But unlike energy in general, free energy is not conserved. It’s unstable, like an uncoiling spring. As it does work, it loses both its structure and the ability to do more work. When the water from a waterfall smashes into rocks at the bottom, it turns into the scattered, incoherent energy of heat. Every molecule jiggles around more or less independently. The energy is still there; it’s still conserved (that’s the first law). But the molecules push in so many directions that their energy can no longer drive a turbine. Free energy has turned into heat energy. The second law of thermodynamics
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