Ohm’s Law is an empirical law. It is obeyed until it is not.

If a component obeys Ohm’s Law, it must have a constant *V*-to-*I* ratio. What if a component does not obey Ohm’s Law? It just means that the component does not have a constant resistance.

Suppose you are given an unidentified two-terminal component or device. To investigate its behaviour, you can apply different voltages across its terminals, and measure the resulting currents passing through it. If you plot the data as a current-voltage graph, you get the so-called *I-V* characteristic curve of this component.

The above *I-V* characteristic curve shows a non-ohmic component. It has a different *V*-to-*I* ratio when different voltages are applied across it. For illustration, let’s calculate the resistance of this component at three different operating points A, B and C.

Now let me introduce this visual aid of mine which I call the “wiper lines”. They are these lines that join the origin and the operating points (drawn in blue in the graph). Realize that the lower a wiper line leans towards the *V*-axis, the larger the *V*-to-*I* ratio, so the larger the resistance. Conversely, the higher the wiper line leans towards the *I*-axis, the smaller the *V*-to-*I* ratio, hence the smaller the resistance. This visual aid should help you see with just a glance that .

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**Video Explanation**

It is *V/I* not *dV/dI*!

xmPuzzle 005

**Concept Test**

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