Spend hours cleaning your desk, your basement, your attic, and it seems to spontaneously revert back to disorder and chaos before your eyes. If you assert that nature tends to take things from order to disorder and give an example or two, then you will get almost universal recognition and assent. Entropy is a crucial microscopic concept for describing the thermodynamics of systems of molecules, and the assignment of entropy to macroscopic objects like bricks is of no apparent practical value except as an introductory visualization. If the contributions to entropy involves the multiplicity of the ways that the vast number of molecules in the two stacks of bricks can be arranged, then the fact that the macroscopic orientations of the bricks is different is a negligible contribution to the total entropy. The ordered bricks vs the jumbled pile may be a useful introductory visualization, but if these two piles of bricks were at the same temperature, then the numerical value of the entropy would be almost identical for the two stacks. Objections to this kind of introduction to entropy come from the fact that the useful applications of entropy are thermodynamic ones and involve nature on the atomic and molecular scale. Again, the idea of multiplicity is the key point. The bottom diagram is again a picture with which common experience immediately identifies - there are more ways to create a jumbled pile of bricks than a neatly stacked arrangement. So multiplicity is the key concept - molecular ensembles will spontaneously tend to evolve from configurations of lower multiplicity to configurations of greater multiplicity. The diagram at left depicts a more random or disordered configuration, but the key point is that there is a vast number of ways that such configurations could be achieved. If it occurred, it would be for a brief instant and then the molecules would move to some other configuration. At ordinary temperatures, the internal energy of a gas would give the molecules high velocities, and it is evident that this orderly arrangement would be very rare because there are only a few ways to do it. The top diagram depicts time's arrow as pointing from order to disorder, but one must admit that the apparent tendency to move from order to disorder is not the most fundamental way to look at the top diagram. The very fact of differences of opinion on the use of order and disorder can itself be instructive. Chemists, on the other hand, often protest this approach because in chemical applications order vs disorder doesn't communicate the needed ideas on the molecular level and can indeed be misleading. It is typical for physicists to use this kind of introduction because it quickly introduces the concept of multiplicity in a visual, physical way with analogies in our common experience. The diagrams above have generated a lively discussion, partly because of the use of order vs disorder in the conceptual introduction of entropy. In teaching thermodynamics, I have always tried to keep in mind the words of one of the great physics teachers of the mid-20th century, Mark Zemansky: "Teaching thermal physics There are some subtleties in the nature of entropy and other thermodynamic quantities, subtleties that we try to put into word pictures and sketches which sometimes oversimplify. For an isolated system, entropy always increases or remains the same, so if you compare states of different entropy, the one with the greater entropy will be later in time. Using Newton's laws to describe the motion of the molecules would not tell you which came first. This tells us that the right hand box of molecules happened before the left. ( uncountable ) The tendency of a system that is left to itself to descend into chaos.One of the ideas involved in the concept of entropy is that nature tends from order to disorder in isolated systems.( statistics, information theory, countable ) A measure of the amount of information and noise present in a signal.The dispersal of energy how much energy is spread out in a process, or how widely spread out it becomes, at a specific temperature.The capacity factor for thermal energy that is hidden with respect to temperature.( thermodynamics, countable ) A measure of the amount of energy in a physical system that cannot be used to do work.( Boltzmann definition ) A measure of the disorder directly proportional to the natural logarithm of the number of microstates yielding an equivalent thermodynamic macrostate.A measure of the disorder present in a system.Entropy ( countable and uncountable, plural entropies)
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