May
17
2009
Light Bulbs
Author: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.
When the bulb is hooked up to a power supply, an electric current flows from one contact to the other, through the wires and the filament. Electric current in a solid conductor is the mass movement of free electrons (electrons that are not tightly bound to an atom) from a negatively charged area to a positively charged area.
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 inside the vibrating atoms can be temporarily enhanced to a higher energy level and when they receed back to their normal energy level, they release extra energy in the form of photons. Atoms of metal mostly release infrared light, which human eyes cannot see. However if they are heated enough (in the case of a light bulb around 2,200 degrees C / 4,000 degrees F) they will emit lots of clearly visible light.
Within the light bulb itself, the filament is a long, very thin wire of tungsten metal. This is used is very nearly all incandescent light bulbs, and this is because tungsten is simply ideal to use for the filament material. Most other metals would melt under such high temperatures, the structural bonds between the atoms would be broken apart by the vibrations caused by the heat, hence the material becomes liquid. Tungsten is the favoured metal to use in light bulbs because of its high melting temperature. However with that said, tungsten will set on fire when it reaches such extreme temperatures, in the right conditions. Combustion is the end result of two particular chemicals reacting with each other, which occurs when one of said chemicals reaches it’s ignition heat. On earth at least, combustion is almost always a reaction between the atmosphere’s oxygen, and a given heated material, but there are other chemical combinations that will ignite as well. The lightbulb’s filament is encased in a sealed chamber free of oxygen, therefore avoiding combustion. In the early days of light bulbs, a near vacuum was created inside the bulb by sucking out all (or nearly all) of the air, as a result of this, there was barley any oxygen in the bulb, preventing combustion.
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.
However, in modern day light bulbs, inert gases are used inside them to reduce the loss of tungsten, usually argon. So when an atom of tungsten seperates itself from the filament (evaporates), it is highly likely that it will bounce off an atom of argon and be pushed back towards the filament to rejoin the tungsten structure. Because inert gases are normally unreactive with all other elements, it will not create a combustion reaction within the light bulb.
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.
