V-I Characteristics: Ohmic and Non-ohmic Conductors

The $V$-$I$ characteristic of a conducting device is simply a graph of the potential difference $V$ applied across it against the resulting current $I$, usually obtained by varying $V$ (using a rheostat or a variable supply) and recording the corresponding $I$ with an ammeter, keeping the device's temperature effectively constant throughout the measurement.

Ohmic conductors. A device is said to be ohmic if its $V$-$I$ graph is a straight line passing through the origin, for both directions of current flow. Common examples include a metallic wire, a carbon or wire-wound resistor, and a rheostat, all held at constant temperature. For such a device, the slope of the $V$-$I$ graph is CONSTANT and is precisely the resistance $R = V/I$ of the conductor, independent of the particular value of $V$ or $I$ chosen; equivalently, plotting $I$ against $V$ gives a straight line of slope $1/R$.

Non-ohmic conductors. Many practical devices do NOT give a straight-line $V$-$I$ graph, so their ratio $V/I$ is not a fixed constant but instead varies with the operating point. Such devices are called non-ohmic, and Ohm's law, strictly speaking, does not apply to them (though a "resistance" $R = V/I$ can still be quoted at any one particular operating point, this value then changes if $V$ or $I$ is changed). A p-n junction diode is the standard example: it conducts current easily in one direction (forward bias, beyond a small threshold voltage) but conducts only a tiny leakage current in the reverse direction (reverse bias), so its $V$-$I$ curve is markedly non-linear and asymmetric about the origin. Other common non-ohmic devices include a torch-bulb filament (whose resistance rises sharply as it heats up, curving the graph -- see Section 3.7) and an electrolytic cell undergoing dissociation. …