Systems and thermodynamic property

1 Systems and thermodynamic property

Thermodynamic Property

A property is a macroscopic characteristic of a system such as mass, volume, energy, pressure, and temperature to which a numerical value can be assigned at a given time without knowledge of the previous behavior (history) of the system. Many other properties are considered during the course of our study of engineering thermodynamics. Thermodynamics also deals with quantities that are not properties, such as mass flow rates and energy transfers by work and heat.

The word state refers to the condition of a system as described by its properties.

Process

When any of the properties of a system change, the state changes and the system is said to have undergone a process. States of a thermodynamic system can be changed by interacting with its surrounding through work and heat. When this change occurs in a system, it is said that the system is undergoing a process.

 Some sample processes:

o Isothermal process: temperature is constant T=C

o Isobaric process: pressure is constant, P=C

o Constant-volume process, v=C

o Adiabatic process: no heat transfer, Q=0

Isothermal and Adiabatic Process

Thermodynamic Cycle

A thermodynamic cycle is a sequence of processes that begins and ends at the same state. At the conclusion of a cycle all properties have the same values they had at the beginning. Consequently, over the cycle the system experiences no net change of state. Cycles that are repeated periodically play prominent roles in many areas of application. For example, steam circulating through an electrical power plant executes a cycle.

Extensive and Intensive Properties

Thermodynamic properties can be placed in two general classes: extensive and intensive.

A property is called extensive if its value for an overall system is the sum of its values for the parts into which the system is divided. Mass, volume, energy, and several other properties introduced later are extensive. Extensive properties depend on the size or extent of a system.

Intensive properties are not additive in the sense previously considered. Their values are independent of the size or extent of a system and may vary from place to place within the system at any moment. Specific volume, pressure and temperature are important intensive properties.