Heating & Electrical Resistance

[Jimmy87]

When you increase the resistance of a filament bulb more work is done on the bulb which means its temperature increases. This causes the ionic lattice to vibrate with a greater amplitude (since it has more kinetic energy). The conduction electrons now encounter more collisions, hence the resistance goes up. What I was wondering is why this does NOT work for a fixed carbon resistor. If you increase the voltage across a fixed resistor it heats up (you can feel it) but the voltage and current remain in direct proportion. What is the reason for this? If a resistor is hot surely the atoms in the carbon resistor are vibrating more (as temperature is proportional to the average kinetic energy)? So how can the resistance not be affected?

 

[nsaspook]

The resistance is affected by heat in a regular carbon resistor but the change is small compared to a metal like tungsten. One factor is the difference in the collisional process in each material as it heats.
http://hyperphysics.phy-astr.gsu.edu/hbase/tables/rstiv.html
http://www.phy.gonzaga.edu/downloads/phys102L/electricalEquivalent.pdf

 

[Jimmy87]

The resistance is affected by heat in a regular carbon resistor but the change is small compared to a metal like tungsten. One factor is the difference in the collisional process in each material as it heats.
http://hyperphysics.phy-astr.gsu.edu/hbase/tables/rstiv.html
http://www.phy.gonzaga.edu/downloads/phys102L/electricalEquivalent.pdf

Thanks for the help and the links. Do you know what it is specifically about the structure of carbon that makes its resistance remain fairly constant over a wide range of temperatures as appose to tungsten?

 

[nsaspook]

Thanks for the help and the links. Do you know what it is specifically about the structure of carbon that makes its resistance remain fairly constant over a wide range of temperatures as appose to tungsten?

I sure others here could give you an exact explanation but I suspect it would have something to do with the anisotropic behavior of carbon expansion in the graphites commonly used in resistors.

http://simscience.org/cracks/glossary/isotropic.html