Question

The image below shows a three-way bulb that can glow at three different brightness. The bulb is rated to operate at 120 V power supply. 

 

Determine how the bulb can have three different brightnesses. Determine the total current through the circuit which causes the bulb to glow in each of the three cases.

Answer 1

Case (i) - when switch S₁ is closed, only the 75 W filament glows. 

Current through the circuit = P/V = 75/120 = 0.625 A 

Case (ii) - when switch S₂ is closed, only 100 W filament glows

Current through the circuit = P/V = 100/120 = 0.833 A 

Case (iii) - when both switches are closed, both 75 W and 100 W filament glows. 

Current through the circuit = 0.625 + 0.833 = = 1.458 A