LESSON - 2 PROPERTY, THERMODYNAMIC EQULIBRIUM, STATE, PROCESS, CYCLIC PROCESS, QUASI-STATIC PROCESS AND NON QUASI-STATIC PROCESS

 INRODUCTION

 In thermodynamic analysis of system, the work/heat interactions are quantified by finding the change in some of the characteristics of the system.

To make it clear, consider the arrangement shown in Fig. 2.1. The work done by an engine on the water pump is quantified by increase in water/system pressure from P₁ to P₂.

 

Fig. 2.1.

 2.1. PROPERTY

It is the relevant measureable characteristics of a system that can be used to define the system.

For example: pressure, temperature, volume etc.

These can further be classified as intensive and extensive properties.

2.1.1. Intensive Properties

The intensive properties are characteristics of a system that remain the same for part of the system as they were as well as for the whole system when the system is divided into parts (Fig. 2.2).

For example: pressure, temperature, density etc.

Fig. 2.2. Illustration of intensive properties.

Example for temperature:

A heated iron bar of 10 cm length is at 100°C. When the rod is cut into five equal parts, then each part of the rod will be at the same temperature of 100°C. Hence the property temperature is an intensive property.

Exercise: Think of examples for pressure and density.

2.1.2. Extensive Properties

The extensive properties are characteristics of a system that depend on the matter content of the system as shown in Fig. 2.3. In other words, the value of the extensive properties for the whole system is the sum of the values of its parts into which the system is divided.

 For examples: volume, energy, internal energy, enthalpy, entropy etc.

Fig. 2.3. Illustration of extensive properties.

2.2. SPECIFIC PROPERTY

A property defined per unit mass is called specific property. A specific property is always an intensive property.

For examples: specific volume, specific heat capacity, specific energy etc.

2.3. THERMODYNAMIC EQUILIBRIUM

A system is said to be in thermodynamic equilibrium when it satisfies all the following conditions of equilibrium.

2.3.1. Mechanical Equilibrium

If its properties like, pressure, elastic stress etc. do not change at any point of the system with time, we say that the system is in mechanical equilibrium.

2.3.2.  Thermal Equilibrium If the temperature is the same throughout the entire system, we say that it is in thermal equilibrium (Fig. 2.4). 2.3.3.  Chemical Equilibrium If the chemical reaction in the system ceases and the chemical composition of the system does not change with time, we say that the system is in chemical equilibrium.

Fig. 2.4. A closed system reaching thermal equilibrium.

 2.3.4.  Phase Equilibrium

If there is no change in the mass of each phase of the constituents, then the system is in phase equilibrium.

 2.4. STATE

As described earlier, the state of a thermodynamic system can be described by using thermodynamic properties. The unique values assigned to these properties will define state of a system. Only two independent properties are necessary and sufficient to determine the state of a system. The state of a system can be described by using thermodynamic property of a system. Hence, thermodynamic property is a point function.

Mathematically, it can be represented as an exact differential. When a system undergoes a change of state from 1 to 2 it can be denoted by

where,  Ø is a property.

So, the change in pressure and volume, during change of state from 1 to 2, are given as

                   

Example: State 1 and state 2 of air in a reciprocating compressor (system) corresponding to before and after air being compressed (Fig. 2.5).

State 1: Air in cylinder has volume V₁ and pressure p₁.

State 2: Air in cylinder has volume V₂ and pressure p₂.

 

  Fig. 2.5. Reciprocating compressor (a system) at two different states

2.5. PROCESS It is defined as transition in which a system changes from one equilibrium state ‘1’ to another equilibrium state ‘2’ (Fig.2.6). It gives information about intermediate equilibrium state points (a, b, c, d, e, f) on the thermodynamic plane for the change of state from ‘1’ to ‘2’.  2.5.1. Non-flow process The process in which the mass of the system remains the same during a change of state. 2.5.2. Flow process The process in which there is mass interaction between system and surrounding.

Fig. 2.6. A process between states 1 and 2 and the process path.

 2.6.  PATH

It represents a series of intermediate states through which a system passes during a process is called the path of the process.

2.7. CYCLIC PROCESS If a system after having under gone a number of processes returns to its original state, then we say that the system has undergone a cyclic process. Cyclic processes completed by two and four processes are shown in Fig. 2.7(a) & Fig. 2.7(b), respectively.

(a)                                     (b)                      Fig. 2.7. Systems performing cyclic process

2.8 QUASI-STATIC PROCESS If a process is carried out in such a way that at every instant, (intermediate states-‘a’, ‘b’, ‘c’, ‘d’) the system remains infinitesimally close to thermodynamic equilibrium state, then such a  process is called Quasi-static process. In other words, It is an idealized process where a system passes through a series of equilibrium intermediate states. Such a process is represented by a full line on property relation diagrams.

Fig. 2.8. Slow compression process (Quasi-static)

Example: Slow compression process is the example of quasi-static process (Fig. 2.8). In slow compression process, the equilibrium is attained at any intermediate state and we are able to characterize the entire system by a single state at every intermediate state.Therefore, the intermediate states for the whole system can be determined and process path can be drawn on the thermodynamic plane.

2.9 NON QUASI-STATIC PROCESS

If a process is carried out in such a way that at every instant the system departs finitely from thermodynamic equilibrium state, such a  process is called  Non quasi-static process.

Example: Fast compression process is an example for Non quasi-static process (Fig. 2.9). In fast compression process, the equilibrium is not attained at any intermediate state and we are not able to characterize the entire system by a single state at intermediate states (As shown in Fig. 2.9, at intermediate states ‘a’ there is variation of temperature in the system so we cannot characterize the whole system by a single state). Therefore, the intermediate states for system for process 1-2 cannot be determined and the process path cannot be drawn on thermodynamic plane.

Fig. 2.9. Fast compression process (Non quasi-static )

 A Non quasi-equilibrium process is denoted by a dashed line between the initial ‘1’ and final states ‘2’ instead of a solid line.