Not all components are designed to obey Ohm’s Law (V will not be proportional to I)
They can control when the current can flow and how much can flow.
R These components are also known as SEMI CONDUCTORS or Non-Ohmic Devices
(1) Filament Bulb Voltage Current In the bulb as the voltage goes up , so will the heat energy (vibrations of the atoms) and therefore the electrons find it difficult to flow. Resistance (shown by the gradient of the graph) is seen to increase as current increases Current is not proportional to voltage!
(2) Diode Current Voltage Ohm’s Law is clearly not obeyed as voltage is not directly proportional to current. The diode allows current to flow in one direction only – known as forward bias but only at a definite voltage (usually about 0.6V).
Negative temperature coefficient (n.t.c.) thermistors have a high resistance when cold but a much lower resistance when hot . They are used as electrical thermometers to detect temperature changes. As the temperature rises, more charge carriers become available and the resistance falls.