Question

Three light bulbs of 40W, 60W and 100W are connected in series with 220 volt source. Which of the bulbs will grow brightest?

• A. 60W
• B. 40W
• C. 100W
• D. All with same brightness

Answer 1

The correct option is B 40W

Resistance of $100Wbulb={200}^2/100=400ohms$

Resistance of $60Wbulb={200}^2/60=666.67ohms$

Resistance of $40Wbulb={200}^2/40=1000ohms$

Therefore, total resistance in series = $(400+666.67+1000)=2066.67ohms$

Current in the circuit =$200/2066.67=0.0967A$

Therefore, actual power consumed by $“40W”bulb={0.0967}^{2}x1000=9.35W$ (much lesser than any of the original)

The 40W bulb will grow the brightest as the current is constant in all three and it has the maximum resistance. But it would consume much less than 40W as the bulbs are connected in series, and voltage would be divided across all three filaments depending upon resistances.