The Laws of Thermodynamics

If one looks around the Internet, he or she can find a multitude of pages that state the laws of thermodynamics; so why add one more?  I've been writing a lot of posts on thermodynamics, but nowhere have I given a concise statement of the laws; also, I am not always happy with how the laws are described.  So this post is my contribution.

The Zeroth Law of Thermodynamics

The zeroth law states:

If two bodies are in thermal equilibrium with a third body, then they are in thermal equilibrium with each other.

The Second Law, Radiative Transfer, and Global Warming

This post is part of a series, Nonsense and the Second Law of Thermodynamics. The previous post is entitled Spontaneous Change and Equilibrium.

Does global warming violate the second law of thermodynamics?  Such a claim may seem strange.  The idea that the vast majority of physical scientists would subscribe to an idea that somehow violates a fundamental law of thermodynamics on its face seems odd.  Yet, such a claim is often made by people calling the science behind global warming into question.

Free Energy

This post is part of a series,Nonsense and the Second Law of Thermodynamics. The previous post is entitled Entropy as Religious, Spiritual or Self-Help Metaphor.

You cannot get something for nothing, and the term "free energy" does not mean energy that has no cost.  Rather it refers to energy that is available to do something useful.  The use of the word "free" is in the sense of "liberated."  Free energy is energy that can be liberated to do something useful.

We know that the second law places limitations on how much energy can be used to do useful work.  In most situations, some of the energy must be dissipated as heat that cannot be used to do something useful.  The portion of the energy that can do something useful is the free energy.

At constant volume, the free energy is the Helmholtz Free Energy, given the variable A.

Enthalpy

This post continues a tangent  from my series on the second law of thermodynamics.  It discusses another quantity in thermodynamics, but it is necessary before I can get to the next post in the series, which is on free energy.

This post discusses the term enthalpy.

At constant pressure the change in enthalpy is the heat transferred to a system.

      ΔH = q  at constant pressure.

Heat is not a state function, but enthalpy is.

The First Law of Thermodynamics

I need to take a tangent from my series on the second law of thermodynamics and discuss the first law of thermodynamics. 

Heat is not a conserved quantity.  Work is not a conserved quantity, but the sum of heat and work is a conserved quantity. The first law is related to the law of conservation of energy; in fact it is one case of that law.

Partition Functions

This post is part of a series,Nonsense and the Second Law of Thermodynamics. The previous post is entitled Fluctuations.

In previous posts it was shown that entropy is related to the the number of ways that a system can arrange itself subject to constraints such as constant energy.

The previous post on fluctuations showed that for very large numbers that fluctuation from the most probable distribution, are insignificant.  Most of the distribution is contained within the square root of N of the most probable result.

Fluctuations

This post is part of a series,Nonsense and the Second Law of Thermodynamics The previous post is entitled Entropy and Statistical Dynamics.

The second law of thermodynamics works because of the statistics of very large numbers. Consider a bouncing ball: as it bounces, it dissipates heat and eventually does not bounce as high.  

Entropy and Statistical Thermodynamics

This post is part of a series, Nonsense and the Second Law of Thermodynamics. The previous post is entitled The Second Law and Swamp Coolers.

A previous post discusses the macroscopic thermodynamic definition of entropy, but there is another, statistical way of describing entropy.  Consider an isolated macroscopic system of interacting molecules.  Without knowing much about what is going on with the individual molecules, it is possible to measure macroscopic thermodynamic properties such as the pressure, the temperature etc.

                                                                                         (Figure Source)

Consider that the system is isolated; so that the total energy of the entire system of molecules is a constant.  Energy is free to move from one molecule to another, and each molecule has multiple electronic, vibrational, rotational, and translational energy states that it could be in.  There are many distinguishable ways that the system could be arranged to achieve the this energy.

The Second Law and Swamp Coolers

This post is part of a series, Nonsense and the Second Law of Thermodynamics. The previous post is entitled Heat Can Be Transferred From a Cold Body to a Hot Body: The Air Conditioner.

When I was an undergraduate, I had a physical chemistry professor who claimed that air conditioners that were completely inside a room could not possibly work.  Opening the refrigerator door on a hot day will not make your house cooler.

The refrigerator gives off more heat than it transfers from inside itself; a refrigerator is actually heating the house.  If the door is left open, the refrigerator works harder to try to maintain a cool temperature in accordance with its thermostat setting.  As the refrigerator works harder, it releases more heat into the house.

My professor, however, was not correct.  It is possible to have a cooling unit that does more cooling than it releases heat to the environment.  The trick with indoor coolers is that the process by which they operate is not cyclic.

Heat Can Be Transferred From a Cold Body to a Hot Body: The Air Conditioner

This post is part of a series, Nonsense and the Second Law of Thermodynamics. The previous post is entitled The Hydrogen Economy.

As of 10/26/2010, a survey on this site shows that 25% (Final result 21%) of the respondents thus far think that the second law of thermodynamics says that heat cannot be transferred from a cold body to a hot body.

Not only are these people mistaken, but they are also ignoring their own common experience of the world.

It is possible to transfer heat from a cold reservoir to a hot reservoir! 

The Hydrogen Economy

This post is part of a series, Nonsense and the Second Law of Thermodynamics. The previous post is entitled Perpetual Motion.

The media often perpetuate the idea that the so-called hydrogen economy is the solution to all of our energy needs.  Hydrogen is abundant everywhere; in fact there are oceans full of hydrogen in the form of water, just waiting to be extracted, oxidized and used as an endless source of energy, right?

Perpetual Motion

This post is part of a series, Nonsense and the Second Law of Thermodynamics. The previous post is entitled The Definition of Entropy.

It is a consequence of conservation of energy and the second law of thermodynamics that it is impossible to build a perpetual motion machine. There are many types of proposed perpetual motion machines.

There is a post that goes into a lot of detail of the various sorts of perpetual motion machines by Kevin T. Kilty, entitled Perpetual Motion.   Rather than go into arcane detail about different types of perpetual motion machines, I think it suffices to refer the interested reader to Kilty's post.

No machine can generate more more energy than put in (first law of thermodyanics, conservation of energy, Noether's theorem).  The first law of thermodynamics states that work can be converted into heat, and heat can be converted into work, but that the sum, the so-called internal energy (E or U) is a conserved quantity.

The Definition of Entropy

This post is part of a series, Nonsense and the Second Law of Thermodynamics. The previous post is entitled: The Carnot Cycle.  This post is heavily dependent on the previous post; so I recommend reading it first. 

Let q represent the heat transferred in a process, and qrev represent the heat transferred in a reversible process. Let T be the absolute temperature (in Kelvin).

The sum of  qrev/T for all steps of the process over a full Carnot cycle is equal to zero.  In fact, it is true for any reversible cyclic process.

The Carnot Cycle

This post is part of a series, Nonsense and the Second Law of Thermodynamics.  The previous post is entitled:  Reversible Processes.

In 1824, Nicolas Léonard Sadi Carnot tried to explain how heat could be converted into useful work. He came up with a four-step cycle that is known as the Carnot cycle.

Entropy Is Not a Measure of Disorder

This post is part of a series, Nonsense and the Second Law of Thermodynamics.  The previous post is entitled:  What the Second Law Does Say.

Entropy is not a measure of disorder.  Entropy is not a measure of disorder.

To paraphrase Stanford  Professor H.C. Anderson, there are a lot of sentences in the English language that contain the words "entropy" and "disorder," and most of them are wrong.  There are many reputable text books and sources that say that entropy is disorder; nevertheless, entropy is not a measure of disorder.

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.

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

Introduction

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.

Thinking About Oil Rain

Recently, there has been discussion on the Internet of a video that purportedly shows evidence of it raining oil in Louisiana.



Is this evidence of oil rain? Is oil rain possible?

Radiative Transfer

If you are following this primer on infrared spectroscopy and global warming you already have some of the basics of radiative transfer.  The previous post in this series develops a simple multi-layer model of the carbon dioxide in the troposphere.  It leaves out many important features but shows conceptually how absorption and emission behave in layers of the troposphere.

The current post is intended to wrap up the topic and touch upon a few issues that were not discussed. It is possible to teach a year-long course in radiative transfer (or even multiple courses); so of course this post does not do the topic justice, but perhaps it provides some basic principles that give the reader a cursory understanding of the topic.