Thermodynamics: Systems, Equilibrium, Laws, and Processes
Thermodynamic Systems
A thermodynamic system is a specific portion of matter with a definite boundary on which our attention is focused. The system boundary may be real or imaginary, fixed or deformable.
There are three types of systems:
- Isolated System – An isolated system cannot exchange energy and mass with its surroundings. The universe is considered an isolated system.
- Closed System – Across the boundary of the closed system, the transfer of energy takes place, but the transfer of mass doesn’t take place. Refrigerator, compression of gas in the piston-cylinder assembly are examples of closed systems.
- Open System – In an open system, mass and energy can both be transferred between the system and surroundings. A steam turbine is an example of an open system.
Thermodynamic Equilibrium
A system is said to be in thermodynamic equilibrium when the following three conditions of equilibrium are satisfied simultaneously: mechanical equilibrium, chemical equilibrium, and thermal equilibrium.
Mechanical Equilibrium
- For a system to be in mechanical equilibrium, there should not be any unbalanced forces acting within the system and between the system and its surrounding.
- Also, the pressure in the system should be the same throughout the system and should not change with time.
Chemical Equilibrium
- For a system to be in chemical equilibrium, there should be no chemical reactions going on within the system.
- There is no transfer of matter from one part of the system to the other due to diffusion.
- A system to be in chemical equilibrium, its chemical composition has to be the same throughout and should not change with time.
Thermal Equilibrium
For a system to be in thermal equilibrium, the temperature of the system should be uniform throughout and it should not change with time. A system when in thermal equilibrium is described in terms of state variables.
Thermodynamic Processes
A system undergoes a thermodynamic process when there is some energetic change within the system that is associated with changes in pressure, volume, and internal energy.
There are four types of thermodynamic processes that have their unique properties, and they are:
- Adiabatic Process – A process where no heat transfer into or out of the system occurs.
- Isochoric Process – A process where no change in volume occurs and the system does no work.
- Isobaric Process – A process in which no change in pressure occurs.
- Isothermal Process – A process in which no change in temperature occurs.
Laws of Thermodynamics
Zeroth Law of Thermodynamics
The Zeroth law of thermodynamics states that if two bodies are individually in equilibrium with a separate third body, then the first two bodies are also in thermal equilibrium with each other.
First Law of Thermodynamics
The First Law of Thermodynamics, also known as the law of conservation of energy, states that energy can neither be created nor destroyed, but it can be changed from one form to another.
Second Law of Thermodynamics
The Second Law of Thermodynamics states that the entropy in an isolated system always increases. Any isolated system spontaneously evolves towards thermal equilibrium—the state of maximum entropy of the system.
Third Law of Thermodynamics
The Third Law of Thermodynamics states that the entropy of a system approaches a constant value as the temperature approaches absolute zero.
Carnot’s Theorem
Carnot’s theorem states that:
- Heat engines that are working between two heat reservoirs are less efficient than the Carnot heat engine that is operating between the same reservoirs.
- Irrespective of the operation details, every Carnot engine is efficient between two heat reservoirs.
Maximum efficiency is given as:
Where,
TC: absolute temperature of the cold reservoir.
TH: absolute temperature of the hot reservoir.
𝜼: the ratio of work done by the engine to heat drawn out of the hot reservoir.
Processes involved in thermodynamics can be carried out in the following two ways:
Reversible Engine
The efficiency of all reversible engines remains the same that works between two same heat reservoirs.
Where,
ΔS: change in entropy
T: temperature
Irreversible Engine
There is no irreversible engine that is more efficient than the Carnot engine working between two same reservoirs.
Example of an irreversible engine are:
- Plastic deformation
- Friction
- Spontaneous chemical reaction
Applications of Carnot’s Theorem
Carnot’s theorem finds application in engines that convert thermal energy to work.
Refrigeration: Method of removal of heat from the at low temperature and dissipating it to a higher temperature. This is a reversible process.
