1 Thermodynamic Systems, Properties, and States
Learn how thermodynamic systems and boundaries are defined, how properties describe states, and how equilibrium and processes are distinguished.
Define systems and boundaries
A is the quantity of matter or region in space selected for study. Everything outside the system is its surroundings, and the separates the two. A can be a physical surface or an imaginary surface chosen for analysis.
For gas inside a piston-cylinder device, the gas may be selected as the system, with the cylinder walls and piston forming its . As the piston moves, part of that moves too. Choosing a different can simplify a problem without changing the physical situation.
Boundaries can be described as fixed or moving according to whether their position changes, and as real or imaginary according to whether they correspond to a physical surface. They can also be adiabatic or diathermal: in the model, an adiabatic prevents heat transfer, while a diathermal permits it.
Classify systems by exchange
Systems are commonly classified by what can cross their boundaries. A contains a fixed amount of matter, so matter does not cross its , although energy may. Gas in a sealed piston-cylinder is an example.
An allows matter and energy to cross its . Water flowing through a pump and steam leaving a kettle are examples. The selected region is often called a control volume, and its a control surface.
An exchanges neither matter nor energy with its surroundings. It is an idealization; a well-insulated, sealed container approximates one. Even when matter cannot cross a system , energy may cross as heat or work. Which exchanges are possible depends on the and the system’s interactions with its surroundings.
Describe properties and states
A is a measurable macroscopic characteristic of a system, such as pressure, temperature, volume, mass, or internal energy. It depends on the system’s present condition, not on the route by which that condition was reached; properties are therefore also called properties.
Intensive properties are independent of system size; examples include temperature, pressure, and density. Extensive properties depend on system size and are additive for combined subsystems; examples include mass, volume, and total internal energy.
Dividing an by mass gives a , which is intensive. Specific volume is volume per unit mass, and specific internal energy is internal energy per unit mass.
The of a system is its condition as described by a sufficient set of independent properties. For a simple, compressible substance of fixed composition, pressure and temperature often suffice to specify the when they are independent. An expresses a relationship among properties. For an ideal gas, one such relationship is
Here, is pressure, is volume, is mass, is the gas constant, and is absolute temperature.
Recognize forms of equilibrium
means there is no tendency for a spontaneous change in the system’s macroscopic . At equilibrium, properties are uniform where needed to describe the system as a whole, and there are no unbalanced driving forces for change.
Equilibrium includes several forms:
Thermal equilibrium: no temperature differences drive heat transfer.
Mechanical equilibrium: no unbalanced pressure or force differences drive motion.
Chemical equilibrium: no net chemical reaction or transfer of species changes the system’s composition.
Phase equilibrium: there is no net change in the amounts of the phases present.
A system can be in one kind of equilibrium but not another. For example, a closed container may have uniform pressure while a temperature difference still drives heat transfer. Real systems may have gradients; in such cases, dividing the system into smaller regions or using local properties can be more useful than treating it as one uniform .
Distinguish processes from states
A thermodynamic process is a change from one to another, and the sequence of states followed is its path. Processes are often named for a that remains constant:
Isothermal: temperature remains constant.
Isobaric: pressure remains constant.
Isochoric (or isovolumetric): volume remains constant.
Adiabatic: no heat crosses the .
These names identify a constraint but do not, by themselves, specify every detail of the process. In particular, adiabatic does not necessarily mean that temperature remains constant.
A proceeds slowly enough for the system to remain close to equilibrium at each stage. This idealization allows the process to be described as a continuous sequence of equilibrium states. Many real processes are too rapid or involve strong gradients to be quasi-equilibrium, but the approximation is useful for analysis.
properties describe the endpoints, while a process describes the path between them. Pressure and temperature are properties; heat and work are transfers associated with processes, not properties stored in a system. Different processes can connect the same initial and final states even when their paths differ.