Question

5. A three-way lamp has four switch positions: off, low, medium, and high brightness settings when used with a working three-way light bulb. Such a bulb has only two filaments; when connected to 120-V household circuit one filament dissipates 60W of power while the other filament dissipates 120W. (a) Calculate the resistances of the two separate filaments. b) Explain how current must flow through the bulb to dissipate 180W at the fourth switch position. c) The most common way for a three-way bulb to fail is for the lower resistance filament to break. When this happens, describe the resulting brightness output for each of the four switch positions. d) Explain fully how much power the bulb will dissipate at each of the four switch positions if the filament with the higher resistance burns out?

Answer 1

Remember that power is: V2 PER a) The first resistance: v2_120 R1 = =30=240 12 The second resistance: y2 1202 12 = 120 = 1201 b) To get this power R and R2 must be in parallel configuration, so the current will be different in each resistor: = 0.5 A 12 = = = 1A c) Off (no current, no bright) Low (bright, because this correspond for R1) Medium (no bright because R2 failed) Hight (like low (R2 failed) d) Off: P = OW Low: 60W Medium: OW High: 60 W