Question

What are ohmic and non-ohmic resistances? Write down one example of each.

Hint:

Ohm's law: \(V \propto I\). Ohmic resistors follow this; non-ohmic resistors don't.

Answer 1

Ohmic Resistance:
A resistance is called ohmic if it obeys Ohm's law, which states that the current \( I \) flowing through a conductor is directly proportional to the applied voltage \( V \) across it, provided the temperature remains constant. Mathematically, this can be expressed as: \[ V \propto I \quad \Rightarrow \quad V = IR, \] where \( R \) is the resistance and remains constant for ohmic materials.
The graphical representation of voltage versus current (the \( V-I \) characteristic) for an ohmic resistor is a straight line passing through the origin, indicating a constant resistance.
Example: Common conductors such as copper wires and metallic resistors exhibit ohmic behavior under normal conditions.
Non-Ohmic Resistance:
A resistance is termed non-ohmic if it does not obey Ohm's law, meaning the current flowing through the device is not directly proportional to the applied voltage. In such cases, the resistance changes with voltage or current, often due to changes in temperature or other physical properties.
The \( V-I \) characteristic curve for a non-ohmic device is nonlinear, indicating that the resistance varies with applied voltage or current.
Example: Devices like diodes and filament bulbs are non-ohmic. For instance, in a filament bulb, the filament heats up as current passes through it, increasing its resistance and causing a nonlinear \( V-I \) curve.