# Energy and Entropy

The word energy is widely used in everyday life. "I do not have the energy to exercise today" or "the world has an energy crisis" are familiar sayings. We use the word energy so much that it is taken for granted that we know what it means (?). The first law of thermodynamics tells us that energy can neither be created nor destroyed. When hot and cold water are mixed together the heat energy lost by the hot water equals the heat energy gained by the cold water. Another word in the vocabulary of physicists and engineers is entropy. Entropy is an indicator of the disorder of a system and the second law of thermodynamics states that natural processes tend to move toward a state of greater disorder (entropy) as time passes. The entropy of a system can never decrease. Students sometimes think that entropy is mysterious since it can be created, whereas energy cannot. When hot and cold water are mixed together the entropy of the hot water decreases and the entropy of the cold water increases by a greater amount. The total entropy of the mixture increases during the process. The mixing process created the entropy. The mixture does not naturally separate back into hot and cold components as this would involve a decrease in total entropy. Entropy is nature's arrow, always increasing, pointing in the direction in which systems evolve in time.

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