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Ohm's law is the first formula most students learn about circuits, so it is easy to think it applies to everything. Board papers and JEE questions then show a V-I graph that curves, or a diode, and ask whether Ohm's law holds. This post sets out what the law claims, the three ways it fails, and how to handle the numbers.
What exactly does Ohm's law claim?
Ohm's law says that for a given conductor the current is proportional to the potential difference, so V = IR with R constant. The ratio V/I can be worked out for anything, even a lamp or a diode, but the law only holds if that ratio stays the same as V changes. The CBSE 2026-27 curriculum asks for "V-I characteristics (linear and non-linear)", and the JEE (Main) 2026 syllabus asks for "I-V characteristics of Ohmic and non-ohmic conductors". Check your year's syllabus.
What are the three limitations?
NCERT Class 12 Chapter 3, Current Electricity, section 3.6, lists them:
V ceases to be proportional to I. The V-I graph curves.
The relation depends on the sign of V. Reverse the voltage and you do not get the same current in the other direction. NCERT gives a diode as the example.
The relation is not unique. There is more than one V for the same current. NCERT gives GaAs (gallium arsenide) as the example.
The chapter summary adds that a rectifier combines features 1 and 2, and that Ohm's law "is not a fundamental law of nature". NCERT also notes that devices that do not obey it are widely used in electronic circuits.
Worked example 1: an ohmic resistor (sample readings)
Here are sample readings for a carbon resistor: 1.0 V gives 20 mA, 2.0 V gives 40 mA, 3.0 V gives 60 mA. V/I is 1.0/0.020 = 50 Ω, 2.0/0.040 = 50 Ω and 3.0/0.060 = 50 Ω. The ratio is constant, so the V-I graph is a straight line through the origin and the resistor is ohmic over this range.
Worked example 2: a silicon diode
NCERT Chapter 14, Semiconductor Electronics, Example 14.4, reads a silicon diode's forward curve as 0.7 V at 10 mA and 0.8 V at 20 mA. Work out V/I at each point:
At 10 mA: 0.7/0.010 = 70 Ω.
At 20 mA: 0.8/0.020 = 40 Ω.
The ratio falls as current rises, so the diode fails limitation 1. NCERT defines a separate quantity, dynamic resistance r = ΔV/ΔI, and gets 0.1 V/10 mA = 10 Ω for this stretch of the curve. In reverse bias, the same chapter says the current stays very small, so the diode also fails limitation 2.
Worked example 3: a filament lamp (sample readings)
Sample readings for a small torch bulb: 1.0 V gives 0.10 A, 2.0 V gives 0.15 A, 3.0 V gives 0.18 A. V/I rises from 10 Ω to about 13.3 Ω to about 16.7 Ω. The graph bends towards the voltage axis.
Here the reason is temperature. As current grows, the filament gets hotter, and NCERT section 3.8 shows that a metal's resistivity rises with temperature. A lamp rated 60 W at 220 V has a working resistance of 220²/60 ≈ 807 Ω when hot; measured cold with a multimeter, it reads far lower. Our post on the temperature coefficient of resistance shows how to put numbers on that change.
How should you read a V-I graph in an exam?
Straight line through the origin: ohmic, R = 1/slope if I is on the y-axis.
Curve: non-ohmic. Check which axis is which before saying whether resistance rises or falls.
Different shapes for +V and −V: direction-dependent, like a diode.
A curve that doubles back so one current has two voltages: non-unique, like GaAs.
Which mistakes cost marks?
Saying R = V/I is "wrong" for a lamp. It is the resistance at that point. What fails is the claim that it is constant.
Using the slope of a curved graph as V/I. The slope at a point gives the dynamic resistance, not V/I.
Assuming a lamp keeps its rated resistance at every voltage. Problems often assume this for simplicity, as in our post on which bulb glows brighter; say so when you use it.
A practice routine
Write NCERT's three limitations from memory with one example each.
For any table of V and I readings, compute V/I for each row before drawing the graph.
Do two diode questions that ask for both V/I and ΔV/ΔI.
Sketch the V-I curves of a resistor, a lamp and a diode on one set of axes.
If a V-I graph question still confuses you, the Fermi Concept Library covers every concept in physics, so you can review the idea behind the curve before trying more questions.
Next step: take any three V-I readings from your lab notebook and decide, using V/I, whether the device is ohmic.
