Simulation of I-V graph of Non-Ohmic conductor
I–V GRAPH OF A NON-OHMIC CONDUCTOR (FILAMENT BULB)
A filament bulb does not obey Ohm’s Law. As voltage increases, the tungsten filament heats up — and as it heats up, its resistance rises. This means current increases more slowly than voltage, producing a curve that bends away from the straight line you would expect from an ohmic conductor. The graph starts steep (cold filament, low resistance) and gradually flattens (hot filament, high resistance).
The colour of the plotted curve tells the story: cyan at low voltage (cold, low resistance), through yellow in the middle, to deep red at high voltage (glowing hot, high resistance). The faint dashed line shows what the graph would look like if the bulb were ohmic — the difference between that line and the curve is the effect of temperature on resistance.
Key points the graph demonstrates:
- The I–V graph is a curve, not a straight line — confirming non-ohmic behaviour
- The curve passes through the origin (no voltage = no current) but bends as V increases
- Resistance is not constant — it increases as the filament gets hotter
- The gradient of the curve decreases as voltage increases, meaning 1/R is falling (R is rising)
Class activity:
- Plot from cold to hot — drag the slider slowly from 0V to 12V, releasing at every 2V to plot a point. Record V, I, and R in a results table. Notice R roughly doubles from cold to hot.
- Compare to the ohmic reference — ask: at 2V, how close is the bulb to ohmic behaviour? What has changed by 12V?
- Calculate R at each point — use R = V ÷ I for each plotted coordinate and confirm the resistance column is not constant.
- Exam question: “Explain why the I–V graph of a filament bulb is a curve and not a straight line.” Students should reference temperature, filament resistance, and the meaning of gradient.