Entropy: An engineering approach

Forget about Boltzmann or a room full of ping-pong balls flying everywhere or a little devil called Maxwell’s Demon. As an engineer, I  have always heard enough about the nano/microscopic description of things in order to learn (or at least listen to an explanation of) how stuff works, especially important stuff such as matter or energy.

 Don’t get me wrong, I have a healthy physicist’s curiosity as every other Mechanical Engineer. I drink coffee, sit on a chair and read the news without bothering about the fact that the coffee, the chair or the newspaper are actually human perceptions of small bursts of mass and empty spaces between. In the Engineering disciplines, we work with tools and devices that are within a certain human length scale, not micro/nanoscale.  This article attempts to give Thermal Engineers a chance to understand Entropy without resorting to intricate descriptions about chance and disorder.

Entropy is an inherent property of systems. Everyone and everything has an entropy value. Entropy is the measure of how scattered the energy is in a given system. Energy has two features: quantity and quality. This issue is absolutely vital in understanding the Second Law of Thermodynamics and it is a point that is often paid minimal attention.

 A 2 kJ hot-air balloon can be different from a 2 kJ hot-air balloon. No, it is not a mistake. The amount of energy for each air balloon is 2 kJ, of course, but in quality, they can be different. Quality of energy simply means how well that energy can do something useful. There is a simple relation between this quality and the subject of this article, entropy. High quality means low entropy and low quality means high entropy.

 Well, ok. So what? Actually this gives us certain perspective about the problem of something wrongly called energy crisis. Energy is constant, no matter what, which is a consequence of the First Law of Thermodynamics. How can there be a crisis (that is, a shortage) of something that is constant no matter what? Our villain here is entropy, the disaggregation as Clausius called it. Most of the times when energy is transformed from one form to another, entropy steps in and lowers the quality of energy. Again, energy will be constant, but every transformation, unless performed in a very specific way, reduces the ability of that energy to do some useful task. There is a huge amount of energy all around us that will never be able to do anything useful again because entropy has reached a maximum. This condition is called equilibrium. To sum up, men will go great lengths to find sources of energy of low entropy, like oil, uranium or coal. High entropy sources of energy are a complete waste of energy, literally.

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  1. Took me 30+ years of mechanical engineering practice, but I finally figured out what entropy really is, too. It is a measure of the ability of the universe, or some portion thereof, to allow for energy conversions. A simple example: Suppose we had a ball sitting at the top of a hill, about to roll down the hill. At the top of the hill, the stationary ball has potential energy. As it rolls down the hill, that potential energy is converted into kinetic energy. If there is another hill on the other side of the valley, the kinetic energy of the rolling ball will be converted back into potential energy as it rolls up the other side to a stop. That section of the universe at the top of the hill is somehow different from conditions at the bottom – the universe of my hill is heterogeneous, not homogenous. The existence of the hill (in a gravitational field) is entropy. The fact that there are gradients, one area different than another, is what allows for the observed energy conversion. Entropy is a quantitative way of describing the universe’s gradients. Back to our example: Each time the ball rolls down the hill and across the valley and up the other side, then back again, if it wears away a little bit of the hill (2nd Law of Thermo at work), eventually there will be no gradient of hill versus valley, and the conversion between potential and kinetic energy will cease. My hill universe will be homogenous, at a state of maximum entropy. So, without the universe’s built-in characteristic of entropy, there would be no energy transfer, and without energy transfer, there would be no life. Cool, huh?

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