Resistors & I–V Characteristics

Ohmic and non-ohmic components

• For some resistors $R$ stays constant as the current changes; for others it changes with the current
• An ohmic conductor at constant temperature: current is directly proportional to potential difference, so the resistance is constant and the I–V graph is a straight line through the origin
• Filament lamp: as the current increases the filament gets hotter, and its resistance increases. The I–V graph curves, getting shallower at high current
• Diode: current flows in one direction only. In the reverse direction the resistance is very high, so the current is zero. In the forward direction current flows once the potential difference passes about 0.7 V
• Thermistor: resistance decreases as temperature increases. Used in a thermostat: as the room warms, the thermistor's resistance drops, changing the current and switching the heater off
• LDR (light dependent resistor): resistance decreases as light intensity increases. Used to switch lights on when it gets dark: in the dark its resistance is high, and the circuit turns the lamp on

• Reading the graphs: a steeper line means a lower resistance (more current per volt). Where a lamp's line flattens, its resistance is rising. The graphs are symmetrical for resistors and lamps because reversing the potential difference reverses the current