The shape of an I-V graph depends on how the component behaves. A wire at constant temperature gives a straight line through the origin, so its resistance is constant. A longer or thinner wire has a greater resistance, which gives a less steep line, and at the same p.d. it carries less current. A wire with a resistance of 5 ohms carries more current than a wire of 10 ohms at the same p.d.
A filament lamp gives a curve that gets less steep as the p.d. increases. As the current increases the filament gets hotter, the ions in the metal vibrate more, and the electrons collide with them more often. The resistance of the lamp therefore increases.
A diode lets current flow in one direction only. In the forward direction there is almost no current until the p.d. reaches a small value, then the current rises quickly. In the reverse direction its resistance is very high, so almost no current flows.
An NTC thermistor is a resistor whose resistance decreases as its temperature rises. A light dependent resistor (LDR) has a resistance that decreases as the light intensity increases. Both are used as sensors.
