Question
In an experiment to verify Ohm’s law, the following readings were obtained:
| Voltage (V) | 1.0 | 2.0 | 3.0 | 4.0 | 5.0 |
|---|---|---|---|---|---|
| Current (A) | 0.25 | 0.50 | 0.75 | 1.00 | 1.25 |
Plot the V-I graph. From the graph, determine the resistance of the resistor. Is the resistor ohmic?
(CBSE 2024, similar pattern)
Solution — Step by Step
Take voltage (V) on the x-axis and current (I) on the y-axis. Choose a suitable scale — for example, 1 cm = 1 V on x-axis and 1 cm = 0.25 A on y-axis.
Plot the five data points: (1.0, 0.25), (2.0, 0.50), (3.0, 0.75), (4.0, 1.00), (5.0, 1.25).
All five points lie on a straight line passing through the origin. This confirms a linear relationship between V and I — exactly what Ohm’s law predicts.
Ohm’s law states , which means .
Pick any point on the line. Using (4.0, 1.00):
We can verify with another point: . Same answer — consistent.
Yes. Since the V-I graph is a straight line through the origin, the resistance is constant at all voltages. A device that follows this linear relationship is called an ohmic conductor.
Non-ohmic devices (like diodes or filament bulbs) would give a curved V-I graph.
Why This Works
Ohm’s law () says that for a conductor at constant temperature, the current through it is directly proportional to the voltage across it. The proportionality constant is the resistance .
On a V-I graph, this proportionality shows up as a straight line through the origin. The slope of the graph (if I is on y-axis and V on x-axis) gives , and the inverse of the slope gives . A steeper line means lower resistance (more current for the same voltage).
The “constant temperature” condition matters because resistance changes with temperature. That’s why a filament bulb (which heats up as current increases) doesn’t obey Ohm’s law — its V-I graph curves.
Alternative Method — Calculating slope directly
The slope of the I vs V graph is:
Since slope , we get .
In the CBSE practical exam, always draw the graph on graph paper with a sharp pencil, mark the plotted points as small circles (not dots), and draw a smooth best-fit line. Write the scale clearly. The examiner checks all of this. Also, the resistance should be calculated from the graph line, not just from the table — that’s the whole point of the graphical method.
Common Mistake
Many students plot V on the y-axis and I on the x-axis (the reverse of what CBSE expects). While both are technically correct physics, the CBSE marking scheme typically expects V on x-axis and I on y-axis. Check your question paper — if it says “plot V-I characteristics,” V comes first (x-axis). Getting the axes wrong can cost you graph-related marks.