Thermodynamics: Basic Concepts

• Thermodynamics is the study of energy, its transformations, and its relation to matter.
• It is governed by a set of laws that describe the behavior of energy and matter at a macroscopic level.
• Thermodynamics is crucial in various fields, including engineering, chemistry, and physics.

Basic Concepts

• System: A defined region of space or quantity of matter under consideration.
• Surroundings: Everything outside the system that can affect its behavior.
• Boundary: The surface that separates the system from its surroundings. It can be fixed or movable.
• Types of Systems:
  • Isolated system: No exchange of matter or energy with the surroundings.
  • Closed system: Exchange of energy but not matter with the surroundings.
  • Open system: Exchange of both energy and matter with the surroundings.
• Properties of a System:
  • Intensive properties: Do not depend on the size or extent of the system (e.g., temperature, pressure, density).
  • Extensive properties: Depend on the size or extent of the system (e.g., mass, volume, energy).
• State of a System: The condition of a system, defined by its properties.
• Process: Any change that a system undergoes from one state to another.
• Thermodynamic Equilibrium: A state where no changes occur in the system's properties when it is isolated from its surroundings. Requires thermal, mechanical, phase, and chemical equilibrium.
• Quasi-static Process: A process that occurs slowly enough to allow the system to remain in equilibrium at all times.

Thermodynamic Properties

• Temperature (T): A measure of the average kinetic energy of the particles in a system.
• Pressure (P): The force exerted per unit area.
• Volume (V): The amount of space a substance occupies.
• Energy (U): The capacity to do work.
• Enthalpy (H): A thermodynamic property of a system, defined as H = U + PV.
• Entropy (S): A measure of the disorder or randomness of a system.
• Gibbs Free Energy (G): A thermodynamic potential that measures the amount of energy available in a thermodynamic system to perform useful work at a constant temperature and pressure. G = H - TS.

Laws of Thermodynamics

• Zeroth Law: If two systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each other. This law establishes the concept of temperature.

• First Law: The change in internal energy of a system is equal to the heat added to the system minus the work done by the system (ΔU = Q - W). Deals with the conservation of energy.

  • Energy can neither be created nor destroyed.
  • Energy can only be converted from one form to another.
  • For a cyclic process, the net heat added is equal to the net work done.

• Second Law: The total entropy of an isolated system can only increase over time or remain constant in ideal cases (reversible processes).

  • Entropy is a measure of the disorder of a system.
  • Heat cannot spontaneously flow from a colder body to a hotter body.
  • No heat engine can have a thermal efficiency of 100%.

• Third Law: As the temperature approaches absolute zero, the entropy of a system approaches a minimum or zero value.

  • It is impossible to reach absolute zero in a finite number of steps.
  • At absolute zero, all processes cease, and entropy is at its minimum value.

Thermodynamic Processes

• Isothermal Process: A process that occurs at constant temperature.
• Isobaric Process: A process that occurs at constant pressure.
• Isochoric (or Isometric) Process: A process that occurs at constant volume.
• Adiabatic Process: A process in which no heat is exchanged between the system and its surroundings (Q = 0).
• Reversible Process: A process that can be reversed without leaving any trace on the surroundings; it is an idealization.
• Irreversible Process: A process that cannot be reversed without leaving a trace on the surroundings; all real-world processes are irreversible.
• Polytropic Process: A process that follows the relation PV^n = constant, where n is the polytropic index.

Heat Engines and Refrigerators

• Heat Engine: A device that converts thermal energy into mechanical work.
  • It operates in a cycle, receiving heat from a high-temperature reservoir, converting part of it into work, and rejecting the remaining heat to a low-temperature reservoir.
  • Thermal Efficiency (η) = (Work Output) / (Heat Input).
• Refrigerator: A device that transfers heat from a low-temperature reservoir to a high-temperature reservoir, requiring work input.
  • Coefficient of Performance (COP) = (Desired Effect) / (Required Input). For a refrigerator, COP = (Heat Removed from Cold Reservoir) / (Work Input).
• Carnot Cycle: A theoretical thermodynamic cycle that provides the maximum possible efficiency for a heat engine operating between two temperatures.
  • It consists of two isothermal processes and two adiabatic processes.
  • Carnot Efficiency (η_carnot) = 1 - (T_cold / T_hot), where temperatures are in Kelvin.

Applications of Thermodynamics

• Power Generation: Design and optimization of power plants, internal combustion engines, and gas turbines.
• Refrigeration and Air Conditioning: Design of refrigeration systems, heat pumps, and air conditioning units for cooling and heating purposes.
• Chemical Processes: Analysis and optimization of chemical reactions, phase equilibria, and separation processes.
• Materials Science: Understanding the thermodynamic properties of materials, phase transformations, and material stability.
• Aerospace Engineering: Design of propulsion systems, thermal management systems for spacecraft, and aerodynamic analysis of aircraft.

Thermodynamic Cycles

• Carnot Cycle: As previously described, a theoretical cycle with maximum possible efficiency.
• Otto Cycle: Idealized cycle for spark-ignition internal combustion engines.
  • Consists of two isochoric processes and two adiabatic processes.
• Diesel Cycle: Idealized cycle for compression-ignition internal combustion engines.
  • Consists of an isochoric process, an isobaric process, and two adiabatic processes.
• Rankine Cycle: Idealized cycle for steam power plants.
  • Consists of two isobaric processes and two isentropic processes.
• Brayton Cycle: Idealized cycle for gas turbine engines.
  • Consists of two isobaric processes and two isentropic processes.

Phase Transitions

• Phase: A state of matter that is uniform throughout, both in chemical composition and physical state (e.g., solid, liquid, gas).
• Phase Transition: The transformation of a substance from one phase to another, such as melting, boiling, or sublimation.
• Vaporization: Liquid to gas.
• Condensation: Gas to liquid.
• Melting (Fusion): Solid to liquid.
• Freezing (Solidification): Liquid to solid.
• Sublimation: Solid to gas.
• Deposition: Gas to solid.
• Triple Point: The temperature and pressure at which three phases of a substance coexist in equilibrium.
• Critical Point: The temperature and pressure above which distinct liquid and gas phases do not exist.

Mixtures and Solutions

• Ideal Gas Mixture: A mixture of gases that behaves as if each gas were alone in the volume. The total pressure is the sum of the partial pressures of each gas.
• Partial Pressure: The pressure exerted by a single gas in a mixture of gases.
• Humidity: The amount of water vapor present in the air.
  • Absolute Humidity: The mass of water vapor per unit volume of air.
  • Relative Humidity: The ratio of the partial pressure of water vapor in the air to the saturation pressure of water vapor at the same temperature, expressed as a percentage.
• Psychrometrics: The study of the thermodynamic properties of moist air.

Statistical Thermodynamics

• Statistical Thermodynamics: A branch of thermodynamics that connects the macroscopic properties of systems to the microscopic behavior of their constituent particles.
• Boltzmann Distribution: Describes the probability of a particle being in a particular energy state as a function of temperature.
• Partition Function: A function that summarizes the statistical properties of a system in thermodynamic equilibrium.