Question about ohmic conductors

Question

I'm having some trouble understanding Ohm's law. My trouble is with the different ways it is described when referring to ohmic and non-ohmic conductors. If someone can answer this question I think it will clear up my doubts. (I made up this question myself -- it's not homework!)

Which one of A and B is wrong, and why?:

A:

A non-ohmic conductor is one whose resistance changes with increasing
  temperature, while an ohmic conductor is one whose resistance doesn't change
  with increasing temperature.

B:

A non-ohmic conductor is one whose temperature changes with increasing
  voltage, while an ohmic conductor is one whose temperature doesn't change
  with increasing voltage.

steve · Accepted Answer

Ohm's law assumes the temperature remains constant.

An Ohmic conductor is one in which the current flowing through it is proportional to the voltage applied across it.

A non-ohmic conductor is one in which the voltage and current are not linear.

A) The resistance of most conductors increases as the temperature increases, however being ohmic and not ohmic is not the reason.

B)What causes heating in a conductor is the current flowing through a conductor. The Power = current^2 * Resistance = voltage^2 / resistance

This power is converted into heat which increases the temperature as time goes on. The actual voltage does not matter as much as the how long it has been on the conductor.

Guy Inchbald · Answer

Application of the term "non-Ohmic" can be a bit messy and neither A nor B is wholly correct.
Ohm's law strictly applies only for constant temperature, however in practice resistance always varies with temperature.
An ohmic device such as a resistor is designed to remain within tolerance over a wide range of temperatures, working voltages and currents. Any variation in resistance due to such factors will be either ignored or compensated for.
A thermistor is a non-ohmic device in which the resistance variation with temperature is the characteristic property of the device.
Other passive non-ohmic devices include the voltage-dependent resistor or VDR, the photocell whose resistance which varied with the amount of light falling on it, and I forget what all else.
Such passive non-ohmic devices still obey Ohm's law provided the characteristic parameter (temperature, voltage, light) remains constant, in that they still have a specific resistance for that value of the parameter. Moreover the variation with parameter value is usually linear over the operating range of the device.
Other non-ohmic devices can be highly non-linear. Diodes and transistors are classic examples.

Question about ohmic conductors

2 Answers

Add your own answers!

Ask a Question