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Ohm's law, resistivity, and how resistance changes with temperature sheet
3-mark answer frame

State Ohm's law. Distinguish between ohmic and non-ohmic conductors, with a graph and an example of each.

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

  1. 1 · State the law. The current through a conductor is directly proportional to the potential difference across it, if the temperature and other physical conditions stay the same: V = IR.
  2. 2 · Ohmic conductor. Obeys Ohm's law, so R stays constant. Example: a copper or nichrome wire at a steady temperature.
  3. 3 · Non-ohmic conductor. Does not obey Ohm's law, so V/I changes with V. Example: a semiconductor diode or a transistor.
  4. 4 · Draw the graphs. [I against V: a straight line through the origin labelled 'ohmic, slope 1/R'; a curve for a diode rising steeply after a small forward voltage]
  5. 5 · Real life. A bulb filament heats up as it glows, so its resistance rises and it does not follow Ohm's law over its full range.
  6. 6 · Close in one line. So Ohm's law is not universal: it holds only for ohmic conductors under steady conditions.

Scoring tip: Always add the condition 'temperature remaining constant' to the statement. Without it the statement loses its mark.