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The Science Behind Light Bulbs |
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Light bulbs have a very simple structure. At the base, they have two metal contacts, which connect to the ends of an electrical circuit. The metal contacts are attached to two stiff wires, which are attached to a thin metal filament. The filament sits in the middle of the bulb, held up by a glass mount. The wires and the filament are housed in a glass bulb, which is filled with an inert gas, such as argon.
by PedroStrovalinski
Light bulbs have a very simple structure. At the base, they have two metal contacts, which connect to the ends of an electrical circuit. The metal contacts are attached to two stiff wires, which are attached to a thin metal filament. The filament sits in the middle of the bulb, held up by a glass mount. The wires and the filament are housed in a glass bulb, which is filled with an inert gas, such as argon.
An electric current flows from one metal contact to the other when the bulb is connected to a power supply, moving through the wires and filament. Electric current inside a solid conducter is defined by the mass movement of free electrons (electrons that aren't strongly attached to an atom) from an area of negative charge, to an area of positive charge.
Electrons constantly collide with the atoms in the filament as the zip through it. The energy produced by each collision vibrates the atoms, which heats them up. A thinner electrical conductor heats up easier and quicker than a thicker one, this is because it has a far greater resistance to electron movement.
Bound electrons in the vibrating atoms may be boosted temporarily to a higher energy level. When they fall back to their normal levels, the electrons release the extra energy in the form of photons. Metal atoms release mostly infrared light photons, which are invisible to the human eye. But if they are heated to a high enough level -- around 4,000 degrees Fahrenheit (2,200 degrees C) in the case of a light bulb -- they will emit a good deal of visible light.
The filament in a light bulb is made of a long, incredibly thin length of tungsten metal. Tungsten is used in nearly all incandescent light bulbs because it is an ideal filament material. Most metals will actually melt before reaching such extreme temperatures -- the vibration will break apart the rigid structural bonds between the atoms so that the material becomes a liquid. Light bulbs are manufactured with tungsten filaments because tungsten has an abnormally high melting temperature. But tungsten will catch on fire at such high temperatures, if the conditions are right. Combustion is caused by a reaction between two chemicals, which is set off when one of the chemicals has reached its ignition temperature. On Earth, combustion is usually a reaction between oxygen in the atmosphere and some heated material, but other combinations of chemicals will combust as well. The filament in a light bulb is housed in a sealed, oxygen-free chamber to prevent combustion. In the first light bulbs, all the air was sucked out of the bulb to create a near vacuum -- an area with no matter in it. Since there wasn't any gaseous matter present (or hardly any), the material could not combust.
The main flaw with this method was that the tungsten atoms were evaporating. At the high temperatures required, occasionally a tungsten atom vibrates heavily enough to seperate itself from the surrounding atoms and launches itself into the air. In the old style vacuum bulbs, the free atoms of tungsten shoot out in straight line and build up on the inside of the glass bulb. As this happens over and over again, the filament is losing atoms and therefore starts to disintegrate, and the glass gets darker and darker as it fills up with atoms. This has a massive effect on the lifespan of a light bulb.
In a modern light bulb, inert gases, typically argon, greatly reduce this loss of tungsten. When a tungsten atom evaporates, chances are it will collide with an argon atom and bounce right back toward the filament, where it will rejoin the solid structure. Since inert gases normally don't react with other elements, there is no chance of the elements combining in a combustion reaction.
Incandescent light bulbs give off most of their energy in the form of heat-carrying infrared light photons -- only about 10 percent of the light produced is in the visible spectrum. This wastes a lot of electricity. Cool light sources, such as fluorescent lamps and LEDs, don't waste a lot of energy generating heat -- they give off mostly visible light. For this reason, they are slowly edging out the old reliable light bulb.
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