Thermodynamic Systems – The General and Zero Principles of Thermodynamics

States And Thermodynamic Processes. General Principle.

The thermodynamics study the physical processes which take place in large particles number systems, wherein interfere thermic phenomenon situations. A thermodynamics system is a random portion of Universe which may interact with the surrounding medium (the exterior). A system is called isolated in case this one cannot interact with outside ambience. An enclosed system exchanges only energy with the exterior. A system is opened in case this one exchanges substances with the outside medium.

The state of a thermodynamics system at a certain time is determined by a finite number of state parameters. The pressing and volume are mechanical parameters, while the temperature is one specific to thermodynamic systems. The intensive parameters as pressure, temperature, concentration, electric tension, etc. do not depend on the number of particles in the system, having the same value for all the elements of the system. The extensive parameters as volume, internal energy, entropy, electric charge, etc. are proportional with the number of particles in the system, being additive measures.

To express the elementary mechanical work, we associate to each contact a pair of parameters form the system and exterior, ones of force, others of position (e.g. : pressure and volume). The thermodynamic balance state is a state wherein state parameters are constant through time and there are no fluxes inside the system. The general principle of thermodynamics shows that an isolated thermodynamics system is at a moment disturbed, then after the disturbance cessation, the system spontaneously evolves to a thermodynamic balance state it reaches after a time T, named rest time. The system can never be disturbed by itself. 

Temperature. The Zero Principle of Thermodynamics.

Two thermodynamic systems in contact make a global system which in time reaches the thermic balance state. The balance does not break in case we remove the contact between systems and after a while we reconnect these. It shows that the global system is in state of balance, then each component system will be in thermic balance itself. From here is deduced the third principle of thermodynamics, the zero principle, or the law of thermic balance transitivity: two systems in balance with the third are between in balance.

This experimental law allows the introduction of temperature as macroscopic parameter, a common feature of the systems in balance. The zero principle of thermodynamics can be rephrased with the help of temperature notion: there is a state function named temperature; the equality of the temperatures in all the spots is the thermic balance condition for the two thermodynamic systems or two parts of the same system. Since the objects in thermic balance have the same temperature, it is possible to measure their temperature using a thermometer, which is in thermic balance with these. As thermometric object can be any item with measurable property that varies with temperature in reproductive way. For the appreciation of temperature we can choose: volume variation, pressure variation, electric resistance variation, etc.