Final answer:
The resistance of a lightbulb filament initially decreases when turned on due to a lower resistance at a cooler temperature, drawing more current. As the temperature rises, the resistance increases, reducing the current flow. This behavior is a characteristic of the filament material's temperature-dependent resistivity.
Step-by-step explanation:
The resistance of a lightbulb filament changes after the light has been turned on due to the relationship between resistance and temperature. Typically, resistance increases as the temperature of the filament increases. However, an incandescent lightbulb's filament has a lower resistance when it is first switched on, causing it to draw more current during its warm-up period. As it reaches its operating temperature, its resistance increases significantly, often more than tenfold, which in turn reduces the current flowing through the filament. This phenomenon is directly related to the material properties of the filament and its Resistivity changes with temperature.
In terms of electrical power, the power dissipated by a resistor or a filament is given by P = V2/R or P = I2R. Although it seems contradictory, there is no contradiction because these two equations are just different transformations of Ohm's law. Initially, when a filament has low resistance, it allows more current, and therefore, both the current and power are higher during the warm-up period.
The design of light bulbs, including the use of evacuated or inert gas-filled spaces around the filament, is engineered to minimize heat conduction and protect both the filament and the glass from the effects of extreme temperatures. This also helps in maintaining the efficiency of the lightbulb throughout its lifespan.