[Physics] Why does brightness of the bulb decrease when a heater is connected in parallel to it


Whenever we connect electric bulb in parallel with a electric heater, the brightness of the bulb decreases, but later it acquires its original brightness in spite of parallel combination (in which voltage across bulb remains same).Why?

Best Answer

When the heater is cold, its resistance is low, and a large current will flow through it. This causes a voltage drop in the room because the copper wiring connection to the mains grid also has some resistance.

After a while, the heater gets hot. This causes an increase in the resistivity of the heater wire. It will draw less current, the voltage drop will become smaller, the light bulbs will get a bit brighter again.

How large the voltage drop is depends on the "internal resistance" or the "output impedance" of the connection to the mains grid.