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I just started learning about Ohm's law and the textbook defines it as:

Provided the temperature is constant, then for an Ohmic conductor the current through it is directly proportional to the potential difference across it.

My question is how to interpret this. Do I interpret it as:

  1. "Provided we can get the conductor to keep the same temperature, then if it is Ohmic, Ohm's law will apply."

OR

  1. "Ohmic conductors always keep the same temperature."

Because I also just learned about I-V Characteristic graphs, and under the one for the filament lamp it says that its graph isn't straight because of temperature. So if I somehow managed to keep the filament lamp at a constant temperature, would the graph become a straight line? But then wouldn't that make a filament lamp an Ohmic conductor if I interpret Ohm's law as #1 above? So I'm assuming Ohm's law should be interpreted as #2. (But I suppose #1 would still make sense if it wasn't possible to keep the filament lamp's temperature constant in the first place.)

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Yes, if you kept the filament at a constant temperature, the IV curve would be straight.

Ways to do this: uncoil the filament, put it in an oil bath. If that is not enough: flatten the wire so that it becomes more like a ribbon, to increase the cooling surface.

2) is not correct. Ohm's law is valid because there is dissipation. There will always be heat production by a current in a resistor.

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  • $\begingroup$ Thanks! So Ohmic conductors are Ohmic because they manage to keep their temperature constant by dissipation? $\endgroup$ – kesra Dec 27 '16 at 12:40
  • $\begingroup$ @kesra Ohm's law is valid because the speed of the electrons is constant; they are moving at "terminal velocity" in an electric field (like raindrops in the Earth's gravitational field in the atmosphere). This implies some kind of internal dissipation mechanism, "friction", which generates heat. If temperature is constant in time, this means a steady state, heat is conducted (or radiated) away. $\endgroup$ – Pieter Dec 27 '16 at 13:10
  • $\begingroup$ @kesra The rate of change of resistivity of materials with temperature is very different for different materials. For the tungsten filament of a light bulb, the resistivity changes by about 0.5% per degree C, but for the metal alloy manganin the change is only 0.0002% per degree C. So manganin is very close to being an ohmic material even if its temperature changes by hundreds of degrees, but a light bulb filament is not. $\endgroup$ – alephzero Dec 28 '16 at 6:00

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