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Third Law of Thermodynamics · Acedra
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Third Law of Thermodynamics
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Professor Dave here, let's talk about the third law of thermodynamics.
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We've gone through three of the laws of thermodynamics, which leaves just one to go, and the third law of thermodynamics deals with another strange concept: the idea of a temperature of absolute zero. We know that temperatures can get very hot and very cold, but not everyone is familiar with the idea of the coldest possible temperature. However, with our new understanding of heat and temperature, it should make perfect sense. We know that temperature is a measure of
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available heat energy, which is just a collection of kinetic energies of motion. We know that as the temperature decreases, the internal energy of a system decreases, as does the kinetic energy of each particle. As the temperature becomes very cold, particles slow to a complete stop, and there is no more kinetic energy. No more kinetic energy of motion means no more temperature by any real definition of the term, so we will call this absolute zero. At absolute zero, every substance
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would be a solid, even things like hydrogen and helium, and one way of summarizing the third law of thermodynamics is by saying that the entropy of any crystalline substance at absolute zero will be equal to zero. This is a necessary consequence of Boltzmann's equation, S equals k times the natural log of W. If there is no kinetic energy, nothing is moving, so there is only one possible microstate that the particles can inhabit. With a value for W equal to one, the natural log
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of 1 is 0, so at absolute zero the entropy of any substance must also be zero. However, absolute zero can never truly be reached, which is communicated in a rephrasing of the third law which states that it is impossible to lower the temperature of any system to absolute zero in a finite number of steps.
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Satisfactory analysis of this law will require the quantum mechanical zero-point energy, and will thus have to wait until the modern physics course. This concludes our analysis of the laws of thermodynamics, a near mythical accomplishment in the history of science. It also means we have covered some of the harder stuff in physics, so now we get to move on to fun things like waves and magnets. But first, let's check comprehension.
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