What causes a surge of current upon turning on incandescent lamps or electric heaters?

The phenomenon of a surge of current when turning on incandescent lamps or electric heaters is primarily linked to the behavior of the filaments in these devices. Initially, when the lamps or heaters are switched on, the filaments are at a room temperature and consequently have a low resistance. As the electric current begins to flow through them, they start to heat up, causing their resistance to increase significantly.

This increase in resistance as the filament heats up reduces the current flow after the initial surge. However, at the moment of activation, the very low resistance results in a high inrush of current, much larger than what is expected during normal operation. This is due to Ohm’s Law, where current (I) is inversely proportional to resistance (R), given constant voltage (V). Thus, when the resistance is low, the current increases sharply.

The ability of the filament to withstand this inrush and ultimately settle into a higher resistance state as it heats up is crucial to the functionality of the incandescent lamp or heater.