## Monday, September 27, 2010

### What the Second Law Does Say

This post is part of a series, Nonsense and the Second Law of Thermodynamics.  The previous post is entitled:  What the Second Law Does Not Say.

There are multiple valid ways to state the second law of thermodynamics.  Some ways of expressing the law do so in terms of macroscopic notions such as heat and temperature.

Other descriptions employ the concept of entropy, which is based upon a statistical approach to thermodynamics. Some alternative macroscopic statements include:
• There can be no process with the sole result of absorbing heat and completely converting it into work.
• It is impossible to convert heat completely into work in a cyclic process.
• It is impossible to carry out a cyclic process using an engine connected to two heat reservoirs that will have as its only effect the transfer of a quantity of heat from the low-temperature reservoir to the high-temperature reservoir.
Other descriptions involve a quantity called entropy:
• The entropy of a closed system must increase or remain the same.
• For any irreversible process, the entropy of a system and its surroundings must increase; for a reversible process, the entropy of a system and its surroundings must remain the same.
To understand these statements, it is necessary to understand the term "entropy." The few sections explain entropy from a macroscopic point of view.  The section on statistical mechanics describe it from a statistical point of view.

A convenient way to remember the laws or thermodynamics is:
• First Law: You cannot win.
• Second Law: You cannot break even.
• Third Law:  Let's keep score, or alternatively: you cannot leave the game.
The next few posts start the explanation of entropy from a macroscopic thermodynamic perspective. The next post is entitled: Entropy is Not a Measure of Disorder.

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
• Wikipedia: The Second Law of Thermodynamics
• Anderson, H.C., Stanford University, Lectures on Statistical Thermodynamics, ca. 1990.

#### 1 comment:

Anonymous said...

"There can be no process with the sole result of absorbing heat and completely converting it into work." - of course (in theory) there is such process, isothermal expansion. "process" is not "cycle" in thermodynamics.