What is the difference between linear and non linear resistance?

Answer 1

Materials that obey Ohm's Law are called 'linear' or 'ohmic'; those that don't are called 'non-ohmic' or 'non-linear'.

Ohm's Law isn't by any means a universal law; it doesn't apply to all conductors! Ohm's Law simply states that 'the current flowing through a wire, at constant temperature, is directly proportional to the potential difference across the ends of that wire'. This doesn't apply to, for example, a tungsten filament lamp, whose ratio between voltage and current changes as the voltage increases (due to its resistance changing as its temperature increases).

The so-called 'Ohm's Law equation' (R = V/I) is, in fact, derived from the definition of the ohm, and not from Ohm's Law. For this reason, the equation applies even when Ohm's Law does not. If the ratio of voltage to current remains constant over a wide range of voltages, then Ohm's Law applies for that range of voltages. If the ratio of voltage to current changes over a range of volages, then Ohm's Law does not apply.

In the case of resistors (as opposed to 'resistance'), 'linear' and 'non-linear' describes the way in which variable resistors have been wound. 'Non-linear' variable resistors are those that have been wound to produce specific characteristics, such as logarithmic values of resistance, as they are adjusted, whereas 'linear' variable resistors produce values of resistance that are directly proportional to how far the adjustment wiper has been moved.