= Thermodynamics
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Thermodynamics studies <energy> transfers and the constraints on macroscopic changes. Its vocabulary connects <internal energy>, <entropy>, <temperature>, <pressure> and <volume> to <heat> and <work>. The <first law of thermodynamics> balances <energy>; the <Second law of thermodynamics> decides which changes can occur spontaneously and limits how much <work> a <heat engine> can deliver. These principles let us bound performance before choosing a material or a detailed machine design. Applications include power generation, propulsion and <refrigerators>. https://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node10.html[MIT, What thermodynamics describes]{ref}
Start by choosing a system boundary. A <closed system> can exchange <energy> but no matter; an <isolated system> exchanges neither. A <state function>, such as <internal energy> or <entropy>, depends on the equilibrium state. A process specifies how that state changes. Thus a body's <temperature> is a property it has, whereas <heat> records <energy> crossing its boundary during a process. A body contains <internal energy>, rather than a stored quantity of <heat>.
Two insulated bodies initially at different positive <temperatures> approach <thermal equilibrium> when brought into thermal contact. Their combined <energy> is conserved, but their combined <entropy> increases. They do not spontaneously separate again into a hotter body and a colder one. This familiar asymmetry makes <thermodynamics> interesting: conserving <energy> leaves many imaginable changes available, and <entropy> supplies a further constraint. The same distinction underlies both <entropy maximization under thermal contact> and the <thermal efficiency> limit in <Carnot's theorem>.
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