First Law of Thermodynamics Total energy transferred to

First Law of Thermodynamics Total energy transferred to a system by macroscopic forces exerted on it by other systems Work: Work done by a gas on a piston A Work done by a fluid as it expands from V 0 to Vf Fluid (gas) F =P A 0 x x Energy leaves the system A x Energy feed in the system

First law of thermodynamics: All systems have an internal energy state function that is changed by heat transferred to or from the system and by the work done by or on the system. nothing but conservation of energy closed system internal energy state function Energy transferred by macroscopic mechanical means Energy transferred by non-macroscopic mechanical means A Q

Joule’s experiment: the mechanical equivalent of heat What do we learn from the experiment 1 -dissipative work Wd is done -no heat flow Q measured -change in by change in temperature (V=const. ) 2 Joule's Paddle-Wheel Apparatus 1849 -same change in could be created by a heat flow Q Change of internal energy U in a dissipative process is the same as if there had been a heat flow Q into the system of equal magnitude to Wd. click for virtual experiment

Application of 1. law in a free expansion experiment V 1 , Ts V 2 , Tf Thermal insulation gas no heat flow =: adiabatic process fixed walls no work done by the container W=0 1. law: ideal gas U=U(T) for the ideal gas only