INTRODUCTION TO THERMODYNAMICS, Mechanical Engg 3rd Semester Notes, ME3001

 

1. INTRODUCTION TO THERMODYNAMICS

Thermodynamics is the branch of physics that deals with the study of energy, its transformations, and the properties of matter. It plays a key role in engineering and science by explaining how energy can be converted from one form to another and how various systems behave under different conditions.

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1.1 Role of Thermodynamics in Engineering and Science

Thermodynamics is crucial in various fields of engineering and science, as it governs the principles of energy conversion and utilization. Some major roles are:

• In Engineering: It helps in designing engines, refrigerators, heat pumps, and energy systems. Thermodynamics is vital in determining the efficiency of power plants, refrigerators, and air conditioners.
• In Chemistry: Thermodynamics helps explain chemical reactions, equilibrium, and the energy changes involved in chemical processes.
• In Physics: It explains the physical properties of matter, energy transfer, and phase changes.
• In Biology: Thermodynamics is important in understanding metabolic processes and energy transformations within living organisms.

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1.2 Basic Concept of Thermodynamic Laws

Thermodynamics is based on a few fundamental laws that govern energy and matter. These laws form the foundation of thermodynamic analysis and help engineers and scientists understand energy systems.

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1.2.1 Types of Systems, Thermodynamic Equilibrium, Properties (Basic Concept Only)

• System: A system is a specified portion of matter or space under study, and everything else is considered the surroundings.

  • Open System: Can exchange both energy and matter with surroundings (e.g., boiling water in an open pot).
  • Closed System: Can exchange only energy, not matter (e.g., a sealed pressure cooker).
  • Isolated System: Cannot exchange energy or matter with surroundings (e.g., a thermos bottle).

• Thermodynamic Equilibrium: A system is said to be in thermodynamic equilibrium if its properties (pressure, temperature, and volume) are uniform throughout and do not change with time. It can be in several types of equilibrium:

  • Thermal Equilibrium: No temperature gradient exists within the system.
  • Mechanical Equilibrium: No unbalanced forces exist within the system (no motion).
  • Chemical Equilibrium: No net chemical reactions occur within the system.

• Properties of a System: Properties are the characteristics of a system that describe its state and behavior.

  • Intensive Properties: Independent of the quantity of matter (e.g., temperature, pressure).
  • Extensive Properties: Dependent on the quantity of matter (e.g., volume, energy).

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1.2.2 Elementary Introduction to Zeroth Law, First Law, Heat and Work

• Zeroth Law of Thermodynamics: If two systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each other. This law provides the foundation for temperature measurement.

  Example: If A is in equilibrium with C, and B is in equilibrium with C, then A is in equilibrium with B.

• First Law of Thermodynamics (Conservation of Energy): Energy cannot be created or destroyed, only transferred or converted from one form to another.

  $$\Delta U = Q - W$$

  Where:

  • $\Delta U$ is the change in internal energy of the system.
  • $Q$ is the heat supplied to the system.
  • $W$ is the work done by the system.

  Heat is energy transferred due to a temperature difference, while Work is energy transferred due to a force applied over a distance.

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1.2.3 Second Law of Thermodynamics: Kelvin-Planck and Clausius Statements

• Kelvin-Planck Statement: It is impossible to construct a heat engine that operates in a cycle and produces no other effect except transferring heat from a cold reservoir to a hot one.

  • In simple terms, it states that it is impossible to create a perfect heat engine, and some energy will always be lost as waste heat.

• Clausius Statement: It is impossible to construct a refrigerator or heat pump that operates in a cycle and has no effect other than transferring heat from a cold reservoir to a hot one.

  • This statement emphasizes the concept of the direction of heat flow: heat will naturally flow from hot to cold without external work.

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1.2.4 Concept of Heat Engine, Heat Pump & Refrigerator, Efficiency/COP

• Heat Engine: A device that converts heat energy into mechanical work. It operates by absorbing heat $Q_H$ from a hot reservoir, doing work, and rejecting some heat $Q_C$ to a cold reservoir.

  • Efficiency of Heat Engine:
  $$\eta = \frac{W}{Q_H} = 1 - \frac{Q_C}{Q_H}$$

  Where:

  • $W$ is the work output, $Q_H$ is the heat absorbed from the hot reservoir, and $Q_C$ is the heat rejected to the cold reservoir.

• Heat Pump: A device that uses mechanical work to transfer heat from a cold space to a hot one (e.g., air conditioners).

• Refrigerator: A device that operates by removing heat from a cold space and rejecting it to a hot space. It requires work input to operate. The Coefficient of Performance (COP) is the ratio of heat removed from the cold space to the work input.

  $$COP_{refrigerator} = \frac{Q_C}{W}$$

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1.2.5 Carnot Cycle, Carnot Efficiency, T-S and P-V Diagrams

• Carnot Cycle: A theoretical thermodynamic cycle that is the most efficient possible cycle for a heat engine. It consists of two isothermal processes (constant temperature) and two adiabatic processes (no heat exchange).

  • The Carnot cycle is reversible and provides an upper limit for the efficiency of any heat engine.

  • Carnot Efficiency:

  $$\eta_{Carnot} = 1 - \frac{T_C}{T_H}$$

  Where:

  • $T_H$ is the temperature of the hot reservoir.
  • $T_C$ is the temperature of the cold reservoir.

• T-S (Temperature-Entropy) Diagram: This diagram shows the relationship between temperature and entropy during the Carnot cycle. The area under the curve represents the heat transferred.

  Diagram:

  • The process consists of two isothermal processes (horizontal lines) and two adiabatic processes (curved lines).

• P-V (Pressure-Volume) Diagram: In the P-V diagram, the Carnot cycle is represented as a series of processes that form a rectangle or loop, showing how pressure and volume change during the cycle.

  Diagram:

  • Two isothermal lines are curved, and the adiabatic processes appear as steep curves.