Thermodynamic System: Select Definitions | Thermodynamics - Mechanical Engineering PDF Download

Thermodynamic System: Select Definitions

It may be evident from the foregoing introduction, that for the purpose of any thermodynamic analysis it is necessary to define a ‘system’. A system, in general, is any part of the universe which may be defined by a boundary which distinguishes it from the rest of the universe. Such a thermodynamic system is usually referred to as control volume as it would possess a volume and would also contain a definite quantity of matter. The system boundary may be real or imaginary, and may change in shape as well as in size over time, i.e., increase or decrease. A system can either be closed or open. A closed system does not allow any transfer of mass (material) across its boundary, while an open system is one which does. In either case energy transfer can occur across the system boundary in any of its various forms; for example, heat, work, electrical / magnetic energy, etc. However, for most real world systems of interest to chemical engineers the primary forms of energy that may transfer across boundaries are heat and work. In contrast to closed or open systems, a system which is enclosed by a boundary that allows neither mass nor energy transfer is an isolated system.

All matter external to the system constitutes the surroundings. The combination of the system and surroundings is called the universe. For all practical purposes, in any thermodynamic analysis of a system it is necessary to include only the immediate surroundings in which the effects are felt. A very common and simple example of a thermodynamic system is a gas contained in a piston-and-cylinder arrangement derived from the idea of steam engines, which may typically

exchange heat or work with its surroundings. The dotted rectangle represents the ‘control volume’, which essentially encloses the mass of gas in the system, and walls (including that of the piston) form the boundary of the system. If the internal gas pressure and the external pressure (acting on the moveable piston) is the same, no net force operates on the system. If, however, there is a force imbalance, the piston would move until the internal and external pressures equalize. In the process, some net work would be either delivered to or by the system, depending on whether the initial pressure of the gas is lower or higher than the externally applied pressure. In addition, if there is a temperature differential between the system and the surroundings the former may gain or lose energy through heat transfer across its boundary.  

This brings us to a pertinent question: how does one characterize the changes that occur in the system during any thermodynamic process? Intuitively speaking, this may be most readily done if one could measure the change in terms of some properties of the system. A thermodynamic system is, thus, characterized by its properties, which essentially are descriptors of the state of the system. Change of state of a system is synonymous with change in the magnitude of its characteristic properties. The aim of the laws of thermodynamics is to establish a quantitative relationship between the energy applied during a process and the resulting change in the properties, and hence in the state of the system.  

Thermodynamic properties are typically classified as extensive and intensive. A property which depends on the size (i.e., mass) of a system is an extensive property. The total volume of a system is an example of an extensive property. On the other hand, the properties which are independent of the size of a system are called intensive properties. Examples of intensive properties are pressure and temperature. The ratio of an extensive property to the mass or the property per unit mass (or mole) is called specific property. The ratio of an extensive property to the number of moles of the substance in the system, or the property per mole of the substance, is called the molar property.     

Specific volume (volume per mass or mole) V =V ^t / M                          ..(1.1) 
Molar Volume (volume per mole) V =V ^t / N

where,

V ^{t =} total system volume (m³ )
M = total system mass (kg)
N = total moles in system (kg moles)