Timeline for Helmholtz energy and thermal equilibrium
Current License: CC BY-SA 4.0
17 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Mar 15, 2023 at 23:11 | history | bounty ended | Jimmy Yang | ||
| Mar 15, 2023 at 23:11 | vote | accept | Jimmy Yang | ||
| Mar 14, 2023 at 22:44 | comment | added | Themis | Here is how to think about free energy. It is analogous to the potential energy difference between the tob and the bottom of a waterfall: the maximum amount of work that can be priduced by the waterfall is $\Delta E_\text{pot}$, but this work will be produced only if we place a turbine in the path of the waterfall. | |
| Mar 14, 2023 at 22:42 | comment | added | Themis | Yes, if the process is completely irreversible. In the picture in my answer above, if we simply remove the wall between the two parts the system will come to equilibrium without producing any work whatsoever. | |
| Mar 14, 2023 at 21:09 | comment | added | Jimmy Yang | It seems like all the work done by the system is caused by the free energy change but are there any situations when the system does no work but its free energy still changes? | |
| Mar 14, 2023 at 11:18 | comment | added | Themis | Absolutely, any of these types of work are included. Indeed, this is how we analyze electrochemical cells; in this case we would rather use the Gibbs energy $G=U+PV-TS$ since we typically study the problem under constant $T$ and $P$ (Helmholtz requires constant $T$ and $V$). | |
| Mar 14, 2023 at 2:02 | comment | added | Jimmy Yang | I think it could be electrical work from the change in chemical potential...? | |
| Mar 14, 2023 at 1:00 | comment | added | Themis | @JimmyYang Yes, it refers to any type of work. But can you give an example of what you have in mind? | |
| Mar 13, 2023 at 22:19 | comment | added | Jimmy Yang | Thank you. One more thing, $W$ refers to any kind of work, not just compression/expansion mechanical work, right? | |
| Mar 13, 2023 at 20:31 | comment | added | Themis | @JimmyYang (1) Yes, minimization od $A$ requires thermal as well mechanical and chemical equilibrium (2) For the calculation of $\Delta A$ (or any other property) all states must be in equilibrium. More specifically, each part must be in internal equilibrium, though parts need not be in equilibrium with each other. Equilibrium thermodynamics can only deal with states in internal equilibrium. | |
| Mar 13, 2023 at 18:53 | comment | added | Jimmy Yang | When calculating the difference in $A$, does the final state have to be the equilibrium state? Can it take non-equilibrium states? | |
| Mar 13, 2023 at 18:03 | comment | added | Jimmy Yang | I mean you provided an example of the case where although the system is in thermal equilibrium with the surroundings but not necessary mechanical and chemical equilibrium. The Helmholtz energy is minimized when there’s thermal, mechanical and chemical equilibrium. | |
| Mar 13, 2023 at 13:41 | comment | added | Themis | @JimmyYang Not sure what you mean by "two kinds of equilibrium states". We distinguish equilibrium into three types: thermal (uniformity of temperature), mechanical (uniformity of pressure) and chemical (uniformity of chemical potential). Each can be established independently, depending on the constraints of the system. Is this what you are referring to? | |
| Mar 13, 2023 at 5:53 | comment | added | Jimmy Yang | I see, I was confused in the first place because I didn't know there are two kinds of equilibrium states: thermal equilibrium state and 'equilibrium' state (minimum energy state). | |
| Mar 13, 2023 at 5:49 | comment | added | Jimmy Yang | Wow thank you, I am starting to get it now. If you don't mind, I have some more questions: 1. So the maximum work that can be extracted from the system is $W=\Delta A\le0$ but what this work can be used for if it cannot be done against surroundings? 2. Is the work extracted $W$ spontaneous or not? I assume it is spontaneous because the process of reaching equilibrium is spontaneous? | |
| Mar 12, 2023 at 21:16 | history | edited | Themis | CC BY-SA 4.0 |
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| Mar 12, 2023 at 21:04 | history | answered | Themis | CC BY-SA 4.0 |