**Date:** Fri, 22 Mar 2002 04:32:38 -0600

**Author:** trappe@physics.utexas.edu (Karl Trappe)

**Subject:** Re: Non-linear Lamp

**Post:**

Geoff: Is the curve a parabola? I would have guessed a 4th power, as well.

What did I overlook? Karl

>"George M. Caplan" wrote:

>>

>> I connect a #47 pilot bulb to a variable power supply.

>> I make a table of V vs. I.

>> I plot the graph. Clearly it is not linear.

>> (This is especially obvious if I include the point [0,0].)

>> The curve is fit very well by a parabola.

>> Can someone please tell me where I read an accurate

>> but understandable explanation of why this is true.

>

>Sure. As the filament gets hot, its resistance gets larger. A

>significant part of the resistance at room temperature comes from the

>electrons scattering off thermal vibrations (phonons). As it gets

>hotter, there are more phonons to scatter off of, so the resistance is

>higher.

>

>You can use this effect as a thermometer to measure the temperature of

>the filament and make a very nice lab demonstrating Stefan's law (power

>radiated is proportional to T^4). Interested parties can contact me for

>details.

>

>-Geoff Nunes

Dr. Karl I. Trappe Desk (512) 471-4152

Lecture Demonstration Office Office (512) 471-5411

Physics Department, Mail Stop C-1600 Home (512) 264-1616

The University of Texas at Austin

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