When wood is burnt at atmospheric pressure, the hot gases need to push the atmosphere during expansion.So is the chemical energy converted into isobaric work and heat transferred to the surrounding?
1 Answer
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Interesting question. Certainly there is heat transfer from the hot gases to the atmosphere. But the chemical energy is not converted into work. I believe the work associated with the hot air rising is done by the atmospheric air and not the hot gas. Let me explain why I think that is so.
The hot gas rises because it is less dense than the air around it. The hot air balloon is an easier scenario to understand. From a thermodynamic standpoint the hot gas constitutes the system, the atmospheric air constitutes the surroundings, and the surface of the balloon is the boundary between the two. Because the hot gas is less dense than the atmospheric air the hot gas pressure is less than the air and thus the air exerts an upward force on the balloon- called buoyancy. The increase in height (increase in gravitational potential energy) of any weight attached to the balloon reflects the work done by the air on the gas.
In your example there is no well-defined boundary between the hot gas and atmospheric air. The gas immediately intermingles with the air and the pressures, temperatures and densities will be vary all over the place and vary. So the exact nature and amount of work being done is ill defined. Hope this helps.
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$\begingroup$ I think I read this example(about the hot gas doing work against atmospheric pressure to expand) in the book "An introduction to the three laws" by Peter Atkins.I asked this queation because I couldnt understand the concept of availability and I felt this scenario(where the surrounding gas absorbs heat from the fire, expands isobarically and when touches the surface of a vessel or a boiler, loses heat and contracts isobarically) has something to do with availability.Because the formula for availability resembles the difference in heat added and heat rejected in case of an engine cycle with iso $\endgroup$– user196272Commented Jul 16, 2018 at 18:24
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$\begingroup$ Because the formula for availability resembles the difference in heat added and heat rejected in case of an engine cycle with isobaric heat addition and isothermal heat rejection(at atmospheric temperature). $\endgroup$– user196272Commented Jul 16, 2018 at 18:27
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$\begingroup$ The heat added (to the engine) is equal to the heat lost by the gas(that absorbs heat from fire and loses it isobarically on contact with a vessel). $\endgroup$– user196272Commented Jul 16, 2018 at 18:31
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$\begingroup$ I googled "an introduction to the three laws" by Peter Atkins but got no hits. $\endgroup$– Bob DCommented Jul 16, 2018 at 21:06
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$\begingroup$ Sorry, I gave you the wrong title.Its, "The laws of thermodynamics" by Peter Atkins. $\endgroup$– user196272Commented Jul 16, 2018 at 21:30