Presentation on theme: "Liceo Scientifico Isaac Newton Physics course The first law of thermodynamics Professor Massimo Patrone Read by Cinzia Cetraro."— Presentation transcript:
Liceo Scientifico Isaac Newton Physics course The first law of thermodynamics Professor Massimo Patrone Read by Cinzia Cetraro
Historical notes James Joule (1818-1889) Until 1600 combustion Industrial Revolution Newton Heat: source of mechanical work Carnot and Joule Thermodynamics
From energy to heat energy: Joule's experiment Until 1700 The prevailing view of scientists was that the flow of heat between two bodies at different temperatures consisted in the transition from the warmer to the colder of a fluid, called caloric. James Joule Confirmed Thompson s intuition around 1843 showing that when mechanical work is transformed entirely into heat, the relationship between work, measured in joules, and heat, expressed in calories, remains constant in any process. Benjamin Thompson Called into question the model based on caloric fluid and understood the nature of heat energy. He suggested that the friction generated by the work of a body on another body produces heat.
masses blades water temperature h Joule's experiment 1 calorie = 4,186 joule = 4,186
The First Law of Thermodynamics A thermodynamic system is any device that exchanges heat and work with the external environment The First Law of Thermodynamics is a special formulation of the energy conservation principle, applied to thermodynamic systems. It express the changes in the internal energy of a system when it performs work or it exchanges heat with the environment.
Internal Energy The internal energy is A state function Sum between kinetic energy and potential energy Measured in joules molecular motion
Internal energy U of a system is the sum of the kinetic energy of molecules and the different types of potential energy associated with the bonds between the atoms that compose the system kinetic energy of molecules potential energy of bond internal energy U
First Law of Thermodynamics U1=U U2>U1 U2<U1 U2=U1+ U LsLs LsLs QaQa QaQa QcQc QcQc LfLf LfLf
Quasi-static processes Piston or elementary machine To understand the processes that involve real thermodynamic systems, which are rather complex, we refer to the simple model of heat engine, consisting of a cylinder with a sliding piston and containing a perfect gas.
The perfect gas U = U(T) Is the only substance of which we can describe the behavior with relatively simple and accurate laws, based on measures of volume, pressure and temperature, said state quantities; these, we add the internal energy U of an ideal gas, which is all kinetic and depends only on the temperature.
The real thermodynamic processes give rise in the PV plane, to a graphic representation which is not possible to describe mathematically because for a given volume of fluid, the pressure has different values. So they are irreversible. Reversible and irreversible transformations In a very slow process, there are a succession off equilibrium states. This transformation is called quasi-static. The quasi-static transformations can be played in reverse.
transformations in the perfect gas 1) isobaric2)isochoric3)isothermal4)adiabatic
1) Transformation at constant pressure: isobaric volume p i =p f ViVi VfVf pressure ΔXΔX
2)Transformation at constant volume: isochoric volume pipi V i =V f pressure pfpf L=0 ΔU=Q-L=Q