Does the internal energy of the combined system change when ice melts?

For the ice as a system, there is a phase change, but no change in volume. For the water as a system, there is no phase change and therefore no change in volume.
  • #1
JoshMG
9
0
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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  • #2
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
 
  • #3
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!
 
  • #4
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]
 
  • #5
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.
 
  • #6
JoshMG said:
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
 
Last edited:
  • #7
JoshMG said:
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.
 

1. What is entropy and why is it important?

Entropy is a measure of the disorder or randomness in a system. In scientific terms, it is defined as the number of microstates that a system can have at a given energy level. It is important because it helps us understand and predict the behavior of systems and how they change over time.

2. How does entropy relate to internal energy?

Entropy and internal energy are closely related. As a system increases in entropy, its internal energy also increases. This means that as a system becomes more disordered, its particles move around more and have more energy. On the other hand, a decrease in entropy results in a decrease in internal energy.

3. Can entropy be reversed?

According to the Second Law of Thermodynamics, the entropy of a closed system will always increase over time. This means that in a closed system, it is not possible to reverse the increase in entropy. However, it is possible for individual processes to decrease in entropy, as long as the overall entropy of the system still increases.

4. How is entropy measured?

Entropy is typically measured in joules per kelvin (J/K) in the SI unit system. In thermodynamics, it is often represented by the symbol S. Entropy can be calculated using the formula S = k ln W, where k is the Boltzmann constant and W is the number of microstates in a system.

5. What is the relationship between entropy and disorder?

Entropy and disorder are closely related concepts. As the entropy of a system increases, the level of disorder also increases. This is because as a system becomes more disordered, there are more possible arrangements or microstates that it can exist in. Therefore, a higher entropy value indicates a higher level of disorder in a system.

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