Thermodynamic equilibrium is the backbone of understanding system behavior. It's when a system's properties stay constant over time, with no energy or matter exchange. This state allows us to use key laws and principles to predict how systems will act.

Different types of processes, like isothermal and isobaric, show how systems change under various conditions. Quasi-static processes help us analyze systems that change slowly, while reversibility concepts explain ideal vs. real-world scenarios in thermodynamics.

Thermodynamic Equilibrium

Concept of thermodynamic equilibrium

• Thermodynamic equilibrium refers to a state in which a system's macroscopic properties remain constant over time with no net exchange of energy or matter between the system and its surroundings
• Characterized by uniform temperature, pressure, and chemical potential throughout the system (thermal, mechanical, and chemical equilibrium)
• Allows for the application of thermodynamic laws and principles to calculate system properties and predict system behavior (ideal gas law, first law of thermodynamics)

Types of thermodynamic processes

• Isothermal process occurs at constant temperature and requires heat exchange between the system and surroundings to maintain constant temperature, with work done by or on the system resulting in a change in internal energy (ideal gas expansion/compression)
• Isobaric process occurs at constant pressure and involves changes in volume and temperature, with heat transfer and work occurring during the process (heating a gas at constant pressure)
• Isochoric (isovolumetric) process occurs at constant volume with no work done by or on the system, and heat transfer results in a change in internal energy and temperature (heating a gas in a sealed container)
• Adiabatic process occurs without heat exchange between the system and surroundings, with work done by or on the system resulting in a change in temperature and internal energy, and rapid processes often approximating adiabatic conditions (compression stroke in an internal combustion engine)

Analysis of quasi-static processes

• Quasi-static process occurs slowly enough for the system to remain infinitesimally close to equilibrium at all times, allowing for the application of equilibrium thermodynamic equations
  • Reversible process is a quasi-static process with no dissipative effects (friction, viscosity)
• Problem-solving approach for quasi-static processes:
  1. Identify the type of process (isothermal, isobaric, isochoric, or adiabatic)
  2. Apply the appropriate thermodynamic equations for the specific process:
     • Ideal gas law: $PV = nRT$
     • First law of thermodynamics: $\Delta U = Q - W$
     • Work done by the system: $W = \int P dV$
  3. Use given information to solve for unknown quantities

Reversibility vs irreversibility in thermodynamics

• Reversible process can be reversed without any net change in the system or surroundings, requires the process to be quasi-static and have no dissipative effects, and represents an ideal limit that actual processes can approach but never achieve (Carnot cycle)
• Irreversible process cannot be reversed without a net change in the system or surroundings, involves dissipative effects (friction, viscosity, heat transfer across a finite temperature difference), and all real-world processes are irreversible to some extent
• Reversible processes establish the maximum theoretical efficiency for energy conversion devices (heat engines, refrigerators), while irreversibilities reduce the efficiency of real-world devices and processes
• The second law of thermodynamics states that the entropy of an isolated system always increases for irreversible processes