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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.

Saturday, September 25, 2010

What the Second Law Does Not Say

This post is part of a series, Nonsense and the Second Law of Thermodynamics.

The Second Law does not say it is impossible for heat to be transferred from a cold body to a hot body.  The second law does not say that "disorder" must increase on the earth or anywhere else.  Life is not a counter-example to the second law; life is an example of the second law in action.

One has to be very careful about applying statistical results to a single molecule or a few molecules and remembering that increasing entropy applies to irreversible changes, not reversible ones. The second law says nothing about disorder. The second law does not prevent evaporative coolers from operating.

The second law does not contradict radiative transfer theory or global warming. The second law does not contradict conservation of energy.  In applying the second law to  cosmology, one should tread cautiously.

Nonsense and the Second Law of Thermodynamics


The Second Law of Thermodynamics is, perhaps, the most abused physical law of all time. It may be rivaled for that distinction by the Uncertainty Principle, Relativity, and Hawking Radiation, but I think the Second Law probably wins the contest.

There is a plethora of nonsense disseminated on the web and elsewhere that misrepresents what the law actually says. This series is an attempt to curb some of that nonsense.  Along the way, I hope to make some sense of what the second law of thermodynamics actually does say, as well as addressing some of the nonsense that people believe about it.

Wednesday, September 1, 2010

Reformatted Post on Beer's Law

I have had to reformat the post on Beer's Law.  The html editor at Blogspot seems to do strange things with <br> tags.