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Entropy and internal energy

by JoshMG
Tags: energy, entropy, internal
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JoshMG
#1
Dec7-08, 08:27 PM
P: 9
PROBLEM:
Some warm water is mixed with ice and the ice melts. Assume that the entire process happens in an insulated box.

Does the internal energy of the combined (ice+water) system change?
Does the entropy of the combined system change?

Considering that the ice and water as separate system, discuss the change in internal energy and in entropy for each.


ATTEMPT:

I know that internal energy is dependent of change in temperature, there is a change in internal energy.

So in this case, I am assuming, it is an isobaric process so the volume is changing. So there is a change in entropy.



Separate systems: also change in internal energy. Again, change in entropy.


There is something definitely wrong with my answer. WHAT IS IT?! I'm going crazy!
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Saladsamurai
#2
Dec7-08, 09:10 PM
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P: 3,016
Change in Internal Energy=Work+Heat Transfer

You say for the entire system there is a change... how? Where is the work? Where is the heat transfer?

Also, this should be in Introductory Physics
JoshMG
#3
Dec7-08, 09:53 PM
P: 9
PROBLEM:
Some warm water is mixed with ice and the ice melts. Assume that the entire process happens in an insulated box.

Does the internal energy of the combined (ice+water) system change?
Does the entropy of the combined system change?

Considering that the ice and water as separate system, discuss the change in internal energy and in entropy for each.

ATTEMPT:

I believe this is a conceptual question, so this is what I think:

I know that internal energy is dependent of change in temperature, there is a change in internal energy.

So in this case, I am assuming, it is an isobaric process so the volume is changing. So there is a change in entropy.



Separate systems: also change in internal energy. Again, change in entropy.


There is something definitely wrong with my answer. WHAT IS IT?! I'm going crazy!

Saladsamurai
#4
Dec8-08, 07:43 PM
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P: 3,016
Entropy and internal energy

Did you read my post?!

If it happens in an insulated box there is no heat transfer. The total volume of the system cannot increase It is contained in a box.

[itex]\Delta U=Q-W[/itex] For the entire system, both Q and W are zero, so [itex]\Delta U_{sys}=0[/itex]
JoshMG
#5
Dec9-08, 05:45 PM
P: 9
I don't agree. This is what I think:

I do agree that there is no heat transfer, so Q=0. But the prompt does say that the ice melts, therefore there is a change in volume and since there is a change in volume, we have work done by the system. Because there is work done, we have a change in internal energy.

I just don't understand what difference it would make if we looked at the ice and water as different systems.
Andrew Mason
#6
Dec9-08, 06:25 PM
Sci Advisor
HW Helper
P: 6,672
Quote Quote by JoshMG View Post
I don't agree. This is what I think:

I do agree that there is no heat transfer, so Q=0. But the prompt does say that the ice melts, therefore there is a change in volume and since there is a change in volume, we have work done by the system. Because there is work done, we have a change in internal energy.

I just don't understand what difference it would make if we looked at the ice and water as different systems.
You are asked whether there is a change in energy of the combined water and ice in the container (we will call that the system). There is no addition of energy from the surroundings to the system. There is no removal of energy from the system to its surroundings.. So Q = 0.

Apply the first law. Unless there is work done on or by the system, there can be no change in internal energy.

I don't see where there is any significant work done here. If it was air-tight, there would be a slight increase in the volume of air/vapour as the ice melted. But it would be small. I don't think the question asks you to take that into account.

AM
Saladsamurai
#7
Dec9-08, 07:32 PM
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P: 3,016
Quote Quote by JoshMG View Post
I don't agree. This is what I think:

I do agree that there is no heat transfer, so Q=0. But the prompt does say that the ice melts, therefore there is a change in volume and since there is a change in volume, we have work done by the system. Because there is work done, we have a change in internal energy.

I just don't understand what difference it would make if we looked at the ice and water as different systems.
Just because the Ice melts does not mean there is a volume change. The box is sealed, so for the entire system there cannot be a change in volume.


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