By James Barrante
Scientists tend to throw words around that we sometimes mistakenly assume the layperson will understand. So it should not surprise us when laypeople or individuals from other areas of science misinterpret a word, or do not use it correctly. One such word is "entropy."
People generally associate the word entropy with some idea of order or disorder. What most laypeople do not understand is that the word only has specific meaning in an area of physics called "thermodynamics," and the systems to which the systems apply are those at the atomic level.
For example, I have heard people say a shuffled deck of cards is at a higher entropy than one ordered by suit and number. Nothing could be further from the truth. Or, similarly, that a messy bedroom is at a higher entropy than a tidy bedroom. They will use as an argument, "What are the chances that if you throw an ordered deck of cards in the air and have it fall, it will return with all the cards ordered by suit and number?" My answer to this is a question. "What are the chances that if you throw a shuffled deck of cards in the air and have it fall, it will return with the cards ordered exactly as they were in the shuffled deck before you threw it in the air?" Any ordering of the deck is equally probable. They are all at the same entropy.
So what is entropy? A simple definition is that it is a measure of lost work. For example, if you drop a book to the floor, it does no work as it falls. You could have done work by dropping the book. That is, you could have rigged the book with a pulley and rope, so that as it fell, it lifted another object, doing work in the process. That work is lost forever by just dropping the book.
You might argue, "I'll lift the book up and then drop it again with the pulley system in place and recover that lost work." Sorry! Once the book has fallen, it will take work to lift the book up again.
No, that work can never be recovered, and the work lost will cause the entropy of the universe to increase.
So every time you design a machine and it doesn't do as much work as thermodynamically possible (that's where efficiency comes in), the entropy of the universe increases by an amount equal to the lost work.
So what about this order-disorder thing? This is a more subtle and, perhaps, a more accurate description of entropy. It has to do with the energy states a system can occupy. All particles, atoms, and molecules occupy energy states. And the more energy states available to be occupied, the higher the entropy.
If we were to cool a system to absolute zero (the lowest possible temperature, equaling minus 460), only one energy state is possible, and the entropy of the system is zero.
As one heats the system up, its entropy increases, since more energy states become available (hot solids have more energy states than cool solids, liquids more states than solids, gases more states than liquids). Liquids are more disordered than solids, gases are more disordered than liquids, entropy increases from solid to liquid to gas.
You can excite an electron (those little negative things zipping around the nucleus of an atom like planets around the sun) in an atom to a higher energy by zapping it with electricity.
The electron will then drop back to a lower energy state, releasing a photon (a particle of light) to the surroundings. Why? Most people would say that it is because the electron is lowering its energy. But the energy lost by the electron is gained by surroundings. It's the emitted photon that drives the process.
There are more energy states for the photon in the surroundings than in the atom. Its entropy increases when it is emitted.
It's entropy, not energy, that drives the process.
James Barrante of Cheshire is a retired college professor of physical chemistry.