Light bulb problem (Power, Reistance Voltage…etc..)
A 3-way light bulb has 3 brightness settings (low, medium, & high), but only 2 filaments.
a.) a particular 3-way lightbulb connected across a 120-V line can dissipate 60W, 120W, or 180 W. Describe how the two filaments are arranged in the bulb, and calculate the resistance of each filament.
b.) suppose the filament w/ the higher resistance burns out. How much power will the light bulb dissipate on each of the 3 brightness settings? What will be the brightbess (l,m,h) on each setting?
c.) Repeat part b for the situation where it is the filament w/ the lover resistance that burns out.
One Response
Eric C
26 Jun 2010
For a three-way bulb, the wattage of the highest setting will always equal the wattage of the two lower ones put together, since the highest setting is selected by simply turning on both filaments.
Essentially, it’s as if you had a 60-watt bulb and a 120-watt bulb. You could turn on the 60, the 120, or both.
If you call the 60-watt filament A and the 120-watt filament B, the switch will activate them in this sequence:
None (dark) –> A –> B –> A and B –> None (dark) .
If the smaller filament (A) burns out, the sequence becomes:
Dark –> Dark –> B –> B –> Dark (since A cannot light).
If the larger filament (B) burns out instead, the sequence becomes:
Dark –> A –> Dark –> A –> Dark etc
You can calculate power and brightness as if the two filaments were separate bulbs.