Meaning of thermodynamical potentials (F,H,G,E)

[sisife]

Hello!

I'm trying to grasp the "intuitive" meaning of the thermodynamical potetials E,F,H and G, or at least of their connections.

As in other threads before mentioned, I learned that you can't real.y give an meaningful definition of energy , so I assume for the other potentials this is also true.

As for what energy 'is', you could just as easily as "What is 'red'? The fact that a simple answer doesn;t exist does not make the concept any less useful.

But at least I have a kind of intuition what energy is, in contrary to the other quantities.

I tried to understand G by considering a quasistatic process. Then Q=TS and W=PV. So G=E-TS-PV is the energychange in a system in a non-quasi-static process (e.g. due to friction etc., or due to a difference in the entropy change (but here I am confused about, how this is connected)). Is this correct?

Are there nice ways to illustrate the other ones? Or is this simply a waste of time? (if I don't have an ituition for what I am doing I always find it difficult to find solutions for a problem).


thanks for your help

 

[Count Iblis]

http://en.wikipedia.org/wiki/Helmholtz_free_energy#Minimum_free_energy_and_maximum_work_principles" This is called "A" instead of the more widely used "F" here.

 

[Mapes]

Hi sisife, welcome to PF. First, G is E-TS+PV. Second, it may be easiest to think of the potentials as "barometers" for spontaneity for processes occurring under different conditions. For processes at constant temperature and pressure, G must be negative for any spontaneous process. At constant temperature and volume, the Helmholtz energy (A or F) must be negative, and so on.

These relationships can all be derived from the required entropy increase during any spontaneous process in a closed system.

 

[olgranpappy]

For processes at constant temperature and pressure, G must be negative for any spontaneous process.

You mean to write: For processes at constant temperature and pressure, *the change in* G must be negative for any spontaneous process.

I.e., G decreases...

To the OP, you can compare the above statement to what happens to the energy in mechanical processes--E.g., a ball rolls *down* a hill--a spontaneous process decreases the potential energy.

 

[Mapes]

Right you are! Boy, and I've even https://www.physicsforums.com/showthread.php?p=1746830#post1746830" for that...

 

[sisife]

That's a good picture, that will help.
Thank you!