Undstanding Thermodynamics

Knowledge of today’s Thermodynamic principles came little by little to our ancestors . As time went on, they made educated guesses based on experiments; sometimes they were spot-on, other times they were wrong. According to MIT, “...Our predecessors faced problems and situations that they couldn't explain with existing principles in Physics and Mathematics. Consequently, they proceeded to create new theories and 'laws' verifying their hypotheses with experiments...”

The Historical Progression of Classical Thermodynamics:

The Preclassical Era (1600 - 1840)
People like Galileo, Black and Count Rumford developed Physics mainly, focusing on experiments. Towards the ending of this era, however, Joule and Carnot proposed the Work and Heat Concepts , leading to the next era---

The Classical Era (1840 – 1900)
Scientists like Maxwell, Clausius, Lord Kelvin and Boltzmann studied and further developed Physics, Mathematics and Mechanical Engineering, focusing on laws and postulates.
Gibbs proposed the Statistical and Chemical Thermodynamics postulates, which led to the next era---

The Modern Classical Era (1900 – 2000s)
Chemical and Molecular Thermodynamics was developed, with focus on non-ideal (real) fluids, phase and chemical equilibrium and stability, and this is where we are presently.

The First Law of Thermodynamics:
Before moving on to the 1st Law, a proper understanding of reversible and irreversible processes is required. I’ll just outline the characteristics of reversible processes (from MIT’s Open Courseware)-

1. If any (real or ideal) system in a non-equilibrium state is isolated, it will tend toward a state of equilibrium.

2. All real or natural processes are not reversible. Hence reversible processes are only idealizations that are very useful in showing limiting behavior. The performance of real processes is frequently compared with ideal performance under reversible conditions.

3. A system undergoing a reversible process is no more than differentially removed from an equilibrium state – the system passes through a set of equilibrium states.

4. "A process will be called reversible if a second process could be performed in at least one way so that the system and all elements of its environment can be restored to their respective initial states, except for differential changes of second order." For example, in a reversible expansion or compression δ(δW) ≈ dPdV

5. If a cyclic process A → B → A is reversible, then when the process is carried out, no changes will occur in any other bodies. For example, if A → B involves the absorption of a quantity of heat Q, then B → A will reject the same quantity Q to the environment.

6. Any reversible process is also quasi-static, but the reverse is not necessarily true (a quasi-static process is…? I will find out).

7. Simple systems undergoing reversible processes have no internal gradients of temperature or pressure.

8. Friction and other dissipative forces are not present in reversible processes. A truly reversible process will always require an infinitesimal driving force to ensure that energy transfer occurs without degradation, hence its rate would be infinitely slow. Therefore, a reversible process always can be shown to require a minimum amount of work or will yield a maximum amount of work.

9. Heat engines in reversible processes operate at maximum efficiency. (Why? Find out in good time)

10. If the processes by which heat is transferred or work done on/by the system are reversible, then we can calculate numerical values of for the Q and W (of the 1st Law equation) and work out the internal energy change.

11. If the processes by which heat is transferred or work done on/by the system are irreversible then, values at the start and end of the irreversible process are required to calculate the change in internal energy.


Much more info at:
1. Introductory Chemical Engineering Thermodynamics; http://www.egr.msu.edu
2. MIT Supplementary Open Courseware notes; http://ocw.mit.edu
3. Reversible Processes; http://lorien.ncl.ac.uk/ming/Webnotes/Therm1/revers/revers.htm
Thermodynamics; http://lorien.ncl.ac.uk/ming/Webnotes/Therm1.htm



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