Reversible Processes

This post is part of a series, Nonsense and the Second Law of Thermodynamics.  The previous post is entitled:  Entropy is Not a Measure of Disorder.

To understand the macroscopic thermodynamic definition of entropy,  it is important to understand something called a reversible process.  A reversible process is just what it sounds like: a process that is reversible.

A reversible process should be thought of as an ideal case. In a reversible process, the system is in equilibrium for every infinitesimal step of the process.  Imagine a balloon filled with gas, and imagine that the balloon is perfect, i.e., we need not concern ourselves with the properties of the balloon itself: we care only about the gas inside the balloon and the gas outside the balloon.

At equilibrium, the pressure on each side of the balloon is equal.  If the pressure outside of the balloon is reduced, the balloon expands until the pressures are equal again.  In a reversible process, the balloon is allowed to expand continuously by infinitesimal steps.  The reversible process acts as a  limit to any real process.

When a reversible process is reversed, it leaves both the system and the surroundings in the same state they started in.  If we define the universe as the system plus the surroundings, the universe remains unchanged after a reversible process has taken place.

Compare a reversible process to an irreversible process.  Consider climbing a mountain, climbing back down and returning to the original position at the base of the mountain.  The climber is at the same place that he or she started, but the surroundings have changed.

The climber gave off heat during the ascent and descent, and the surroundings are now an unnoticeably little bit warmer.  The climber has changed as well, having burnt calories.  One need not go into all of the details to understand that this process is not reversible.

Consider a perfect frictionless pendulum: as it falls it converts potential energy into kinetic energy, as it rises it converts the kinetic energy back into potential energy and the cycle continues indefinitely.

Again, notice that the reversible process is an ideal, but unattainable process.  The next post discusses a cyclical reversible process used to convert heat into work. The next post is entitled The Carnot Cycle.

Sources

• Atkins, P. W. Physical Chemistry, W. H. Freeman and Company, New York, 3rd edition, 1986
• McQuarrie, Donal d A., Statistical Thermodynamics,  University Science Books, Mill Valley, CA, 1973 
• Bromberg, J. Philip, Physical Chemistry, Allan and Bacon, Inc., Boston, 2nd Edition, 1984
• Anderson, H.C., Stanford University, Lectures on Statistical Thermodynamics, ca. 1990.

Contents

1. Introduction
2. What the Second Law Does Not Say
3. What the Second Law Does Say
4. Entropy is Not a Measure of Disorder
5. Reversible Processes
6. The Carnot Cycle
7. The Definition of Entropy
8. Perpetual Motion
9. The Hydrogen Economy
10. Heat Can Be Transferred From a Cold Body to a Hot Body: The Air Conditioner
11. The Second Law and Swamp Coolers
12. Entropy and Statistical Thermodynamics
13. Fluctuations
14. Partition Functions
15. Entropy and Information Theory
16. The Second Law and Creationism
17. Entropy as Religious, Spiritual, or Self-Help Metaphor
18. Free Energy
19. Spontaneous Change and Equilibrium
20. The Second Law, Radiative Transfer, and Global Warming
21. The Second Law, Microscopic Reversibility, and Small Systems
22. The Arrow of Time
23. The Heat Death of the Universe
24. Gravity and Entropy
25. The Second Law and Nietzsche's Eternal Recurrence
26. Conclusion