# Question about ohmic conductors

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.

## 2 Answers

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.

• "however, being ohmic and not ohmic is not the reason": Does that mean that the resistance of an ohmic conductors can change if the temperature increases? Thanks! – user45220 May 13 '15 at 20:18
• Yes ohmic conductors resistances will change with in increase in temperature. But the resistance will not change if the temperature is kept constant, the actual temperature it is at is not important. Its important to remember that nothing is perfect, and most materials have a range in which they are ohmic, meaning voltage and current go up linearly together. – steve May 13 '15 at 20:25
• Thanks. So how do we explain metallic conductors? They are always ohmic according to my textbook. Does that mean they NEVER heat up no matter how much current we put through them? – user45220 May 13 '15 at 20:28
• Metallic conductors heat up.Their resistance is higher when they are hotter. But they are ohmic because the Resistance measured with 1Volt is the same as the resistance measured with 10 volts. However if you measured with 1 volt when it was 20 Celsius, and measured at 1 Volt when it was 200 Celcius, you would have 2 different resistances – steve May 13 '15 at 20:32
• Good answer. Small point: because resistance of ohmic resistors is typically a function of temperature, they will exhibit a small non-linear effect (the resistance measured with 10 V will not be the same as the resistance measured with 1 V, because the device was heated by the additional current). But in practice we ignore this because implicitly we assume "at constant temperature". You more or less said that, I just wanted to emphasize it. – Floris May 13 '15 at 20:53

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.