Free expansion and adiabatic process

In summary, the conversation discusses the calculation of change in entropy for different processes, specifically focusing on adiabatic processes and free expansion. The formula for calculating entropy change is given, but it is mentioned that it is only valid for reversible processes. The speaker has a question about why Q can be equal to zero in a free expansion, but not in an adiabatic process. The expert explains that free expansion is not a reversible process and suggests using a reversible path to calculate entropy change. The speaker is satisfied with this explanation.
  • #1
sita1
2
0
I hope someone in here can help me out of this problem.
I know that for calculating the change in entropy of a system ([tex]\Delta S[/tex]) we can use this formula :

[tex]\Delta S[/tex]=[tex]\int[/tex]δq/T
Well the problem is here, when we calculating ΔS for an adiabatic process we know Q= 0 so ΔS=0 in this process, but why we just can't let Q be equal to zero in a free expansion (while we know that in free expansion W=Q=0) and get to the same result (ΔS=0) for the free expansion process?
I should add that I know ΔS≠ 0 for free expansion process but, you know I just don't know where the problem is in my argument.
 
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  • #2
The formula you wrote is only valid for a reversible process. And the free expansion is not a reversible process. To calculate the difference in entropy in this case, connect the initial and final state by any reversible path, for which generally you won't have dQ = dL = 0, and use your formula on this path.
 
  • #3
Thanks, that's really a convincing answer.
 

1. What is free expansion?

Free expansion is a thermodynamic process in which a gas expands rapidly without any external work being done on it. This means that there is no transfer of energy in the form of heat or work during the expansion.

2. How does free expansion differ from other thermodynamic processes?

The main difference between free expansion and other thermodynamic processes is that in free expansion, there is no transfer of energy in the form of heat or work. This is in contrast to other processes such as isothermal, isobaric, and isochoric processes where there can be a transfer of energy.

3. What is an adiabatic process?

An adiabatic process is a thermodynamic process in which there is no transfer of heat between the system and its surroundings. This means that the change in internal energy of the system is equal to the work done on the system.

4. How does an adiabatic process differ from an isothermal process?

The main difference between an adiabatic process and an isothermal process is that in an adiabatic process, there is no transfer of heat between the system and its surroundings. This results in a change in temperature of the system, whereas in an isothermal process, the temperature remains constant.

5. What are some real-life examples of free expansion and adiabatic processes?

One example of free expansion is the release of compressed air from a container. An example of an adiabatic process is the compression and expansion of air in a bicycle pump.

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