🧊Thermodynamics II Unit 1 – Review of Thermodynamics I Fundamentals

Key Concepts and Definitions

• Thermodynamics studies the relationships between heat, work, and energy in systems
• Thermodynamic properties describe the state of a system and include temperature, pressure, volume, and internal energy
• Thermodynamic equilibrium occurs when a system's properties remain constant over time and there is no net exchange of energy or matter with the surroundings
• Intensive properties are independent of the system size (temperature, pressure) while extensive properties depend on the system size (volume, mass)
• Thermodynamic processes involve changes in the state of a system and can be classified as isothermal, isobaric, isochoric, or adiabatic
  • Isothermal processes occur at constant temperature
  • Isobaric processes occur at constant pressure
  • Isochoric processes occur at constant volume
  • Adiabatic processes involve no heat transfer with the surroundings
• Thermodynamic cycles consist of a series of processes that return the system to its initial state
• Thermal efficiency measures the effectiveness of a thermodynamic cycle in converting heat into work

Laws of Thermodynamics

• The zeroth law establishes the concept of thermal equilibrium and temperature
  • If two systems are in thermal equilibrium with a third system, they are in thermal equilibrium with each other
• The first law states that energy cannot be created or destroyed, only converted from one form to another
  • $\Delta U = Q - W$, where $\Delta U$ is the change in internal energy, $Q$ is heat added to the system, and $W$ is work done by the system
• The second law introduces the concept of entropy and states that the total entropy of an isolated system always increases over time
  • Entropy is a measure of the disorder or randomness of a system
• The third law states that the entropy of a perfect crystal at absolute zero is zero
  • As temperature approaches absolute zero, the entropy of a system approaches a constant minimum value
• The laws of thermodynamics govern the behavior of energy in systems and set fundamental limits on the efficiency of energy conversion processes

Thermodynamic Systems and Processes

• A thermodynamic system is a region of the universe that is the focus of study, separated from its surroundings by a boundary
• Open systems allow the exchange of both energy and matter with the surroundings (a steam turbine)
• Closed systems allow the exchange of energy but not matter with the surroundings (a sealed piston-cylinder device)
• Isolated systems do not exchange energy or matter with the surroundings (an insulated, sealed container)
• Quasi-static processes occur slowly enough that the system remains in thermodynamic equilibrium at each step
• Reversible processes can be reversed without any net change to the system or surroundings
  • Reversible processes are idealized and serve as a benchmark for real processes
• Irreversible processes cannot be reversed without a net change to the system or surroundings (friction, heat transfer through a finite temperature difference)
• Work is the energy transferred by a system to its surroundings through a force acting over a distance
  • $W = \int F dx$, where $F$ is the force and $dx$ is the displacement

Properties of Pure Substances

• A pure substance has a homogeneous and invariable chemical composition (water, nitrogen)
• The state of a pure substance can be described by two independent properties (temperature and pressure, temperature and specific volume)
• Phase diagrams show the equilibrium states of a pure substance as a function of pressure and temperature
  • The triple point is the unique temperature and pressure at which the solid, liquid, and vapor phases coexist in equilibrium
  • The critical point is the highest temperature and pressure at which the liquid and vapor phases can coexist
• Saturation conditions occur when the liquid and vapor phases coexist in equilibrium (boiling, condensation)
• Compressed liquid is a liquid at a pressure higher than the saturation pressure at a given temperature
• Superheated vapor is a vapor at a temperature higher than the saturation temperature at a given pressure
• Specific volume is the volume per unit mass of a substance ($v = V/m$)
• Specific heat capacity is the amount of heat required to raise the temperature of a unit mass of a substance by one degree ($c = Q/(m \Delta T)$)

Energy and Work

• Energy is the capacity to do work and can take many forms (kinetic, potential, internal)
• Kinetic energy is the energy associated with motion ($KE = \frac{1}{2}mv^2$)
• Potential energy is the energy associated with position or configuration (gravitational, elastic)
• Internal energy is the sum of the microscopic kinetic and potential energies of a system's particles
  • Internal energy is a function of the system's state and is independent of the process path
• Heat is the energy transferred between systems due to a temperature difference
  • Heat flows from high-temperature regions to low-temperature regions
• Work is the energy transferred by a system to its surroundings through a force acting over a distance
  • Work can be done by expanding against a pressure ($W = \int P dV$) or by lifting a weight ($W = mgh$)
• The first law of thermodynamics relates changes in internal energy to heat and work ($\Delta U = Q - W$)
• The work done by a system during a process depends on the process path, while the change in internal energy depends only on the initial and final states

Entropy and the Second Law

• Entropy is a measure of the disorder or randomness of a system
  • Entropy increases as a system becomes more disordered or random
• The second law of thermodynamics states that the total entropy of an isolated system always increases over time
  • Spontaneous processes occur in the direction of increasing entropy
• The change in entropy for a reversible process is given by $dS = \frac{dQ}{T}$, where $dQ$ is the heat added to the system and $T$ is the absolute temperature
• The entropy of a system can be calculated using the Clausius inequality ($\Delta S \geq \int \frac{dQ}{T}$) or the Boltzmann equation ($S = k \ln W$, where $k$ is the Boltzmann constant and $W$ is the number of microstates)
• The second law sets limits on the efficiency of heat engines and other energy conversion devices
  • The maximum efficiency of a heat engine operating between two temperatures is given by the Carnot efficiency ($\eta = 1 - \frac{T_L}{T_H}$)
• The second law also explains the arrow of time and the irreversibility of many real-world processes (mixing, heat transfer, chemical reactions)

Thermodynamic Cycles

• A thermodynamic cycle is a series of processes that return a system to its initial state
  • The net work done by the system during a cycle is equal to the net heat added to the system ($W_{net} = Q_{net}$)
• The Carnot cycle is an idealized, reversible cycle that consists of two isothermal and two adiabatic processes
  • The Carnot cycle represents the most efficient possible heat engine operating between two temperatures
• The Otto cycle is a four-stroke combustion cycle used in spark-ignition engines (gasoline engines)
  • The Otto cycle consists of isentropic compression, constant-volume heat addition, isentropic expansion, and constant-volume heat rejection
• The Diesel cycle is a four-stroke combustion cycle used in compression-ignition engines (diesel engines)
  • The Diesel cycle consists of isentropic compression, constant-pressure heat addition, isentropic expansion, and constant-volume heat rejection
• The Brayton cycle is an open gas turbine cycle used in jet engines and power generation
  • The Brayton cycle consists of isentropic compression, constant-pressure heat addition, isentropic expansion, and constant-pressure heat rejection
• The Rankine cycle is a closed vapor power cycle used in steam power plants
  • The Rankine cycle consists of isentropic compression in a pump, constant-pressure heat addition in a boiler, isentropic expansion in a turbine, and constant-pressure heat rejection in a condenser

Real-World Applications

• Thermodynamics plays a crucial role in the design and optimization of energy conversion devices (engines, power plants, refrigerators, heat pumps)
• The efficiency of real-world devices is limited by irreversibilities such as friction, heat transfer, and fluid flow losses
• Cogeneration systems use waste heat from power generation for heating or industrial processes, improving overall energy efficiency
• Regenerative braking in hybrid and electric vehicles converts kinetic energy into electrical energy during deceleration, improving fuel economy
• Heat exchangers are used in a wide range of applications to transfer heat between fluids (radiators, air conditioners, chemical processing)
• Thermal insulation reduces heat transfer and helps maintain desired temperatures in buildings, vehicles, and industrial equipment
• Thermodynamic analysis is used to optimize the performance of combustion processes in engines, furnaces, and power plants
• Refrigeration and air conditioning systems use the principles of thermodynamics to remove heat from a space and maintain a desired temperature
• Thermodynamics is essential for understanding and mitigating the effects of climate change, as it governs the energy balance of the Earth's atmosphere and oceans
• Thermodynamic principles are applied in the design and operation of fuel cells, which convert chemical energy directly into electrical energy with high efficiency