I-V Characteristics
I-V Characteristics
- An I-V graph shows how a component behaves. The exam shows you the graph and asks which component it is. Three shapes cover almost every question.
The three shapes to know
- Metal wire at constant temperature: a straight line through the origin. Same slope both ways round. This is the ohmic one.
- Filament lamp: starts straight, then bends over. More current heats the filament, the hot metal resists more, so each extra volt gives less extra current. Not ohmic.
- Semiconductor diode: current passes one way only. Reversed, it passes almost nothing. Forwards, it passes almost no current until about 0.6 V, its , then the current rises steeply. (An LED is a diode that lights up; its threshold is higher, about 2 V.)
Reading the graphs in the exam
- Try the three shapes below: pick a component, drag V, and see how R = V/I changes. The metal's R stays fixed. The lamp's R rises as V increases. The diode's R is very large, then falls sharply past 0.6 V.
metal / resistor (2.0 Ω)·I = 1.00 A·R = V/I = 2.00 Ω
Common mistake
On a curved I-V graph, the resistance at a point is read from that point's values. It is not the gradient at that point, and not 1 over the gradient either. Gradient methods only work when the line is straight and passes through the origin.
Your turn— tap to reveal the worked answer (9702/11/M/J/25 Q33)
An I-V graph is flat at zero for all negative V, stays at zero up to about 0.6 V, then climbs steeply. Which component is it: a filament lamp, a metallic conductor, a resistor, or a semiconductor diode?
Answer: a semiconductor diode. Zero current one way round plus a threshold near 0.6 V happens only for a diode; no other component behaves like this. (9702/11/M/J/25 Q33)