Question

Which one of the following is non-ohmic resistance?
     A) Mercury
     B) Copper
     C) Nichrome
     D) Bulb of a torch

Answer 1

Hint: Ohmic resistances are resistances that have a linear correlation between the current passing through them and the voltage applied, that is, they follow Ohm’s law. On the other hand non-ohmic resistances do not have such a linear correlation and thus do not follow Ohm’s law.
Formula used:
$V=IR$

Complete step by step solution:
A non-ohmic resistance does not follow Ohm’s law, that is there is no fixed linear correlation between the voltage applied across the resistor and the current passing through it.
According to Ohm’s law, the voltage $V$ across an ohmic resistor of resistance $R$ is related to the current $I$ passing through it by
$V=IR$
Mercury, copper and nichrome are all good conductors that follow Ohm’s law. Hence, they can be termed as Ohmic resistances.
On the other hand, the bulb of a torch, be it an incandescent bulb or an LED bulb, does not follow Ohm’s law perfectly. An LED bulb consists of p-n junction diodes which are semiconductors and do not have a linear correlation between the current passing through them and the voltage across them.
Hence, the correct answer is D) Bulb of a torch.

Note: Students might think that nichrome does not follow Ohm’s law since it is an alloy of nickel, chromium and iron but they must remember that the substance being an alloy has nothing to do with it following or not following Ohm’s law. In fact nichrome is one of the best examples of ohmic resistances and is commonly used as resistance wires in electrical heaters, toasters and ovens.