Have you ever wondered what happens to the heat the iron generates, the sound of words we speak, the songs we sing or the noise we make when we clap or beat a drum? I found answers in Entropy.

These posts are not about the science of Entropy. To learn what Entropy is, you might want to start with the Wikipedia article. The texts I have cited are also useful.

Entropy is a physical property of a substance such as pressure, density, heat capacity, etc. Entropy is dependent on temperature, pressure and phase. Entropy is often presented as a measure of disorder or randomness of a system. However, Entropy has two major attributes: Irreversibility and Disorder. I thought of presenting a series of posts in layman's terms about Entropy explaining my views from the perspectives of Irreversibility and Disorder.

Finding Lost Energy

We know that energy is conserved. The electric iron we use to iron our clothes convert electricity to heat energy. I have often wondered what happens to the heat energy that the iron generates? If energy is conserved, what happened to the heat the clothes absorbed during ironing? Where did it go?

I have often wondered what happens to the sound of words we speak, the songs we sing or the noise we make when we clap or beat a drum. When we speak for example, our vocal cords compress air around it creating sound. Sound essentially is a pressure wave in motion. Sound eventually fades and becomes silent. What happened to the sound energy when it faded? Where did it go?

Simple answer - the lost energies were used to generate Entropy. I will offer a detailed explanation (in layman's terms of course) in a later post.

Disorder

Disorder is an attribute Entropy is commonly associated with. Metaphorically, an organized bookshelf getting messy and disorganized can be thought to be increasing the Entropy of the bookshelf. Because when the bookshelf was organized, finding a book is easy and predictable. But if the bookshelf is disorganized, the chance or the probability of not finding a book is much higher. Thus, when a bookshelf is moved from being organized to being disorganized, it generates Entropy.

If there are rats and cockroaches in the house, if your plumbing is not working, if your shoes are broken or your fridge is not working, it would relate to a condition of disorder in your life.

I am mortally afraid of cockroaches. At the mere site of a cockroach I would scream "MORTEIINNN!!". My behavior would be unpredictable. However, I can tolerate rats. Having a few rats running around would not stress me out. My cousin for example is afraid of rats but not of cockroaches. My mom is mortally afraid of snakes. The site of a rat-snake on TV is enough cause for panic. However, she is not afraid of cockroaches.

These behaviors can help to better understand Entropy. Entropy is a measure of disorder or randomness of a system. In my case, the mere site of cockroaches increases my Entropy, but rats and snakes would not have much effect on my Entropy. In the case of my cousin, rats would increase his Entropy but not snakes or cockroaches. In the case of my mom, rats and snakes increase her Entropy but not cockroaches.

Substances behave differently when they are taken from one state to another.
For example:

• 1kg water at 100 degC at standard atmospheric pressure has an Entropy of 1.31 kJ/K
• 1kg steam at 100 degC at standard atmospheric pressure has an Entropy of 7.35 kJ/K
• 1kg of superheated steam at 200 deg C at standard atmospheric pressure has an Entropy of 7.83 kJ/K

Irreversibility

Let's use some real-life metaphors to explain this part of the story. We often say and do things that we cannot undo, leaving permanent trails of our actions. We face situations where our actions change the course of our lives. Sometimes we regret making bad decisions. These are Irreversibilities in our lives - things we cannot undo. When we do such acts, we increase the Entropy of our system. We may move from one situation to another, but how we move can be attributed to Entropy.

We know now that when a system is taken from one state to another, there is a change in Entropy. This is called the Change in Entropy of the System. If the process in which the system was taken between these two states were Reversible, then there will not be any Entropy Generated by the System. However, if the process is Irreversible, then there will be some Entropy Generated by the System. This is based on the Second Law of Thermodynamics. I will explain this theory in the next post.

So now we have two concepts to deal with: Change in Entropy of the System and the Entropy Generated by the System.

A scene from Pyramus & Thisby: Bottom is transformed to an ass. Then he is transformed back to a man. A Reversible process.

Consider cooking. We transform food from uncooked vegetables and meat to a tasty meal by cooking it. Sometimes we add too much or too little salt, sauces, spices, etc. Most of the time these actions cannot be undone. If we add too much salt while cooking noodles, we cannot undo this action. Thus the process is Irreversible.

The difference in the two processes, Reversible and Irreversible, is in how much Entropy they generate while transforming uncooked food to a meal. A Reversible process generates much less Entropy than an Irreversible process does.

Consider war. Two parties battle over a dispute. During this process, there might be actions that cannot be undone. Collateral damage for example. A life lost is a tragedy. It cannot be undone and thus Irreversible. However, buildings damaged can be rebuilt and thus reversible.

During war there is chaos and disorder and thus a nation at war has a higher Entropy than if they were at peace. If war is to bring about peace, then this means moving from a higher Entropy state (war) to a lower Entropy state (peace). If the path taken to move between these states have Irreversibilities, the system would have generated Entropy, despite peace having a lower Entropy at the end of the process.

In the next post we will evaluate the Second Law of Thermodynamics and discuss Change in Entropy of a System and the Entropy Generated by a System.

To be continued...

References

1. T.D Eastop, A. McConkey (2003). Applied Thermodynamics for Engineering Technologists, 5th Edition
2. Yunus A. Cengel (1997). Introduction to Thermodynamics and Heat Transfer, International Edition