Resistors

• The electrical current through an ohmic conductor (at constant temperature) is directly proportional to the potential difference across the ohmic conductor (resistor).

  So the resistance stays constant as the current changes.

  This means that a graph of current against potential difference (called an IV graph) like the graph below

  
• Non-ohmic components like lamps, diodes, LEDs, LDRs and thermistors do not have constant resistance. Their resistance changes when the current through them changes.
• Filament bulbs contain a thin coil of wire, this called the filament. When an electric current passes through the filament it heats up and emits light. When the temperature of the filament in a lamp increases due to the electric current passing through it, its resistance increases too. The graph below shows that the current is not directly proportional to the voltage

  
• The IV graph for a diode and an LED is very similar - see the graph below. Diodes and LEDs both let current flow in only one direction. This means that the resistance of diodes and LEDs is very high in the reverse direction

  
• The resistance of a thermistor decreases when its temperature increases. Thermistors are used in thermostats in your home to "tell" your heating system when to turn on/off, or in your computer to turn on its fan when it is getting too hot
• The resistance of an LDR decreases when the intensity of light shone on it increases. LDRs are used in street lamps to turn them on at night