What are ohmic and non-ohmic resistors? Give one example of each?
1 Answer
Ohm's Law states that the voltage ($V$) across a conductor is directly proportional to the current ($I$) flowing through it, provided all physical conditions and temperature remain constant.
Mathematically, this is expressed as:
$V = I \cdot R$
where $R$ (Resistance) is the constant of proportionality.
Ohmic Resistors
An ohmic resistor (or ohmic conductor) is a component that strictly obeys Ohm's Law.
Key Characteristics:
Constant Resistance: Its resistance ($R$) remains constant regardless of the voltage applied across it or the current flowing through it.
Linear V-I Graph: If you plot a graph of Voltage (V) on the y-axis against Current (I) on the x-axis, the result is a straight line that passes through the origin. The slope of this line ($V/I$) is equal to the resistance.
* Predictable Behavior: They behave in a simple, predictable way, making them ideal for use in circuits where a stable resistance is required.
Example: Carbon Resistor
A standard carbon resistor or a metal-film resistor (the small, color-coded components found in most electronic circuits) is an excellent example of an ohmic resistor. These are specifically designed to have a very stable resistance value over a wide range of operating currents and temperatures. Their primary job is to provide a specific, constant amount of opposition to current flow.
Non-Ohmic Resistors
A non-ohmic resistor (or non-ohmic conductor) is a component that does not obey Ohm's Law.
Key Characteristics:
Variable Resistance: Its resistance ($R$) changes as the voltage and current change. This is often because a physical property, like temperature, changes as more current flows.
Non-Linear V-I Graph: The V-I graph for a non-ohmic component is a curved line. The resistance at any given point is the ratio of V/I at that point, which is not constant.
* Dynamic Behavior: Their resistance is dynamic and depends on the operating conditions. This property is often harnessed for specific applications (e.g., in sensors or switches).
Example: Filament of a Light Bulb
The tungsten filament in an incandescent light bulb is a classic example of a non-ohmic component.
When the bulb is off and the filament is cold, its resistance is very low.
When you turn the bulb on, current flows through the filament, causing it to heat up to thousands of degrees Celsius.
As the temperature of the tungsten filament dramatically increases, its resistance also increases significantly.
Therefore, the resistance of the filament is not constant; it depends on how much current is flowing through it (and thus how hot it is). The V-I graph for a filament bulb is a curve that gets steeper as the current increases, indicating a rising resistance.
Other common examples of non-ohmic devices include diodes, thermistors, and transistors.
Summary Table
| Basis of Comparison | Ohmic Resistors | Non-Ohmic Resistors |
| :--- | :--- | :--- |
| Ohm's Law | Obey Ohm's Law | Do not obey Ohm's Law |
| Resistance (R) | Constant | Changes with voltage/current |
| V-I Graph | A straight line through the origin | A curved line |
| V & I Relationship | Voltage is directly proportional to current | Voltage is not directly proportional to current |
| Example | Carbon Resistor, Nichrome Wire | Filament Light Bulb, Diode, Thermistor |