IV Graphs

A current-potential difference graph, or IV graph, shows how the current through a component changes as the p.d. across it is varied. The p.d. is reversed to find how the component behaves in both directions.An ohmic conductor, such as a fixed resistor at constant temperature, gives a straight line through the origin. Its resistance is constant.A filament lamp gives a curve that becomes flatter at higher p.d. As the current increases, the filament gets hotter and its resistance increases, so the current rises less for each extra volt.A semiconductor diode conducts only in forward bias, and only once the p.d. is above a threshold of about 0.6 V. In reverse bias its resistance is very high and almost no current flows.The resistance at any point is V/I, calculated from the coordinates of that point, not from the gradient of the tangent. The resistance of a thermistor or LDR falls as the temperature or light intensity rises, so its graph becomes steeper.