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Heating & Electrical Resistance

  1. Nov 20, 2014 #1
    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?
     
  2. jcsd
  3. Nov 20, 2014 #2

    nsaspook

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    Last edited: Nov 20, 2014
  4. Nov 20, 2014 #3
    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?
     
  5. Nov 20, 2014 #4

    nsaspook

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    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
     
    Last edited: Nov 20, 2014
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