Heating & Electrical Resistance

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Discussion Overview

The discussion revolves around the relationship between heating and electrical resistance in filament bulbs versus fixed carbon resistors. Participants explore the effects of temperature on resistance and the underlying mechanisms that differentiate materials like carbon and tungsten.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant notes that increasing the resistance of a filament bulb leads to increased temperature due to more work being done, which causes greater vibrations in the ionic lattice and more collisions for conduction electrons, thus increasing resistance.
  • Another participant states that while resistance in a carbon resistor is affected by heat, the change is minimal compared to metals like tungsten, suggesting differences in collisional processes as materials heat up.
  • A participant questions what specific structural characteristics of carbon allow its resistance to remain relatively constant over a wide temperature range compared to tungsten.
  • Another participant speculates that the anisotropic behavior of carbon expansion in graphite may contribute to its resistance characteristics.

Areas of Agreement / Disagreement

Participants generally agree that resistance is affected by temperature in carbon resistors, but they acknowledge that the extent of this effect is less significant than in metals like tungsten. The discussion includes multiple viewpoints regarding the mechanisms behind these differences, and no consensus is reached on the specific reasons.

Contextual Notes

There are unresolved questions regarding the specific structural properties of carbon and how they influence resistance, as well as the collisional processes in different materials as they heat up.

Who May Find This Useful

This discussion may be of interest to those studying electrical engineering, materials science, or anyone curious about the thermal properties of different conductive materials.

Jimmy87
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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?
 
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nsaspook said:
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?
 
Jimmy87 said:
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
 
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