"In terms of making things happen, energy is an indispensable consideration. Systems spontaneously tend towards the lowest-energy state. When a system reaches equilibrium, no further energy can be extracted. That maximum entropy, lowest energy state is the inevitable end-state of the Universe. But until that moment arrives, reactions of all kinds will occur, continuing to liberate energy. In our bodies, chemical bonds break and reform: releasing energy."
"Aerobic respiration releases 2.88 megajoules of energy per mole of sugar. Only 0.0000000094% of the initial fuel's mass converts into energy. Chemical reactions are more efficient, like combustion. TNT's heat of combustion is 14.5 megajoules per kilogram: just 0.000000016% efficient. Rocket fuels, like RP-1 and mixing liquid hydrogen/oxygen, are only ~10 times more efficient. When enriched uranium undergoes nuclear fission, efficiencies are much higher. Each U-235 kilogram liberates 72 trillion joules: 0.08% efficient."
Energy drives physical change as systems progress toward lower-energy, higher-entropy states, and equilibrium prevents further energy extraction. The universe's eventual end-state is maximum entropy and minimum usable energy. Chemical processes continually liberate energy: aerobic respiration yields 2.88 megajoules per mole of sugar while converting only about 0.0000000094% of fuel mass into energy. Chemical combustions and explosives release more energy per mass but remain extremely inefficient. Rocket propellants are roughly ten times better. Nuclear fission and fusion convert larger fractions of mass: U-235 yields ~72 trillion joules per kilogram (0.08%), fusion yields ~630 trillion joules per kilogram (~0.7%), and matter-antimatter annihilation converts mass fully via E = mc².
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