How many of the following observations show that a conductor does not obey Ohm’s law?
(i) The graph of V against I is a straight line through the origin.
(ii) V is not proportional to I.
(iii) Reversing the voltage, with its size unchanged, gives a current of a different size.
(iv) Two different values of V give the same current.
(v) The relation V = IR can be used to find its resistance at a given current.
(vi) Its resistivity depends on the size and direction of the applied electric field.
(vii) Its resistivity changes when its temperature changes.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Four
A conductor obeys Ohm’s law when its resistance does not depend on \( V \), so the graph of \( I \) against \( V \) is a straight line. Any of the departures listed below shows that the law fails.
The observations that show a departure:
(ii) V not proportional to I — departure
(iii) current depends on the sign of V — departure
(iv) more than one V for the same current — departure
(vi) resistivity depends on the applied field — departure
The observations that do not:
(i) straight line through the origin — this is ohmic behaviour
(v) V = IR only defines resistance and holds for every device
(vii) resistivity of metals changes with temperature — this alone does not show a departure
Number of observations that show a departure = 4
NCERT (2026-27):
Summary – “10. Ohm’s law is obeyed by many substances, but it is not a fundamental law of nature. It fails if (a) V depends on I non-linearly. (b) the relation between V and I depends on the sign of V for the same absolute value of V. (c) The relation between V and I is non-unique.”
Points to Ponder – “… This equation defines resistance and it may be applied to all conducting devices whether they obey Ohm’s law or not. The Ohm’s law asserts that the plot of I versus V is linear i.e., R is independent of V. Equation E = ρ j leads to another statement of Ohm’s law, i.e., a conducting material obeys Ohm’s law when the resistivity of the material does not depend on the magnitude and direction of applied electric field.”