I-V Characteristics

An I–V characteristic is a graph of current against potential difference for a component, usually plotted for both directions of p.d. Its shape identifies the component and shows how its resistance behaves.

Resistor at constant temperature — a straight line through the origin. Current is proportional to p.d., so the component is ohmic and its resistance is constant. The gradient is 1/R, so a steeper line means a smaller resistance.

Filament lamp — an S-shaped curve through the origin. At low p.d. the filament is cool and the graph is nearly straight. As the p.d. rises the current heats the filament, the ions vibrate more, electrons collide more often, and the resistance increases — so the graph bends over and the gradient falls. It is symmetric about the origin, because the behaviour is the same whichever way round the p.d. is applied.

Diode — almost no current in reverse bias, and no appreciable current in forward bias until the threshold of about 0.6 V, after which the current rises very steeply. The graph is strongly asymmetric, which is the whole point of the component.

Thermistor — resistance falls as it warms, so as the current heats it the gradient increases: the curve bends the opposite way from a filament lamp.

Reading resistance off the graph — at any point, R = V/I, which is the p.d. divided by the current at that point. It is not the gradient unless the line passes through the origin and is straight.

Example: this catches people out on the filament lamp. At a particular point the resistance is V/I for those coordinates, not the gradient of the tangent there. The two only coincide for an ohmic component.

Getting the data — vary the p.d. with a variable resistor or potential divider, recording current and p.d. at each setting, then reverse the supply connections to obtain the negative half. Take readings quickly, or in short bursts, so that components do not heat up beyond the effect you intend to measure.