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The first law of thermodynamics states that if a system gains/loses some amount of energy, the surroundings must lose/gain the same amount of energy. The second law of thermodynamics states that entropy does not spontaneously decrease. Entropy is a measure of how spread out energy is. As an example, if two systems at different temperatures are in contact, heat will flow from the warmer system to the cooler system until they reach thermal equilibrium (equal temperatures).

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