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ohmic conductors

andieje
andieje used Ask the Experts™
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Hi

Just a quick question about ohmic conductors. I asked a question recently about why a light filament isn't ohmic. Apparently as you increase the voltage in a filament the temperature increases which causes the resistance to increase.

How can anything be an ohmic conductor? Is there anything where the temperature/resistance doesnt increase as you increase the voltage? More importantly, how can metal wire be an ohmic conductor? Doesn't the temp increase with V? What's a light filament made out of then as i thought it was just a metal wire. Obviously not :)

Many thanks
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Jose ParrotGraphics Expert

Commented:
Ohmic conductors are the materials which conducts electricity by obeying the Ohm Law.
Ohm Law stabilishes that
       R = E / I
being R the resistence to the electric flow, E the potential difference and I the intensity of the flow.
R is resulted in Ohms, E is given in Volts and I is given in Amperes.
Metals, in general, are ohmic conductors, because the Ohm Law applies in linear mode, in both directions. Semi-conductors (like germanium, silicon) aren't ohmic, because they conducts the electricity by obeying the Ohm Law in one direction and close to nothing in the other.
Non-ohmic materials have non linear conduction depending on temperature or light. These materials (as semiconductors as well) are applicable to sensors and probes to measure temperature or light levels. CCD sensors use these materials.
Ceramic are very interesting materials. Some are quasi perfect insulators (like the ones in the high voltage towers); others are piezo electric ceramic, which senses pressure variations (some pickups of the old vinil discs use them); other are a very very special kind of material: the Super-conductors. These "super" are capable of "contradict" the gravitation law: they levitate!!! Also are important to MRI technology! Take a look at http://www.superconductors.org/Uses.htm
Jose

Author

Commented:
Hi

Thanks for your answer. However I don't feel you addressed the specific points I asked in my question which were.

How can anything be an ohmic conductor? Is there anything where the temperature/resistance doesnt increase as you increase the voltage? More importantly, how can metal wire be an ohmic conductor? Doesn't the temp increase with V? What's a light filament made out of then as i thought it was just a metal wire.

 Could anyone else comment?
How can anything be an ohmic conductor?
strictly speaking nothing is ohmic
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Is there anything where the temperature/resistance doesnt increase as you increase the voltage?
no
But what do you mean by "temperatureresistance"?
Ohm's law says that when you put a current through anything energy is supplied by that current. The energy has to go someplace. That is the heat which increases the temperature.
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 More importantly, how can metal wire be an ohmic conductor?
because in most cases the resistance does not change all that much. (the temperature change of a light bulb is from 100 degrees to 8000 degrees)
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 Doesn't the temp increase with V?
In a simplified way, no  the temp increases because the voltage increases not the other way around
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What's a light filament made out of then as i thought it was just a metal wire.
It is made of metal (tungsten)      The first ones were made of carbon.
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My answers can be made more general at the cost of greater complication. Ohm's Law is so useful because many conductors have a very small temperature coefficient and in most cases only small temperature changes are encountered.




Ohm's Law is an approximation useful over a range of parameters.

Resistivity of materials can be a function of temperature, magnetic field, and pressure (among other variables).

All resistors have temperature coefficients.  And even the best resistors in the world have operating limits.

     http://www.vishaypg.com/docs/63003/vhp100.pdf

So no matter what you pick for a resistor, you can find operating limits where Ohm's Law will break down.

Alternatively, if you use  1 mA to measure the resistance of a light bulb filament.
Ohm's Law is a fine model from 1 uA to 10 ma.
An engineering joke:

  V = I*R       is a good model for a 1W resistor at 1 W of input power.

  PV = nkT    is a better model for a 1W resistor at 1 MW of input power.

Author

Commented:
Ok, so to summarise to check i understand:

 nothing is truly ohmic but certain materials can obey ohm's law over a range of parameters because the temperature change they experience is minimal over that range so their resistance stays fairly constant over that range?

Jose ParrotGraphics Expert
Commented:
The engineering joke help us to remember that math equations never are perfect for all physical natural phenomena...
Every class on Physics begins with the teacher standing "for normal conditions of temperature and pression"... The wider the range of the phenomena, the more complicated are the formula...
In other approach: if a copper wire is 10inch diameter, 1A or 5A is "absolutelly" the same... "Absolutely" because few instruments are capable of perceive the resistence or thermal difference, although we know that it exists...
Jose

Author

Commented:
thanks