Physics · Current Electricity · NEET
No. Ohm's law is NOT a fundamental or universal law of nature. It is only an experimental result that holds for many metals at constant temperature. Many materials and devices (diodes, GaAs, electrolytes, gases) do not obey it. So never assume V = IR works for every element in a NEET circuit.
In a diode the relation between V and I depends on the SIGN (direction) of V. If a voltage V gives current I one way, then reversing V to -V does not give -I of the same size. Current flows easily in forward bias but almost none in reverse bias. Because the V-I graph is not a straight line and is not symmetric, V is not proportional to I, so Ohm's law fails.
A non-ohmic conductor is one whose V-I graph is NOT a straight line through the origin. This means its resistance R = V/I is not constant - it changes with voltage or current. Examples: diode, transistor, filament bulb, GaAs, thermistor. An ohmic conductor (like a metal wire at fixed temperature) gives a straight V-I line and constant R.
Yes. You can still define resistance at any point as R = V/I (called static or DC resistance). But for a non-ohmic device this value is NOT constant - it depends on the operating point on the V-I curve. So resistance exists, but there is no single fixed R for the whole device the way there is for a metal wire.
For GaAs the V-I relation is not unique. In one region of its curve, increasing V actually decreases I (negative resistance region). Because of this fold in the curve, a single value of current I can correspond to more than one value of V. That breaks the one-to-one V = IR relation, so it is non-ohmic. This is NCERT deviation type (c).
Try the real previous-year questions from this chapter — each with the answer and a full solution.
(a) V ceases to be proportional to I (V-I graph bends). (b) The V-I relation depends on the sign of V, so reversing V does not reverse I equally - this happens in a diode. (c) The V-I relation is not unique, so one current I can give more than one V - this happens in GaAs.
Semiconductor diode, transistor, GaAs, thermistor, filament bulb (its resistance rises as it heats), electrolytes and gases. These are all non-ohmic because their V-I graph is not a straight line through the origin.
Non-ohmic. As current increases, the filament heats up and its resistance increases, so the V-I graph curves and V is not proportional to I. Only at a fixed temperature would a metal behave ohmically.
Yes. Ohm's law assumes constant temperature. If current heats the conductor, resistance changes and the V-I graph is no longer a straight line. This is why heating devices deviate from V = IR.
