Thermodynamics

If two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other. This law establishes the concept of temperature as a measurable property.

Energy is conserved. The change in internal energy of a system equals the heat supplied to the system minus the work done by the system. It rules out the possibility of a perpetual motion machine of the first kind.

Temperature remains constant (T = const). For an ideal gas, internal energy does not change (ΔU = 0), so all heat absorbed equals work done by the gas. The curve on a PV diagram is a hyperbola.

No heat exchange with surroundings (Q = 0). The first law gives ΔU = −W. For rapid processes or thermally insulated systems. The PV curve is steeper than the isothermal curve.

The Carnot engine operates between two heat reservoirs in a reversible cycle: isothermal expansion → adiabatic expansion → isothermal compression → adiabatic compression. It is the most efficient engine possible operating between those temperatures.

Kelvin-Planck: No process is possible in which the sole result is the absorption of heat from a reservoir and conversion entirely into work. Clausius: Heat cannot spontaneously flow from a colder to a hotter body.