Table of contents |
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System |
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Types of System |
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Thermodynamic Equilibrium |
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Important Terms in Thermodynamics |
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Reversible and Irreversible Process |
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A system can be defined as a quantity of matter (control mass) or a region (control volume) in space selected for the study.
(i) Open System
When there is mass as well as energy transfer across the boundary, that type of system is called an Open system.
Example - air compressor, boiler, pump, IC engine with the valve open, etc. The majority of engineering devices come under this category.
(ii) Closed System
When in a system, the mass remains fixed or constant, but there may be energy transfer into or out of the system i.e., no mass transfer occurs across the system boundary, but only energy transfer.
Example: Tea in the kettle, automobile engine with the valve closed etc.
(iii) Isolated System (Special case of Closed System)
An isolated system is a thermodynamic system that cannot exchange either energy or matter outside the boundaries of the system.
There are two ways in which this may occur:
CONCEPT OF CONTINUUM- The concept of the continuum is the idealization of the continuous description of matter where the properties of the matter are considered as continuous functions of space. The space between the molecules (mean free path) is almost zero or negligible when compared to the size of the system.
When no change in macroscopic properties is observed, a system is said to be in a state of thermodynamic equilibrium. A system will be in a state of thermodynamic equilibrium if the following conditions are met:
(i) Mechanical Equilibrium - without the presence of an unbalanced force within the system itself and also between the system and the surroundings.
(ii) Chemical Equilibrium - an absence of any chemical reaction or transfer of matter from one part of the system to another.
(iii) Thermal Equilibrium - When a system exists in mechanical as well as a chemical equilibrium when separated from its surroundings by a diathermic wall (diathermic means ‘which allows heat to flow’).
⇨ Even when one of these conditions is not met, the system can't be in thermodynamic equilibrium.
⇨ The thermodynamic properties are defined only for thermodynamic equilibrium states.
1. Process: Any change of state that a system undergoes, from one equilibrium state to another equilibrium state is known as a process.
2. Path: The succession of states passed through during a change of state from an initial condition to the final required condition, is called the path of the change of state.
3. Cycle: A series of changes in states of a system, such that the final point of the system coincides with the initial point is termed as a cycle.
4. Quasi-static Process: The meaning of ‘Quasi’ is ‘almost’ and the meaning of ‘Static’ is ‘at rest’. The characteristic feature of a quasi-static process is its infinite slowness. A process that is the locus of all the equilibrium states the system passes through from an initial condition to the final desired condition is known as a quasi-static process. Every state of the system through which it passes during this process is an equilibrium state.
Example: P–V, T–S, P–T diagrams
Reversible Process
Example: Heat transfer to finite temperature difference, Free expansion.
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1. What are the different types of systems in thermodynamics? | ![]() |
2. What is thermodynamic equilibrium? | ![]() |
3. What are some important terms in thermodynamics? | ![]() |
4. What is the difference between reversible and irreversible processes in thermodynamics? | ![]() |
5. How are the basics of thermodynamics relevant to the SSC JE exam? | ![]() |