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Chapter 1 – Fundamental Concepts (06 Periods)
Thermodynamic state and system, boundary, surrounding, universe, thermodynamic systems – closed, open, isolated, adiabatic, homogeneous and heterogeneous, macroscopic and microscopic, properties of system – intensive and extensive, thermodynamic equilibrium, quasi – static process, reversible and irreversible processes, Zeroth law of thermodynamics, definition of properties like pressure, volume, temperature, enthalpy, internal energy
Thermodynamic system, boundary and surroundings
System : The thermodynamic system may be defined as a definite area or a space where some thermodynamic process is taking place.
Surroundings : Anything outside the boundaries which affects the behaviour of the system is known as surroundings.
Boundaries : The system and the surroundings are separated by the system boundary. The system boundary may be real or imaginary
Types of thermodynamic systems
Thermodynamic systems may be classified as follows:
- Closed system 2. Open system and 3. Isolated system
- Closed system
A closed system permits the transfer of heat and work across its boundaries; but it does not permit the transfer of mass. The mass of the working substance in a closed system remains constant. The system boundary is determined by the space occupied by the working substance.
The piston and cylinder arrangement shown in the figure is an example of closed system. The gas in the cylinder is considered as system. If the heat is supplied to the cylinder, the temperature of the gas will increase and the piston will move.
As the piston moves, the boundary of the system moves. In other words, the heat and work energy crosses the boundary of the system during this process, but there is no addition or loss of the original mass of the working substance.
- Open system
In this system, the mass of the working substance crosses the boundary of the system. Heat and work may also cross the boundary. The mass within the system may not be constant during the process. An open system may be called as control volume.
The compressor unit shown in the figure is an example of open system. In this system, the low pressure air enters the compressor and leaves the high pressure air. Thus the mass of working substance crosses the boundary of the system. The work crosses the boundary of the system through the driving shaft and the heat is transferred across the boundary from the cylinder walls.
- Isolated system
A system which is not influenced by the surroundings is called an isolated system. In an isolated system, there is no mass, heat or work transfer takes place. This is an imaginary system.
Example : An open system with an universe as its surrounding is an example of an isolated system.
- Homogeneous System
A system which consists of a single phase is termed as homogeneous system. Examples : Mixture of air and water vapour, water plus nitric acid and octane plus heptane.
- Heterogeneous System
A system which consists of two or more phases is called a heterogeneous system. Examples : Water plus steam, ice plus water and water plus oil.
MACROSCOPIC AND MICROSCOPIC POINTS OF VIEW
Thermodynamic studies are undertaken by the following two different approaches.
- Macroscopic approach—(Macro mean big or total)
- Microscopic approach—(Micro means small)
- In this approach a certain quantity of matter is considered without taking into account the events occurring at molecular level. In other words this approach to thermodynamics is concerned with gross or overall behaviour. This is known as classical thermodynamics.
- The analysis of macroscopic system requires simple mathematical formulae.
- The values of the properties of the system are their average values.
- In order to describe a system only a few properties are needed.
- The approach considers that the system is made up of a very large number of discrete particles known as molecules. These molecules have different velocities and energies. The values of these energies are constantly changing with time. This approach to thermodynamics which is concerned directly with the structure of the matter is known as statistical thermodynamics.
- The behaviour of the system is found by using statistical methods as the number of molecules is very large. So advanced statistical and mathematical methods are needed to explain the changes in the system.
- The properties like velocity, momentum, impulse, kinetic energy, force of impact etc. which describe the molecule cannot be easily measured by instruments. Our senses cannot feel them.
- Large number of variables are needed to describe a system. So the approach is complicated.
PROPERTIES OF SYSTEMS
A property of a system is a characteristic of the system which depends upon its state, but not upon how the state is reached.
There are two types of property :
- Intensive properties. These properties do not depend on the mass of the system. Examples : Temperature and pressure.
- Extensive properties. These properties depend on the mass of the system. Example : Volume. Extensive properties are often divided by mass associated with them to obtain the intensive properties. For example, if the volume of a system of mass m is V, then the specific volume of matter within the system is V/m = v which is an intensive property.
A system is in thermodynamic equilibrium if the temperature and pressure at all points are same ; there should be no velocity gradient ; the chemical equilibrium is also necessary. Systems under temperature and pressure equilibrium but not under chemical equilibrium are sometimes said to be in metastable equilibrium conditions. It is only under thermodynamic equilibrium conditions that the properties of a system can be fixed.