# Resistivity of non-ohmic resistor

In layman's terms, how do you determine the resistivity constant of a non-ohmic material having measured voltage and current?

I understand that non-ohmic materials don't follow Ohm's Law, but will this still be able to be used to determine the resistance? Otherwise how can I determine an initial resistivity in order to use R=[rho]l/A to figure out the resistivity? Is there a different formula for this circumstance, or will I need to collect more data?

(Yeah, even I know it's a totally dumb question)

• This page ,I think, is based on differential resistance, which is non ohmic. en.wikipedia.org/wiki/Electrical_resistance_and_conductance. I will be the dumb one if it's no good to you...
– user108787
Commented Oct 30, 2016 at 12:49
• The answer is that this purely depends on the material. Non-ohmic simply means that it does not show the ohmic relationship - we don't know what other relationship it shows instead. Commented Oct 30, 2016 at 12:58

It is not a dumb question.
The gradient of the voltage against current a at given voltage is called the incremental/dynamic/small signal resistance.
This enables you to find out how small changes in voltage/current affect the current/voltage around the voltage where you measured the gradient.
So from this incremental resistance you can define an incremental resistivity but I am not so sure as to what you would use it for.

• I am conducting an EEI in class to find the resistivity of Play-Doh, as it is non-ohmic and exponential will there be no set resistivity? Would siting the derivative of the function calculated (current is on the x-axis if this effects it) perhaps be considered the resistivity equation or some such? (Taking into account this is only Year 11 Physics so it's not really that high-calibre work)
– T. J
Commented Oct 30, 2016 at 13:26
• You could find the incremental resistivity as a function of current and get them to plot the appropriate graph. Then perhaps get them to predict what the incremental resistivity should be at a given current outside their range of readings? Commented Oct 30, 2016 at 13:30
• I'm the student in this scenario... how would I find the resistivity as a function of current? Would I substitute the formula for resistivity into the resistance formula? So use I=VA/[rho]l or something and substitute my values in?
– T. J
Commented Oct 30, 2016 at 13:39
• Get a set of voltage and current readings. Draw a graph of voltage against current. At a number of currents find the gradient (= incremental resistance) and then use these values of incremental resistance to find the incremental resistivity at each of the currents. Commented Oct 30, 2016 at 13:42
• If I used the incremental resistivity as "y values" and current as "x values" and calculated an exponential function using this data, would this be able to calculate or represent resistivity, or is the relationship not uniform enough for this to be accurate?
– T. J
Commented Oct 30, 2016 at 13:56