Tag: incandescent light bulbs

How does a regular lamp (light bulb) work?

A normal incandescent lamp contains a double-wound tungsten filament inside a gas-filled glass bulb. By “double-wound”, I mean that a very fine wire is first wound into a long, thin spiral and then this spiral is again wound into a wider spiral. While the final filament looks about 1 or 2 centimeters long, it actually contains about 1 meter of fine tungsten wire. When the bulb is on, an electric current flows through the filament from one end to the other. The electrons making up this current carry energy, both in their motion and in the forces that they exert on one another. As they flow through the fine tungsten wire, these electrons collide with the tungsten atoms and transfer some of their energy to those tungsten atoms. The tungsten atoms and the filament become extremely hot, typically about 2500° Celsius. Tungsten wire is used because it tolerates these enormous temperatures without melting and because it resists sublimation longer than any other material. Sublimation is when atoms “evaporate” from the surface of a solid. The gas inside the bulb is there to slow sublimation and extend the life of the filament.

Once the filament is hot, it tends to transfer heat to its colder surroundings. While much of its heat leaves the filament via convection and conduction in the gas and glass bulb, a significant fraction of this heat leaves the filament via thermal radiation. For any object that is hotter than about 500° Celsius, some of this thermal radiation is visible light and for an object that is about 2500° Celsius, about 10% is visible light. The light that you see from the bulb is the visible portion of its thermal radiation. However, most of the filament’s thermal radiation is invisible infrared light. While you can feel this infrared light warming your hand, you can’t see it. Because only about 80% of the electric power delivered to the bulb becomes thermal radiation and only about 12% of that thermal radiation is visible, an incandescent light bulb is only about 10% energy efficient. Other types of lamps, including fluorescent and gas discharge lamps, are much more energy efficient.

Is there a better way to construct a light bulb? For instance, is there a way to prevent the surface of the bulb from heating so quickly and generating so much heat? Is glass the best cover?

Unfortunately, there is not much that can be done to increase the efficiency of an incandescent bulb. It emits light by creating a very hot filament. Some of the filament’s heat is emitted as visible light but most ends up as hot air or infrared light (which you cannot see). There are tricks used to increase the bulb’s visible light output slightly (e.g. heating the filament hotter as in a halogen bulb or reducing the heat transport in the bulb gas as in a krypton bulb), but mostly there is nothing that can be done. Glass is about the best material for a bulb: it’s clear and a relatively poor conductor of heat.

On a three-way lamp, what are the switch settings for? Does it pump in more energy?

The lamp has four switch positions: off, filament 1 on, filament 2 on, and both filaments on. The bulb has three electrical connections to its filaments. One contact delivers electrical power to filament 1, another contact delivers electrical power to filament 2, and the third contact returns electricity from both filaments to the power plant. The switch carefully controls the flow of electricity to the two filaments so that at the low light setting, only the small filament is on, at the medium setting, only the large filament is on, and at the high setting, both filaments are on.

Which electric light bulb is best for the money, i.e. uses least electricity and has greatest light. I remember my high school physics teacher saying something like 50 watts -> 100 watts doesn’t double the light, just eats electricity.

For a given type of light bulb, the higher wattage bulbs are more energy efficient. Each light bulb has some “overhead” of wasted power that goes into heating the supporting structure and glass envelope. The higher wattage bulbs produce a little more light per watt of power. But not all types of bulbs are equally efficient. Long life bulbs are the least energy efficient because they run cooler than normal bulbs. The filament lasts a long time, but wastes more power producing infrared light. Some “energy miser” bulbs aren’t as good as normal bulbs. They may have lower wattages (typically 55 W instead of 60 W or 90 W instead of 100 W), but they actually produce significantly less light and thus consume more watts of power for each unit of light they produce. The most efficient incandescent bulbs are halogen lamps. These lamps, with their chemical recycling process, run substantially hotter than normal bulbs and produce more light per watt. They also last longer than normal light bulbs. They also produce whiter light (less red) and are just plain better bulbs than normal light bulbs. They cost more money up front, but it’s worth it in most cases.