Ohmic and Non-Ohmic Devices

Ohm's law, V = IR, is not a universal law of physics — it's a description of how certain materials behave. A resistor made of ordinary metal wire at constant temperature is ohmic: double the voltage across it and the current doubles too, so V/I stays fixed no matter where you measure it. Plot current against voltage and you get a straight line through the origin, and its slope (or inverse slope, depending on your axes) is the resistance.

A light bulb filament breaks this pattern. As current increases, the filament heats up, and hotter metal has more vigorously vibrating atoms for the drifting conduction electrons to collide with. More scattering means a harder time drifting, which means higher resistance. So as you push more current through the bulb, its resistance itself climbs — the I-V graph curves upward less and less steeply, bending away from a straight line. A diode is non-ohmic for a different reason: its internal physics (a semiconductor junction) makes it conduct easily in one direction and barely at all in the other, giving an I-V curve that is sharply asymmetric and far from linear.

None of this means the component is broken. A curved I-V graph is just data telling you that resistance is not constant — it depends on where you are on the curve. You still find resistance at any single operating point the same way: take that point's V and I and divide. This idea sets up the rest of circuit analysis, where you'll often linearize behavior around an operating point or simply be given V and I pairs to work with directly, instead of assuming one fixed R for every situation.