Bloggers Wanted
We're looking for people to help with the main blog. If you are consistent, knowledgeable and you're into it, please drop me a note.
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David S
 Fresh Boarder
Posts: 13 
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Hi, there!
I have a question:
How much volume must a balloon have to lift e.g. one kilo mass?
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Angel-xan
 Fresh Boarder
Posts: 13
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An object is bouyed up by a force equal to the weight of the fluid displaced. If the balloon is at the same temperature as the surrounding air, the bouyant force equals its own weight and it doesn't go up or down. When the balloon is heated, the volume of the balloon goes up proportional to the absolute temperature change (V2/V1) = k * (T2/T1) . So more (V2-V1) air is displaced. Now, if we were at STP, we would know that every mole (6.023*10**23 atoms) would weigh of 29g (if I remember 29 is the molecular weight of 80/20, N2 to O2). At STP each mole will occupy 0.1**3 cu meters. Note: The number of molecules in V2 is the same as in V1, but they are further apart on the average because they have been heated; consequently, lower density.
Let's say the air outside is 20 deg C (293 deg K) and we heat to 30 deg C (303 deg K). a 0.001 cu meter voume will increast to 0.00103 m3 but will still weigh 29 g. This is the same volume as displaced outside air. So, the boyant force is due to the 0.00003 m3 at 29 g per 0.001 m3. 0.00003*29/0.001 = 81/100 gram. Say, 0.8 g. from 0.1m by 0.1m by 0.1m .
You want to like 1kg. 1000/0.80 = 1250 cubic decimeters, roughly 10.6dm by 10.6dm by 10.6dm = slightly bigger than 1 cu meter.
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filip`
Fresh Boarder
Posts: 11
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Thanks William for a great answer!
Kjetil
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Merrill571
Fresh Boarder
Posts: 14
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Where did you get that number? It should be 22.4l or 0.0224 cubic meters. That's the volume of a mole of ideal gas at STP.
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JHollywood
 Junior Boarder
Posts: 20
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That sounds right. I appologize for introducing confusion, but it has been 43 years since high school chemistry class. I'm concerned about 1 ml of a gas at STP occupying 1 cc. Is that right or is that for liquids???
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ArleneBird
Fresh Boarder
Posts: 18
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No, and yes...that's about the density of water. Gases are typically about 1000 times less dense than liquids.
Regards, Jim.
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Filysagur
Junior Boarder
Posts: 21
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I misread the original...I thought he said mg per cc (since ml per cc just wouldn't have occurred to me as being different from unity!)
But my original point on the error in the first posting still applies...
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