The First Law of Thermodynamics•A closed system has a total internal energy Eint.•This energy can be changed in two different ways: A.The system can do workW, or have work done on it, -W.B. Heat Qcan flow into the system, or -Qflow out.•The First Law is just total energy conservation:ΔEint = Q–W•Change in internal energy = heat in – work done.
States of an Ideal Gas•Suppose we have nmoles of an ideal gas in equilibrium in a piston. The “state” of the gas can be defined by giving the state variables P, V. The gas state is a point in the (P, V) plane.•If heat is exchanged or work done, the gas state variables trace a pathin the (P, V) plane.•If the gas moves along an isotherm PV= constant, its internal energy stays the same.•ZPVThe temperature (and therefore internal energy) is constant along an isotherm, PV= const.
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