For IB Physics SL and HL: Ohm's law states that the current through a conductor is directly proportional to the potential difference across it, provided its temperature and other physical conditions remain constant.
For an ohmic conductor, doubling the potential difference doubles the current. Therefore,
and the ratio of potential difference to current is constant:
This gives the familiar equation
where is potential difference in volts (V), is current in amperes (A), and is resistance in ohms (). Resistance describes how strongly a component opposes the movement of charge carriers.
Microscopically, an applied electric field produces a drift velocity of mobile charge carriers. In an ohmic material, drift velocity and therefore current increase proportionally with field strength, while constant temperature keeps resistivity approximately constant. Heating can increase lattice vibrations and alter resistance, so proportionality may fail.
For example, if a resistor has and the potential difference is , then
| Case | Current and potential difference relationship |
|---|---|
| Ohmic resistor at constant temperature | is directly proportional to ; resistance is constant |
| Filament lamp | Heating changes the resistance, producing a curved characteristic |
| Diode | Current depends strongly on direction and potential difference; behaviour is non-ohmic |
A graph of against for an ohmic conductor is a straight line through the origin, with gradient . If is plotted against , however, the gradient is .
The common misconception is that alone defines Ohm's law for every component. The equation can be used to calculate resistance at a particular operating point, but Ohm's law specifically requires a constant (V/I) ratio under constant physical conditions.
In IB Topic B.5, an Explain response should state the proportional relationship and its conditions. For calculations, show equation, substitution, answer, and units. When interpreting a graph, check which variable is on each axis before identifying resistance from the gradient.
