Current-voltage graphs

You must be able to recognise and understand the properties of certain components as demonstrated by their current-voltage graphs:

• Ohmic conductor - this component follows Ohm’s law therefore its current-voltage graph will look like a straight line through the origin. (This is provided physical conditions are kept constant).

• Semiconductor diode - when looking at the current-voltage graph of this component you must consider its forward and reverse bias. The forward bias of a diode is the direction in which it will allow current to flow easily past the threshold voltage, which is the smallest voltage needed to allow current to flow. In the direction of the reverse bias, the resistance of the diode is extremely high meaning that only a very small current can flow.

• Filament bulb - This component contains a length of metal wire, which heats up as current increases, therefore the resistance of this component increases as current increases. At low currents the metal wire will not heat up significantly, therefore for very low currents, Ohm’s law is obeyed. However, as the current increases (in either direction), the graph begins to curve due to the increasing resistance.

• (Negative Temperature Coefficient) Thermistor - This component acts in the opposite way to a filament bulb because as it heats up (due to an increase in current), the resistance across it will decrease. This is because increasing the temperature of a thermistor causes electrons to be emitted from atoms, therefore the number of charge carriers increases and so current increases causing resistance to decrease. Similarly to a filament bulb, at low currents, where temperature is kept constant, Ohm’s law is obeyed.