[mahdialloush]

Dear All,

I have a question. We have plenty forms for the energy equation: total energy, enthalpy, internal energy .. etc

What are the advantages/disadvantages of using each? and are there specific conditions under which we can use any of the equations?

Regards

[LuckyTran]

It's just a matter of using the right tool for a job. You wouldn't use a sledgehammer to drive a nail but you certainly would to build a railroad.

As an aside, the Maxwell relations gives a nice mathematical framework for one potential versus another, which has real physical consequences in applications. Some quantities are easy to measure in one framework and not in another. And certainly there are situations where it is more advantageous to work with one quantity over another whereas sometimes it just cumbersome dealing with a particular quantity.

Consider internal energy versus enthalpy in an example of heat transfer in a simple stationary tube. In the adiabatic case, the internal energy is not constant in the tube but the enthalpy remains constant. So knowing that enthalpy is constant reduces this two parameter problem in P,T to a single parameter problem P w/ H=constant

Now consider a constant heat flux in the tube and let the fluid be exotic so that there are large non-linear changes in specific heat of the fluid (such as near a critical point). The internal energy change is non-linear with position but the change in enthalpy is linear with position.

Sometimes internal energy and enthalpy just don't give enough. For example when species transport is involved. Internal energy and enthalpy doesn't easily explain why the species tends to diffuse into one another. Then you bring in the chemical potential and voila it becomes easy again.

[mahdialloush]

Dear LuckTran,
Thank you a lot. Are you able to provide me with some references that discuss this issue?