Physics · Current Electricity · NEET
No. It must be a straight line AND pass through the origin. If a line is straight but cuts the V-axis or I-axis (does not go through the point 0,0), then V is not directly proportional to I, so the device is non-ohmic even though the graph looks straight. Ohmic needs both: straight and through the origin.
Yes. Ohm's law is V = IR, which means when V = 0, I = 0. So a truly ohmic V-I graph must pass through the origin (0,0). A line with an intercept means there is voltage even at zero current (or current at zero voltage), which V = IR does not allow.
As current increases, the filament heats up. Its temperature rises a lot, so its resistance increases. Because R is no longer constant, V is not directly proportional to I, and the V-I graph curves (it bends toward the V-axis). NCERT lists this as deviation type (a): V ceases to be proportional to I.
A diode is non-ohmic. NCERT deviation type (b): the relation between V and I depends on the sign of V. In forward bias the diode conducts strongly, but in reverse bias almost no current flows. So reversing V does not give the same-size current in the opposite direction. The V-I graph is not symmetric, so it is non-ohmic.
If you plot V on the y-axis and I on the x-axis, the slope (V/I) equals the resistance R. For an ohmic conductor this slope is constant everywhere. If instead you plot I on the y-axis and V on the x-axis (an I-V graph), the slope is 1/R. Always check which quantity is on which axis before reading resistance.
Gallium arsenide (GaAs) shows NCERT deviation type (c): the V-I relation is not unique. For one value of current there can be more than one value of voltage, so the curve folds back on itself (a negative-resistance region). This is clearly non-ohmic and is used in special electronic devices.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
An ohmic conductor is one that obeys Ohm's law V = IR at constant temperature, so its resistance stays constant. Its V-I graph is a straight line through the origin. Metals like copper and nichrome (at fixed temperature) are ohmic.
A non-ohmic conductor does not obey V = IR, so its resistance is not constant and its V-I graph is curved or bent. NCERT examples are a semiconductor diode, gallium arsenide (GaAs), and a torch bulb filament, along with electrolytes and thermistors.
Check two things: is the line straight, and does it pass through the origin? If both are yes, it is ohmic. If the graph curves, bends, has an intercept, or is not symmetric on reversing V, it is non-ohmic.
Yes, it still has resistance at each point, but the resistance is not constant. You find it as V/I at that specific point (called static resistance), and it changes from point to point along the curve.
When V is on the y-axis and I on the x-axis, the slope equals R. So a steeper line means higher resistance, and a flatter line means lower resistance. Remember to confirm the axes first.
