Question 18.AP.3: THREE-WAY LIGHTBULBS Figure 18.11 illustrates how a three-wa...
THREE-WAY LIGHTBULBS
Figure 18.11 illustrates how a three-way lightbulb is constructed to provide three levels of light intensity. The socket of the lamp is equipped with a three-way switch for selecting different light intensities. The bulb contains two filaments. Why are the filaments connected in parallel? Explain how the two filaments are used to provide the three different light intensities.
If the filaments were connected in series and one of them were to fail, there would be no current in the bulb and the bulb would not glow, regardless of the position of the switch. When the filaments are connected in parallel and one of them (say, the 75-W filament) fails, however, the bulb will still operate in one of the switch positions because there is current in the other (100-W) filament. The three light intensities are made possible by selecting one of three values of filament resistance, using a single value of 120 \mathrm{~V} for the applied voltage. The 75 -W filament offers one value of resistance, the 100 -W filament offers a second value, and the third resistance is obtained by combining the two filaments in parallel. When switch \mathrm{S}_{1} is closed and switch \mathrm{S}_{2} is opened, only the 75 -W filament carries current. When switch \mathrm{S}_{1} is opened and switch \mathrm{S}_{2} is closed, only the 100 -W filament carries current. When both switches are closed, both filaments carry current and a total illumination corresponding to 175 \mathrm{~W} is obtained.