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Q & A: Light Bulb Physics

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Most recent answer: 08/19/2011
Q:
WHY DOES THE LIGHT BULB GLOW AND WHAT WAS THE FIRST LIGHT BULB MADE OF?
- Stephanie (age 11)
Singapore American School
A:
Stephanie -

One question at a time. Light bulbs (at least the incandescent kind) glow by taking the little piece of metal inside the bulb (called the filament) and heating it up a whole lot. You may have heard of pieces of metal becoming "white hot" - this is exactly what happens. The scientific term for this is "blackbody radiation", but basically what  it means is that when something gets really really hot, some of the heat energy goes flying off as light energy. When the light bulb does this, you get light that you can see.

The first light bulb was invented by Thomas Edison, who used a carbon filament. In order to keep the carbon from having chemical reactions with the air around it, Edison used a glass bulb with a vacuum inside of it. Nowadays, we use tungsten filaments instead of carbon because tungsten doesn’t evaporate as quickly as carbon does.

-Tamara & Dan

(published on 10/22/2007)

Follow-Up #1: Light bulb lumens and Watts

Q:
My friend Brandon told me the incandescent light bulb (ILB) is a big waste of energy if you want to produce light (output: 15% light,85% heat )as is the internal combustion engine (ICE). But my question is what are the light output in lumens and the heat output in btu's?
- theothenes (age 53)
baltimore,Maryland,USA
A:
Your friend Brandon is correct. Incandescent bulbs are very inefficient in converting electrical power into light, a typical number is about 12 Lumens per Watt.  By the way, one Kilowatt hour is equal to about 3,400 BTU.  
Wikipedia has a very nice article explaining the relationship between Watts, Lumens and efficiencies for various light sources.  Both florescent and LED lights are 4 to 5 times more efficient than incandescents.
LeeH


(published on 08/19/2011)

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