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Careers: Physics/incandescent vs fluorescent bulbs
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Question
Hello Daniel
I applied 120 Vrms to a 60 W incandescent bulb. As expected, my measured current thru the bulb was 0.5 Arms.
I then applied 120 Vrms to a 30 W fluorescent bulb. However, the measured current thru this bulb was 0.5 Arms.
Is it always true that Vrms times Arms gives the absorbed power in watts (assuming a sinusoidal forcing fuction).
Thank you
Ron Herr
Hello Ron,
the absorbed power is indeed always given by Vrms*Arms. The mystery here lies in the power rating of the fluorescent bulb.
The fluorescent bulbs have something called "ballast" included in their circuitry, see paragraph about Electronic Ballast at http://en.wikipedia.org/wiki/Electrical_ballast (for example). If you measure 0.5 Arms flow through both kinds of bulbs, while both are connected to 120 Vrms, they both consume the same amount of power 60 W, no matter what the label says. An interesting insight into our efforts to "save the rain forest", is it not?
It can be argued, however, that the lighting efficiency of the fluorescent bulbs is supposed to be 5 times higher than that of incandescent bulbs of the same rating. If you take that as your base line, then you should compare 30W-rated fluorescent bulb with a 150W incandescent one. Then you still cut the energy consumption down by 2.5 and justify the purchase.
Cheers,
Daniel
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Daniel Mazur
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