Electrical Engineering/thermal energy


QUESTION: Hi - I'm trying to wrap my head around a problem I haven't been able to understand yet: if you have a 100W lightbulb, and only 5% goes to light for example, and 95% warms up the room, what is the sensible gain in the room? Is it 95W? And I ask because I have found an entry in a textbook that shows a conversion from the power input (P) to the thermal energy given off (Q) as Q=Px3.41. Therefore, according to this, the sensible gain in the room would be not 95W but 3.41x95=324W. I want to double check that this is correct, because intuitively it seems bizarre to say that a 100W lightbulb can provide 324W of heating to the room, but I guess it must be in some conversion factors somewhere..? Thanks for any clarification you are able to provide!

ANSWER: I don't know about the 'gain' you are referring to but a 100w lamp will produce 100w.  The ratings of lamps is the electrical heating value; the rating is not reduced by the light that is emitted.

Therefore, a 100 watt lamp produces 100 watts continuously pretty much and it will add that much heat to the interior of the room.  It will not produce more heat than it consumes.  Incadescant lamps are resistive in nature and the voltage times the current passing through the filament gives the power dissipation.  

The equation you give - Q = Px3.41 is strange to me and I don't know where it comes from or what its meaning is.  If you can give me the reference of the title of the book I will investigate it for you.

Hope this helps.

---------- FOLLOW-UP ----------

QUESTION: Thank you for your quick reply. What you have said makes sense to me - now just to figure out what this textbook is referring to.
It is called Mechanical and Electrical Systems in Buildings, by Tao, W.K.Y. and Janis, R.R., published in 1997. The equation comes in the section explaining how to calculate interior cooling loads of rooms, and cites this equation as a conversion from power input to sensible gain in the room for lights and appliances.
Thanks again!

OK. I think it is talking about heat or temperature build up in a room with typical ventilation, air handling, etc.  As you know when a room is closed up and a 100 watt lamp is in the room's center over time the temperature will build up and it gets warmer and warmer as time goes by.

I think the text book is talking about build up and what it takes to keep the temperatures at normal working conditions.  Makes sense to me although I think the formula is probably a gross estimate over many different parameters.

Hope this helps.  

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