb this thermal radiation and experience a rise in temperature. When you turn off the unit, the filament cools rapidly and stops emitting its thermal radiation. The filament itself is protected from oxygen in the air by a heat-resistant glass envelope that's filled with halogen gas. This gas helps to keep the filament intact and prevents it from depositing tungsten atoms on the insides of the glass envelope.

919. Why is an incandescent light bulb hotter than a fluorescent light? — TJ, Woodbridge, VA
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An incandescent light bulb produces light by heating a small filament of tungsten to about 2500° C. At that temperature, the thermal radiation that the filament emits includes a substantial amount of visible light. But the filament also emits a great deal of infrared light (heat light) and it also transfers heat via conduction and convection to the glass bulb around it. When you put your hand near the bulb, you feel both the infrared light and the heat that has worked its way to the surface of the bulb. The bulb feels hot.

In contrast, a fluorescent lamp tries to produce light without heat. It collides electrons with mercury atoms to produce an atomic emission of ultraviolet light. This ultraviolet light is then converted to visible light by the layer of white phosphor powders on the inside of the lamp's glass envelope. In principle, this whole activity can be performed without creating any thermal energy. However, many unavoidable imperfections cause the lamp to convert some of the electric energy it consumes into thermal energy. Nonetheless, the lamp only becomes warm rather than hot.


970. How can one be fire safe while dealing with incandescent and fluorescent light bulbs? — TJ, Woodbridge, VA
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Fluorescent tubes produce relatively little heat, so they're relatively fire safe already. However, incandescent light bulbs become very hot and you have to be careful with them to avoid fires. First, make sure that the bulb can get rid of its waste heat. That means that you shouldn't wrap the bulb in insulation because it needs to transfer its waste heat to the air. Second, keep flammable materials away from the bulb, particularly above the bulb since hot air from the bulb rises upward.

971. What types of gas are used in light bulbs and how do their effects differ? — SF, Westfield, NJ
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The glass envelope of an incandescent bulb can't contain air because tungsten is flammable when hot and would burn up if there were oxygen present around it. One of Thomas Edison's main contributions to the development of such bulbs was learning how to extract all the air from the bulb. But a bulb that contains no gas won't work well because tungsten sublimes at high temperatures—its atoms evaporate directly from solid to gas. If there were no gas in the bulb, every tungsten atom that left the filament would fly unimpeded all the way to the glass wall of the bulb and then stick there forever. While there are some incandescent bulbs that operate with a vacuum inside, most common incandescent lamps contain a small amount of argon and nitrogen gases.

Argon and nitrogen are chemically inert, so that the tungsten filament can't burn in the argon and nitrogen, and each argon atom or nitrogen molecule is massive enough that when a tungsten atom that's trying to leave the filament hits it, that tungsten atom may rebound back onto the filament. The argon and nitrogen gases thus prolong the life of the filament. Unfortunately, these gases also convey heat away from the filament via convection. You can see evidence of this convection as a dark spot of tungsten atoms that accumulate at the top of the bulb. That black smudge consists of tungsten atoms that didn't return to the filament and were swept upward as the hot argon and nitrogen gases rose.

However, some premium light bulbs contain krypton gas rather than argon gas. Like argon, krypton is chemically inert. But a krypton atom is more massive than an argon atom, making it more effective at bouncing tungsten atoms back toward the filament after they sublime. Krypton gas is also a poorer conductor of heat than argon gas, so that it allows the filament to convert its power more efficiently into visible light. Unfortunately, krypton is a rare constituent of our atmosphere and very expensive. That's why it's only used in premium light bulbs, together with some nitrogen gas.

Incidentally, the filament in many incandescent bulbs is treated with a small amount of a phosphorus-based "getter" that reacts with any residual oxygen that may be in the bulb the first time the filament becomes hot. That's how the manufacturer ensures that there will be no oxygen in the bulb for the tungsten filament to react with.


992. How hot is a match when it is ignited? Is the initial point of combustion hotter than when it is just burning? — TB, Excelsior, MN
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You can usually judge the temperature of a hot object by its color—the brighter and whiter the light, the hotter the object. A candle flame has a temperature of roughly 1700° C while an incandescent light bulb has a temperature of about 2500° C. To my eye, a struck match briefly becomes brighter and whiter than a candle flame, so I would guess that its peak temperature is somewhere in the mid 2000° C range. Once the chemicals in the head have been used up, the flame temperature drops to about 1700° C.

1042. How does a halogen lamp get so hot?
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Like all incandescent bulbs, a halogen lamp creates its light as visible thermal radiation from an extremely hot tungsten wire. In fact, the wire in a halogen lamp is allowed to get even hotter than the one in a normal bulb. But while the glass envelope of a normal bulb gets only moderately hot during use, the glass envelope of a halogen bulb gets extremely hot. That's because the halogen bulb is using a chemical trick to keep tungsten atoms from getting away from the filament. Each time one of those tungsten atoms tries to leave, it's picked up by halogen molecules inside the glass envelope and returned to the filament. These halogen molecules can even pick the tungsten atoms up off the glass envelope and return them to the filament, but only if the glass envelope is allowed to get extremely hot. That's why the glass envelope of the halogen bulb is allowed to run so hot—if it weren't, it would accumulate the tungsten atoms permanently and it would darken. And since the tungsten atoms wouldn't be returned the filament, the filament wouldn't last as long.

1060. How does a "touch lamp" work? — LAM, Enosburg Falls, VT
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A touch lamp detects your touch by looking for changes in the electric properties of the lamp's surfaces. It monitors these properties by putting a fluctuating electric charge on them. As electric current flows toward the bulb through the lamp's wires, it passes through an electronic device that places a high frequency (about 60 kHz) alternating current onto those wires. This added current causes the lamp's surfaces to take on a small fluctuating electric charge—first positive, then negative, then positive, over and over again. This surface charging involves electrostatic forces, which extend long distances between charged objects, and occurs even though the lamp's surfaces aren't directly connected to the lamp's wires. The more surface the lamp has, the more easily it can hold that electric charge—the greater the lamp's electric capacitance.

When you plug the lamp in, the electronic device uses its fluctuating charge to determine how easy it is to add or subtract charge from the lamp's surfaces. In other words, it measures the lamp's capacitance. It then begins to look for changes in that capacitance. When you touch the lamp, or even come close to its surfaces, your body effectively adds to the lamp's surface and its capacitance increases significantly. The electronic device detects this increase in capacitance and switches the lamp's state from on to off or from off to on. The fact that you don't have to touch the lamp to affect its capacitance means that a touch lamp can have insulating paint on its metal surfaces yet still detect your touch. You can also buy touch lamp modules that plug into the wall and turn the lamp that's connected to them into a touch lamp. These modules are so sensitive to capacitance changes in the lamp that you can trigger them just by touching the lamp cord.


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