Are Filament Bulbs Ohmic?

No, filament bulbs are non-ohmic.

Filament bulbs do not obey Ohm's Law. Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across the two points, provided the temperature remains constant. For a device to be ohmic, its resistance must remain constant regardless of the voltage applied.

Why Filament Bulbs are Non-Ohmic

The resistance of a filament in a bulb is highly dependent on its temperature. As the voltage applied to the bulb increases, the current flowing through the filament also increases. This increased current causes the filament to heat up significantly.

1. Temperature Dependence of Resistance: The resistance of most metals, including the tungsten used in filaments, increases with temperature. This relationship can be approximated by:

   R = R₀[1 + α(T - T₀)]

   Where:

   • R is the resistance at temperature T
   • R₀ is the resistance at a reference temperature T₀
   • α is the temperature coefficient of resistance

2. Non-Linear Relationship: Because the filament's temperature changes with voltage, the resistance does not remain constant. Consequently, the current-voltage relationship is no longer linear, which is a hallmark of ohmic behavior. As the voltage increases, the resistance increases, leading to a less-than-proportional increase in current.

Example

Imagine a filament bulb at room temperature. Its resistance is relatively low. When you switch on the bulb, the filament heats up to thousands of degrees Celsius very quickly. As its temperature rises, so does its resistance. This means that the increase in current is not directly proportional to the increase in voltage, violating Ohm's Law.

Conclusion

Because the resistance of a filament bulb changes with temperature, which is directly affected by the applied voltage, filament bulbs are non-ohmic devices. Their current-voltage relationship is not linear.