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V-I characteristics: linear and non-linear sheet
3-mark answer frame

Distinguish between ohmic and non-ohmic conductors. State three limitations of Ohm's law, with a sketch of the V-I graph for each.

How to lay out the answer. Follow the frame, then write it in your own words.

  1. 1 · Define both. Ohmic: V ∝ I at constant temperature, a straight V-I graph through the origin. Non-ohmic: the V-I graph is not a straight line through the origin.
  2. 2 · Limitation (a). V stops being proportional to I at large currents, as a metal wire heats up.
  3. 3 · Limitation (b). The relation depends on the sign of V, as in a p-n junction diode.
  4. 4 · Limitation (c). More than one V gives the same I, as in GaAs.
  5. 5 · Sketch the graphs. [Three small labelled graphs: a line bending towards the V-axis; a diode curve with forward and reverse parts; a GaAs curve that rises, dips and rises]
  6. 6 · Close in one line. So, Ohm's law is not universal; it holds only for ohmic conductors under steady conditions.

Scoring tip: Marks go to the shape of each graph. Label both axes with units and name the device beside each curve.