When is a material said to be ohmic?

Answer 1

An ohmic material follows Ohm's law, which states that V = I*R. This means that the potential difference in a system is equivalent to the total resistance multiplied by the current running through the system. This relationship, as you might have noticed, is a linear relationship. Most materials are approximately ohmic, however, there are exceptions. The resistance of a system remains mostly the same except at high temperatures, and Ohm's law assumes that materials will not undergo drastic temperature changes or be subjected to very high or low temperatures. However, it is generally a good approximation.

Another Answer

If you conduct and experiment that allows you to plot a graph of current against variations in voltage, and that graph turns out to be a straight line, then the material through which that current is flowing is said to be 'linear' or 'ohmic', which means that it obeys Ohm's Law.

In essence, Ohm's Law is true when the ratio of voltage to current is constant for variations in voltage.

However, very few materials and electronic devices are 'ohmic'. Repeating the same experiment with most materials will result in a curved-line graph, which tells us that that material/device is 'non-linear' or 'non-ohmic'. So most materials/devices DO NOT obey Ohm's Law. So an argument can be made that Ohm's 'Law' isn't a law at all, but simply describes the behaviour of a limited number of materials.

The equation, R = E/I, which is derived from the definition of the ohm, and NOT from Ohm's Law, applies whether Ohm's Law is obeyed or not, as it tells us what the resistance of a material/device is at a particular value of voltage (as opposed to a range of voltages).

Answer 2

a material which obeys ohm's law is known as ohmic material