A thermodynamic system is a quantity of matter or a region of space selected for analysis in thermodynamics. A boundary separates it from its surroundings and determines which exchanges of matter and energy are included in the analysis. The system may be gas inside a cylinder, material undergoing a reaction, or fluid passing through machinery. Defining the system is a modeling choice: the same physical apparatus can be examined using different boundaries, provided its properties and transfers are accounted for consistently. (web.mit.edu)
Boundaries and surroundings
The boundary is a real or imaginary surface enclosing the system. It may coincide with a container wall, follow a moving piston, or cut across a flowing stream. Everything outside it constitutes the surroundings. A boundary need not be stationary, and a system need not occupy a constant volume. In engineering, a selected region through which matter can flow is called a control volume, while its enclosing boundary is the control surface. (web.mit.edu)
Boundary conditions specify possible interactions. A rigid boundary prevents volume change; a movable boundary permits expansion or compression. An insulating boundary restricts heat transfer, whereas a thermally conducting boundary permits it. These restrictions are independent: a sealed container can exchange heat, and an insulated piston can transmit mechanical work. The distinction between thermal insulation and complete isolation is therefore essential. (openstax.org)
Open, closed, and isolated systems
Systems are commonly classified according to what can cross their boundaries:
- An open system can exchange both matter and energy with its surroundings. A turbine operating with an inlet and outlet is normally analyzed this way.
- A closed system exchanges no matter, although energy may cross its boundary. A fixed quantity of gas beneath a sealed piston is a standard example.
- An isolated system exchanges neither matter nor energy. Isolation is an idealization that can approximate sufficiently well-insulated, sealed arrangements over a specified time interval. (web.mit.edu)
A closed system is also called a control mass, because the analysis follows a fixed quantity of matter. Its volume can change. Conversely, matter can continuously enter and leave a control volume even when its total mass remains constant. Classification concerns permitted transfers, not simply whether the system’s measured contents change with time. (web.mit.edu)
State, properties, and equilibrium
A thermodynamic state is described by macroscopic properties such as temperature, pressure, volume, and composition. A state function depends on the current state rather than the route by which that state was reached. Internal energy is one example. Properties are classified as intensive, such as temperature and pressure, or extensive, such as mass and volume, which scale with the amount of material in an otherwise equivalent system. (web.mit.edu)
For an equilibrium system, an equation of state relates its properties. For example, the ideal gas model uses (pV=nRT), where (n) is the amount of substance and (R) is the gas constant. Such relations mean that not all listed properties can be specified independently. Their applicability depends on the material and the conditions represented by the model. (openstax.org)
Thermodynamic equilibrium requires the absence of unbalanced tendencies toward thermal, mechanical, and chemical change under the imposed constraints. Thermal equilibrium is associated with equal temperatures; mechanical equilibrium involves balanced forces; chemical equilibrium involves no net chemical transformation. The zeroth law of thermodynamics establishes the transitive character of thermal equilibrium. A steady flowing system is not necessarily in equilibrium: its properties may remain constant while transfers continue. (web.mit.edu)
Energy transfers and balances
Heat and work describe energy crossing a boundary, not substances stored inside a system. Heat is transfer caused by a temperature difference. Work includes mechanical and other nonthermal modes of energy transfer. Their values generally depend on the process, unlike changes in state functions. (ocw.mit.edu)
For a closed system, the first law of thermodynamics can be written
[ \Delta E=Q-W, ]
where (Q) is heat supplied to the system and (W) is work done by it. Total energy (E) includes internal energy and any relevant bulk kinetic and potential energy. When the latter contributions do not change, the equation becomes (\Delta U=Q-W). Other sign conventions are valid if used consistently. (openstax.org)
Open-system balances additionally account for energy transported by entering and leaving matter. In fluid-flow analysis, enthalpy, (H=U+pV), incorporates internal energy and the pressure–volume contribution associated with flow. For steady operation, energy entering through heat, work, and incoming streams balances energy leaving through the corresponding channels; kinetic and potential contributions may also matter. (live.ocw.mit.edu)
Processes and entropy
A process changes a system’s state. An isothermal process maintains constant temperature, an isobaric process constant pressure, and an isochoric process constant volume. An adiabatic process involves no heat transfer, but may involve work and temperature change. A quasistatic process proceeds through states arbitrarily close to equilibrium; this condition alone does not eliminate dissipative effects. In a cycle, the system returns to its initial state, so every state function has zero net change. (openstax.org)
Entropy supplies an additional constraint through the second law of thermodynamics. The entropy of an isolated system cannot decrease: it increases in irreversible processes and remains unchanged in the reversible limit. For a nonisolated system, its entropy may decrease through exchange with the surroundings without violating this law. Equilibrium in an isolated system corresponds to maximum entropy subject to its conserved quantities and constraints. (live.ocw.mit.edu)