What's the Difference Between Ohmic and Non-Ohmic Conductors?

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If the potential difference becomes $12\ \text{V}$, an ohmic conductor of unchanged temperature carries $4.0\ \text{A}$, so its resistance remains $3.0\ \Omega$.

In a filament lamp, increasing current raises the filament's temperature. Greater lattice vibrations obstruct charge carriers more strongly, so resistance increases and the current rises less rapidly than the potential difference.

The graph convention matters. On a graph with current on the vertical axis and potential difference on the horizontal axis, the gradient is (I/V), which is conductance, not resistance. On a graph with $V$ vertical and $I$ horizontal, the gradient is resistance. A steeper $I$-$V$ line therefore represents lower resistance, whereas a steeper $V$-$I$ line represents higher resistance.

A common misconception is that $R=V/I$ applies only to ohmic conductors. This equation can calculate resistance at any operating point, but only an ohmic conductor has a constant (V/I) ratio under constant physical conditions.

Exam Technique

In Topic B.5, an Explain question requires the cause-and-effect chain: greater current causes heating, increased temperature causes greater resistance, and this produces a curved characteristic. When discussing a graph, state which quantity is on each axis before interpreting its gradient.