A 40W,60W,100W bulbs are connected in series to a 220V supply.Which one of them will glow brighter?

Question

Considering the fact that all the 3 bulbs are connected in series CURRENT IS CONSTANT for all three bulbs,therefore 100W bulb will glow brighter. But after verification in some books the answer is given that the 40 WATTS will glow brighter ,but why?

Answer 1

I liked both answers above (thumbs up to ya).

I just wanted to add that the hotter the filament in the bulb, the brighter the Tungsten element will glow.

The mechanism is Joule heating and can be calculated as (I^2)*R.  Since the current is the same through all the bulbs, the bulb with the highest resistance will be hottest and will glow the brightest.

The P=(V^2)/R relationship shows that a higher resistance bulb will be rated at a lower power.  So yeah, the 40W bulb will glow the brightest.  Pretty weird huh?

Answer 2

Since Power = E²/R and the 40W bulb has the smallest power dissipation for a given voltage, it must have the highest resistance.

Since they are in series, they all have the same current.

Thus, 40W has the same current and the highest resistance, so it uses the most power. Since it needs the least power to generate light, and uses the most power, must be the brightest.

Answer 3

You really can't answer this question without a lot more information, or making some false assumptions. The false assumptions are that the bulbs are completely linear, the resistance does not vary with voltage, and the brightness is proportional to power.

So under those assumptions, the resistance of each bulb is:

P = E²/R

100 = 220²/R1

R1 = 484 ohms (100)

R2 = 807 ohms (60)

R3 = 1210 ohms (40)

All 3 in series, R = 2501 ohms

I = 220/R = 0.0880 amps

P = 19 watts

P1 (100) = 0.0880²*484 = 3.75 watts

P2 (60) = 0.0880²*807 = 6.25 watts

P3 (40) = 0.0880²*1210 = 9.37 watts

so yes, the 40 watt bulb is brightest.

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