Question

Explain why an electric bulb becomes dim when an electric heater in a parallel circuit is switched on. Why does brightness decrease after some time?

Answer 1

We know that $R\propto \frac{1}{P}$. The power of heater is more than that of bulb. The resistance of a heater coil is less than that of electric bulb. When heater is switched on in parallel, more current start flowing through the heater coil and current through the bulb filament decreases making it dim.

After some time, when heater coil becomes hot its resistance increases. As a result, current through the heater coil decreases and the current through the bulb filament increases and thus dimness of the bulb decreases.