1Principles of Probability
Principles of 1Probability The Principles of Probability Are the Foundations of Entropy Fluids flow, boil, freeze, and evaporate. Solids melt and deform. Oil and wa- ter don’t mix. Metals and semiconductors conduct electricity. Crystals grow. Chemicals react and rearrange, take up heat and give it off. Rubber stretches and retracts. Proteins catalyze biological reactions. What forces drive these processes? This question is addressed by statistical thermodynamics, a set of tools for modeling molecular forces and behavior, and a language for interpret- ing experiments. The challenge in understanding these behaviors is that the properties that can be measured and controlled, such as density, temperature, pressure, heat capacity, molecular radius, or equilibrium constants, do not predict the tenden- cies and equilibria of systems in a simple and direct way. To predict equilibria, we must step into a different world, where we use the language of energy, en- tropy, enthalpy, and free energy. Measuring the density of liquid water just below its boiling temperature does not hint at the surprise that just a few de- grees higher, above the boiling temperature, the density suddenly drops more than a thousandfold. To predict density changes and other measurable prop- erties, you need to know about the driving forces, the entropies and energies. We begin with entropy. Entropy is one of the most fundamental concepts in statistical thermody- namics. It describes the tendency of matter toward disorder. The concepts that 1 we introduce in this chapter, probability, multiplicity, combinatorics, averages, and distribution functions, provide a foundation for describing entropy. What Is Probability? Here are two statements of probability.
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