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There is a closed system that is just a windmill attached to a rotating shaft that can mechanically power something. I know the system produces work on the surroundings from the rotating shaft and this is positive work, but is there also a negative work input from the wind pushing the blades of the windmill?

Also, would there be any heat transfer in this scenario?

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How is this a closed system? Where does the wind come from? I don't see exactly what the question is. – user1631 Nov 14 '11 at 20:49
My bad. I meant to say the system is a windmill. Its an Open system to the environment. The question is asking, is there negative work done on the blades of the windmill by the wind? and also, is there any heat transfer in the scenario? – Greg Harrington Nov 14 '11 at 21:13
I would say the wind is doing positive work on the blades, that that work is being transferred to whatever the shaft is connected to, and also a fraction is converted to heat from friction. – user1631 Nov 14 '11 at 21:35
The convention I learned is that when work is being done on a system by the surroundings, it's meant to be negative. I would assume that since the wind/air is doing work on the blades, this work transfer is negative – Greg Harrington Nov 14 '11 at 21:39
That convention seems a little unconventional to me. – user1631 Nov 14 '11 at 21:45
up vote 1 down vote accepted

Let's start, as per the OP's comments, with the convention that when we take two systems A and B, and A is doing work on B, then: from the perspective of A, the work is positive; and from the perspective of B, the work is negative.

Now, from the perspective of the windmill. the work that the wind does is indeed negative: the wind is adding exergy to our windmill system. And most of that work will then transfer into positive work done by the windmill through the rotating shaft.

However, these transfers of exergy don't come for free: some heat is generated in the process, from friction in the system. As to exactly where the heat transfers are, will depend on the initial temperatures of the surroundings and the windmill, and the amount of heat generated by friction. But typically, the friction would raise the windmill's temperature above the temperature of the surroundings, so heat would get transferred from within the windmill system, to outside it.

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