Engines of Our Ingenuity

No. 732:
A THERMODYNAMICS CLASS

by John H. Lienhard

Click here for audio of Episode 732.

Today, you can help me teach thermodynamics. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

Today, the semester begins. I teach thermodynamics to a new class. That can be fairly arcane. I'll practice by giving you the whole course in three minutes.

Every time anything happens -- anything at all -- both energy and matter are changed. Electricity turns into thermal energy in your stove coils. They glow red hot. That energy flows as heat into your tea kettle. It transforms water into steam.

Every time anything happens -- anything at all. I inhale. My lungs are cooled. I burn the air. I am fueled and empowered. I pull the shade. Solar radiation floods the room. The air expands. My skin warms. My blood burns less oxygen. I'm changed.

Everything that happens has to obey two rules. One is that all that energy constantly changes, but its sum in the universe stays the same. Cool the sun and you warm the earth. Warm an ice cube and cool lemonade. A leaf falls. Its tiny potential energy dissipates in air friction and in a delicate impact with the earth. Earth and air are warmed -- too little to notice.

The second rule is harder. It's asymetrical. It says those changes have direction. Energy will never flow back out of the water to recreate electricity.

Now the plot thickens. Suppose it's your birthday and you get a telegram. You open it. If it says, "Congratulations!" that's nice, but no surprise. But if it says, "Your father has fallen ill," that's information.

Enter a cool room and find a glowing white hot poker in it. That's interesting. Wait an hour. The room is warm and the poker is at room temperature. That's not interesting at all. So we create a measure of lack-of-surprise. We call it entropy. Unopened telegrams and white hot pokers have low entropy.

Energy will never flow from the room to heat the poker back up. That telegram can never be unread. It cannot surprise you a second time. In either case the entropy is now very high.

Now I enter the classroom again. I go to dress these simple ideas in their full mathematical regalia. The students will follow the math and trip on the ideas. They always do.

How can I tell them that their lives are so constrained? How can I make them see any glint of hope behind these limitations? They cannot create energy that wasn't already there. They will, one day, maximize their own entropy and rejoin the earth.

How can I make them see that we would not want things otherwise? How can I make them know that life is a glorious game of optimizing our joy within immutable constraints?

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)


Since thermodynamics is a favorite subject of mine, I've done many episodes that deal with aspects of the subject. For other aspects, use the search function on this website.

Thermodynamics rests upon a set of axioms about nature -- physical laws. We erect mathematical descriptions of nature upon those laws. The problem is that it is impossible to create an axiomatic system that is complete, coherent, and consistent. Consequently all systems of thermodynamic laws are ultimately flawed.

Many systems of thermodynamic laws have been formulated. Each strives to be cleaner than the next. In this episode I've made reference to two of the four axioms that make up the most traditional system. Those laws are,

First Law: Energy can be neither created nor destroyed.
Second Law: (This can be said in many ways. I prefer:)
The entropy of an isolated system can never decrease.
Third Law: The entropy of any system approaches zero as its temperature approaches absolute zero.
Zeroth Law: (After the other three laws were erected, people realized that a fourth principle should precede the other three.)
Any two bodies in equilibrium with a third body are in equilibrium with each other.

We call the Third Law into play only in certain special systems. The Zeroth Law defines thermodynamic equilibrium. Once that's done, it doesn't have much other use. The first and second laws are the ones we use heavily. One wag paraphrased them this way:

First Law: You don't get something for nothing.
Second Law: You can't even break even.

The Engines of Our Ingenuity is Copyright © 1988-1997 by John H. Lienhard.

Previous Episode | Search Episodes | Index | Home | Next Episode