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Tackling Entropy

Paul Kando

Without a basic understanding of entropy (and the laws of thermodynamics), we cannot deal effectively with anything related to energy, like home eating or getting around. So let’s not allow precise scientific definitions deter “the uninitiated”. Let’s begin, instead, with "irreversibility", an idea central to understanding entropy. Popular expressions like "You can't unscramble an egg", "You can't take the cream out of the coffee" and “No use crying over spilled milk” imply a common intuitive understanding of irreversibility. We instantly recognize a home movie running in reverse. It shows impossible events — water jumping out of a glass into a pitcher above it, smoke going down a chimney, water in a glass freezing to form ice cubes, and crashed cars reassembling themselves.

Entropy and Energy

Pouring water from a pitcher, smoke going up a chimney, etc. are "irreversible" processes: they cannot happen in reverse. Nor can physical processes involving systems in everyday life: “in any isolated system, the thermodynamic state variable known as entropy is always increasing”. A movie running in reverse shows processes in which entropy is decreasing, a physical impossibility.

In practical terms, the law of entropy (a.k.a. Second Law of Thermodynamics) means that (1) heat will not flow spontaneously from a cold object to a hot one, so the water in your baseboards must be hotter than the desired temperature of your living room. (2) No heat engine can extract heat and convert it all to useful work. Your car’s MPG is limited by its efficiency. (3) A machine that in its operation converts another form of energy to heat is less efficient than one that is purely mechanical. A heat pump consumes less electricity than an electric resistance heater. Hybrid cars use less fuel because they are mostly powered by an electric motor rather than one burning gasoline. (4) We can spend hours organizing our desk, basement, or attic, only to soon see them revert back to disorder, unless we invest additional effort to maintain order.

In Rudolf Clausius’ original formulation (1865), A system free of external influences becomes more disordered with time and the measure of this “degree of disorder” (a now disputed term) is called entropy. "Multiplicity of possibilities" is another way to express the idea of “degree of disorder”: A pair of dice can produce a 7 by six combinations but there is only one way to produce a 2. A 7 is more probable, has greater multiplicity than a 2. Likewise, CO2 molecules that float in space, are more dissipated than when they are locked into a complex sugar molecule produced by plant.

But if all systems naturally progress from “order to disorder”, isn’t it a violation of the law of entropy, when biological systems develop and maintain such a high degree of order as manifest in a living plant or animal? No, because this order is produced and maintained by an infusion of energy – energy from the Sun.

Armed with these basics of thermodynamics, it should be easier to relate to the energy relationships we encounter in life.