The discussion revolves around the concept of internal energy changes (ΔU) in two mixed water systems, specifically whether the change in internal energy of one system can be considered equal and opposite to that of the other. The scope includes theoretical considerations of thermodynamics and heat exchange between mixed fluids.
Participants express differing views on whether ΔUA and ΔUB can be treated as equal and opposite in the context of mixed fluids. There is no consensus on the appropriateness of this expression when considering different substances or the heat of mixing.
Limitations include the dependence on the definitions of internal energy changes and the complexities introduced by mixing different fluids, which may involve additional thermodynamic considerations such as the heat of mixing.
If they are mixed, then it is not possible to determine the separate identities of A and B in the final mixture. So you you can't determine ΔUA and ΔUB separately. However, you can determine the difference in internal energy between the final mixture and the initial separate internal energies of A and B, and set this difference equal to zero.Henrique Silva said:They are mixed
You can do that, and you would obtain the same result as if you used the method I described in post #4. However, conceptually, if the fluids are mixed, it is not really appropriate to identify separate changes for the internal energies of A and B. After all, at the molecular level, the fluids would be intimately mixed, and you could no longer identify either liquid.Henrique Silva said:If deltaUA and deltaUB=weight . mass thermal capacity . delta temperature, I can determine the deltaU of both A and B even if they are mixed together
That would be OK for mixing two bodies of the same liquid, but what would you do if they were two different liquids, and there was a heat of mixing?Henrique Silva said:So This isn't right deltaUb=-deltaUa?