Comparing lamps

Electricity and Magnetism Level 2

If a 40 W and a 60 W lamps are connected in a series. Which lamp will be brighter?

60 W lamp This will depend on the voltage supply Both will be the same 40 W lamp

Munem Shahriar by Munem Shahriar , 18, Bangladesh

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1 solution

Chew-Seong Cheong
Sep 19, 2017

Light bulbs are rated by standard voltage V V such that it power P = V 2 R P = \dfrac {V^2}R , where R R is the resistance of the lamp filament. This means that the higher the wattage the lower the resistance. That is the resistance of 60-W lamp is lower than that of 40-W lamp or R 60 < R 40 R_{60} < R_{40} .

When the two lamps are connected in series the current I I through them is the same and their powers are I 2 R 60 < I 2 R 40 I^2R_{60} < I^2R_{40} . Therefore, the 40-W lamp will be brighter .

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