# Is this right deltaUA=-deltaUB?

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1. May 7, 2015

### Henrique Silva

If we have 2 systems like water(A) and cold water(B) in contact, if we ignore the energy exchanges between air and those systems, could we consider that the deltaUB=-deltaUA??

2. May 7, 2015

### Staff: Mentor

Are A and B kept separate, and just allowed to exchange heat, or are they mixed with one another?

Chet

3. May 7, 2015

### Henrique Silva

They are mixed

4. May 7, 2015

### Staff: Mentor

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.

Chet

5. May 7, 2015

### Henrique Silva

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

6. May 7, 2015

### Henrique Silva

Now I understand what you were saying, ok but isn't it the same measuring the deltaU with the final mixture?

7. May 7, 2015

### Staff: Mentor

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.

Chet

8. May 7, 2015

### Henrique Silva

So This isnt right deltaUb=-deltaUa?

9. May 7, 2015

### Staff: Mentor

Also, if you were mixing different fluids to form a solution, there might be a heat of mixing, and it would then be problematic to identify the separate contributions of each of the two fluids.

10. May 7, 2015

### Henrique Silva

Thanks

11. May 7, 2015

### Staff: Mentor

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?

Chet

12. May 7, 2015

### Henrique Silva

So if they were two different substances that expression wasn't apropriated. Thanks about your help