3.1 Thermodynamic Systems

Thermodynamic systems are the foundation of understanding energy transfer and transformation. These systems, defined by boundaries, interact with their surroundings through heat and work, leading to changes in state variables like pressure, volume, and temperature.

Thermodynamic processes describe how systems evolve and interact with their environment. The laws of thermodynamics govern these processes, establishing principles of energy conservation and the direction of spontaneous changes, while concepts like entropy help explain the natural world's behavior.

Thermodynamic Systems

Components of thermodynamic systems

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• Thermodynamic system
  • Specific portion of the universe under study can be any size or shape (gas in a container, cup of coffee, living organism)
• System boundary
  • Interface that separates the system from its surroundings can be real or imaginary, fixed or movable
  • Determines the type of system: open allows matter and energy exchange, closed allows only energy exchange, isolated allows neither matter nor energy exchange
• Surroundings
  • Everything outside the system boundary can interact with the system through the boundary (room where a gas container is located, air around a cup of coffee)

Thermal equilibrium and temperature

• Thermal equilibrium
  • State in which two or more systems have the same temperature occurs when there is no net heat transfer between the systems
  • Necessary condition for measuring temperature
• Zeroth Law of Thermodynamics
  • If two systems are in thermal equilibrium with a third system, they are in thermal equilibrium with each other allows for the definition of temperature
• Thermodynamic temperature
  • Measure of the average kinetic energy of the particles in a system directly related to the internal energy of the system
  • Measured in Kelvin (K) or Rankine (°R) with 0 K representing absolute zero, the lowest possible temperature

Equations of state in thermodynamics

• Equation of state
  • Mathematical relationship between the state variables of a system describes the thermodynamic state of a system (ideal gas law, van der Waals equation)
• State variables
  • Macroscopic properties that describe the state of a system (pressure (P), volume (V), temperature (T), internal energy (U), entropy (S))
• Ideal gas law
  • Equation of state for ideal gases relates pressure, volume, temperature, and the number of moles of gas
  • $PV = nRT$, where $n$ is the number of moles and $R$ is the universal gas constant applies to gases at low pressures and high temperatures

Thermodynamic Processes

System-surroundings interactions in processes

• Thermodynamic process
  • Change in the state of a system due to energy transfer or work can be reversible (system can return to its initial state) or irreversible (system cannot return to its initial state)
  • Examples:
    1. Isothermal process: constant temperature
    2. Isobaric process: constant pressure
    3. Isochoric process: constant volume
    4. Adiabatic process: no heat transfer
• Heat transfer ($Q$)
  • Energy transfer due to a temperature difference between the system and its surroundings positive when energy flows into the system, negative when energy flows out of the system
• Work ($W$)
  • Energy transfer due to a force acting through a distance positive when work is done by the system on its surroundings, negative when work is done on the system
• First Law of Thermodynamics
  • Change in internal energy ($\Delta U$) of a system is equal to the sum of heat transfer ($Q$) and work ($W$) done on the system
  • $\Delta U = Q + W$ establishes the conservation of energy principle in thermodynamics
  • Applied to various processes (isothermal, isobaric, isochoric, adiabatic) to determine changes in state variables and energy transfers

Advanced Thermodynamic Concepts

• Second Law of Thermodynamics
  • States that the total entropy of an isolated system always increases over time
  • Introduces the concept of irreversibility in natural processes
• Entropy
  • Measure of the disorder or randomness in a system
  • Helps explain the direction of spontaneous processes
• Heat engines
  • Devices that convert thermal energy into mechanical work (e.g., steam engines, internal combustion engines)
  • Efficiency is limited by the Second Law of Thermodynamics
• Carnot cycle
  • Theoretical thermodynamic cycle that describes the most efficient possible heat engine
  • Consists of two isothermal and two adiabatic processes