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Name: Ellen
Status: student
Grade: other
Location: OR 
Country: N/A
Date: 9/5/2005


Question:
I wanted to know how long the average light bulb filament is. My Current Science Magazine said that the filament was much longer than the science room. (pretty big!)


Replies:
The length of the filament is most easily determined by breaking a light bulb and measuring the filament. The filament "looks" like it is about an inch, or a couple of cm, long.

The diameter of the tungsten wire is very small. It is less than two thousandths of an inch, or about 1/20 mm diameter if you measure it with a micrometer. The mass is 0.0176 grams if you measure it with a sensitive scale. It would take 1,600 filaments to weigh an ounce.

If you use a magnifying glass or microscope you will find that the wire is formed into a coil. The coil is then formed into a larger coil. The filament looks short, but the actual wire is longer.

If you try to stretch the filament you will find that it breaks easily and you cannot find the exact length. The stretched length seems to be several times that of the unstretched length.

If you calculate the length of the filament is based on its mass, size and density, you will find it to be about half a meter, or a foot or two. It is not the size of a room.

Bob Erck


This is tough to generalize because there are so many other contributing factors, but in general specular reflection is more efficient than diffuse reflection, but that depends on "how rough the surface is". The actual surface area of a given 2-dimensional geometric area of a rough surface is much greater than a "smooth" surface because of all the "hills and valleys". This alone provides a greater area for absorption. The complication of the absorptivity as a function of angle of incidence enters in, however. These are just a few of the contributing provisions that prevent sweeping generalizations.

Vince Calder



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