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Medical Definition of law of thermodynamics
1 : conservation of energy
— called also first law of thermodynamics
2 : a law in physics: mechanical work can be derived from the heat in a body only when the body is able to communicate with another at a lower temperature or all actual spontaneous processes result in an increase of total entropy
— called also second law of thermodynamics
3 : a law in physics: at the absolute zero of temperature the entropy of any pure crystalline substance is zero and its derivative with respect to temperature is zero
— called also third law of thermodynamics
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