Glossary

1.2 Thermodynamic systems and surroundings

3 min read•july 23, 2024

Thermodynamic systems are the building blocks of energy analysis. They come in three flavors: open, closed, and isolated, each with unique ways of interacting with their . Understanding these systems is key to grasping how energy and matter move in the world around us.

The boundaries between systems and surroundings are where the action happens. It's here that energy transfers through heat and , and sometimes mass moves too. Knowing how to define and analyze these boundaries is crucial for solving real-world thermodynamic problems.

Thermodynamic Systems and Surroundings

Types of thermodynamic systems

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  • Thermodynamic systems represent a region in space or a quantity of matter bounded by a closed surface
    • Three main types of thermodynamic systems: open, closed, and isolated
  • Open systems allow the transfer of both energy and mass across the system
    • Examples include a pot of boiling water, a turbine, and a compressor
  • Closed systems allow the transfer of energy but not mass across the system boundary
    • Examples include a sealed piston-cylinder device, a closed tank, and a pressure cooker
  • Isolated systems do not allow the transfer of either energy or mass across the system boundary
    • Examples include a perfectly insulated container, a thermos flask, and an

Boundaries in thermodynamic systems

  • System boundary is the real or imaginary surface that separates the system from its surroundings
    • Defined based on the problem under consideration
  • Surroundings encompass everything outside the system boundary
    • Can interact with the system through energy and mass transfer
  • Identifying the system and surroundings involves:
    • Clearly defining the system of interest
    • Determining the appropriate system boundary based on the problem statement
    • Considering the interactions between the system and its surroundings (, work)

System-surroundings interactions

  • Energy transfer occurs through heat (QQ), the transfer of energy due to a temperature difference, and work (WW), the transfer of energy due to a force acting through a distance
  • Mass transfer occurs in open systems and involves the exchange of matter between the system and its surroundings
  • Interactions between the system and surroundings require:
    1. Determining the direction of energy and mass transfer (into or out of the system)
    2. Analyzing the impact of these interactions on the system's properties (temperature, pressure, volume)
    3. Applying the conservation of energy and mass principles

Control volume in thermodynamics

  • Control volume is a fixed region in space through which matter may flow
    • Used to analyze open systems
  • Conservation equations for a control volume include:
    • Conservation of mass: dmcvdt=∑m˙in−∑m˙out\frac{dm_{cv}}{dt} = \sum \dot{m}_{in} - \sum \dot{m}_{out}
    • Conservation of energy: dEcvdt=Q˙cv−W˙cv+∑m˙in(h+V22+gz)in−∑m˙out(h+V22+gz)out\frac{dE_{cv}}{dt} = \dot{Q}_{cv} - \dot{W}_{cv} + \sum \dot{m}_{in}(h + \frac{V^2}{2} + gz)_{in} - \sum \dot{m}_{out}(h + \frac{V^2}{2} + gz)_{out}
  • Steady-state processes involve no change in the properties of the control volume with respect to time
    • Simplifies the conservation equations by setting time derivatives to zero
  • Analyzing thermodynamic systems using control volumes requires:
    1. Defining the control volume and its boundaries
    2. Identifying the energy and mass interactions between the control volume and its surroundings
    3. Applying the appropriate conservation equations based on the problem statement (steady-state or transient)

Key Terms to Review (18)

Adiabatic process: An adiabatic process is a thermodynamic process in which no heat is exchanged between the system and its surroundings. This means that any change in the internal energy of the system is entirely due to work done on or by the system, making it a critical concept in understanding various thermodynamic cycles and processes.
Boundary: In thermodynamics, a boundary is the demarcation that separates a system from its surroundings. It can be real or imaginary and plays a crucial role in determining how energy and matter interact between the system and its environment. Understanding the boundary helps in analyzing how systems exchange heat, work, and mass with their surroundings.
Closed System: A closed system is a type of thermodynamic system that can exchange energy, but not matter, with its surroundings. This means that while energy in the form of heat or work can enter or leave the system, the total mass remains constant as no substances can cross its boundaries. Understanding closed systems is essential for analyzing energy conservation and various thermodynamic processes.
Critical Point: The critical point is the end point of a phase equilibrium curve, where distinct liquid and gas phases cease to exist, and the properties of the substance become indistinguishable. At this point, both the temperature and pressure are at their critical values, leading to a supercritical fluid state that exhibits unique behaviors, connecting various aspects of thermodynamic systems and phase equilibria.
Enthalpy: Enthalpy is a thermodynamic property that represents the total heat content of a system, defined as the sum of its internal energy and the product of its pressure and volume. This concept is crucial in understanding how energy is exchanged in processes, especially in the context of thermodynamic systems and the transformations they undergo.
Equilibrium State: An equilibrium state is a condition in which a thermodynamic system's properties remain constant over time because it is in balance with its surroundings. In this state, there are no net changes occurring within the system or between the system and its environment, leading to stability. Understanding equilibrium is crucial as it helps differentiate between various thermodynamic processes, whether they are reversible or irreversible, and plays a significant role in the concepts of entropy and the Second Law of Thermodynamics.
First Law of Thermodynamics: The First Law of Thermodynamics states that energy cannot be created or destroyed, only transformed from one form to another. This principle emphasizes the conservation of energy within a closed system, illustrating how energy transfers and transformations impact thermodynamic processes and systems.
Heat Transfer: Heat transfer is the process of thermal energy moving from one object or system to another due to a temperature difference. This fundamental concept connects various phenomena, including how systems reach thermodynamic equilibrium and how energy is conserved in different processes.
Internal Energy: Internal energy is the total energy contained within a thermodynamic system, encompassing kinetic and potential energies of all the molecules in the system. This energy is crucial in determining the state of the system and plays a key role in thermodynamic processes, including heat transfer and work done on or by the system.
Isolated System: An isolated system is a type of thermodynamic system that does not exchange matter or energy with its surroundings. This means that both energy transfer and mass transfer are completely restricted, allowing the system to evolve according to its own internal processes without external interference. In this context, understanding isolated systems helps in grasping the fundamental principles of thermodynamics, the interaction between systems and their environments, the behavior of entropy, and the statistical mechanics related to entropy in microcanonical ensembles.
Isothermal process: An isothermal process is a thermodynamic process in which the temperature of the system remains constant while heat is exchanged with the surroundings. This constant temperature implies that any internal energy changes in the system are fully compensated by heat transfer, making it an essential concept in understanding how systems behave under thermal equilibrium and the laws governing energy conservation.
Open System: An open system is a type of thermodynamic system that can exchange both matter and energy with its surroundings. This characteristic allows for the flow of mass and energy, enabling various processes to occur, such as chemical reactions, heat transfer, and fluid movement, all of which are essential in understanding fundamental thermodynamic principles.
Phase Transition: A phase transition is the process where a substance changes from one state of matter to another, such as from solid to liquid or liquid to gas. This change occurs when energy is added or removed, typically through heat, causing the molecules within the substance to rearrange and alter their interactions. Understanding phase transitions is crucial as they relate to fundamental concepts in thermodynamics, the behavior of systems and their surroundings, the heat transfer involved during calorimetry, and the quantitative relationships defined by equations like the Clausius-Clapeyron equation.
Sadi Carnot: Sadi Carnot was a French physicist and engineer who is often referred to as the 'father of thermodynamics' for his foundational work on heat engines and the theoretical understanding of their efficiency. His most notable contribution, the Carnot cycle, established a benchmark for the maximum possible efficiency of a heat engine operating between two thermal reservoirs. This work laid the groundwork for later developments in thermodynamic principles and understanding how systems interact with their surroundings.
Second law of thermodynamics: The second law of thermodynamics states that the total entropy of an isolated system can never decrease over time, and it dictates the direction of thermodynamic processes. This principle establishes that energy transformations are not 100% efficient, highlighting the inherent tendency for systems to move towards a state of greater disorder or randomness, affecting heat transfer, the performance of engines, and various processes in nature.
State Function: A state function is a property of a system that depends only on the current state of the system, not on the path taken to reach that state. State functions are crucial because they allow for a clear description of a system's condition at any given moment, regardless of how it got there, making them fundamental in understanding thermodynamic principles.
Surroundings: Surroundings refer to everything outside a thermodynamic system that can interact with the system and influence its properties. Understanding the surroundings is crucial as they play a key role in energy transfers and thermodynamic processes, often affecting temperature, pressure, and phase changes within the system.
Work: In thermodynamics, work is defined as the energy transfer that occurs when a force is applied to an object and it moves a distance in the direction of that force. This concept is essential to understanding how energy is converted and transferred between thermodynamic systems and their surroundings, and it helps differentiate between various forms of energy such as mechanical, electrical, and thermal energy.
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