4.3.1. Thermodynamic Systems

Table of Contents

Defining a thermodynamic system

Thermodynamics studies how energy moves and changes in matter. To do this clearly, we first decide what part of the world we want to study. That chosen part is called the thermodynamic system. Everything outside it is called the surroundings. The system and surroundings together make up the universe, in the thermodynamic sense.

A thermodynamic system can be something very small, like a gas inside a syringe, or something very large, like an engine cylinder, a lake, or even a planet if we choose to study it that way. What matters is not the object itself, but how we define its boundary.

System, surroundings, and boundary

The boundary is the real or imaginary surface that separates the system from its surroundings. This boundary is very important because it tells us what can cross into or out of the system. Depending on the situation, energy may cross the boundary, matter may cross it, both may cross, or neither may cross.

A boundary can be fixed, like the walls of a sealed metal tank. It can also be movable, like the face of a piston in a cylinder. It can be rigid or flexible, insulating or conducting, permeable or impermeable to matter.

Rendering scientific sketch…

Thermodynamic system and surroundings

A thermodynamic system is the part of the universe chosen for study.
The surroundings are everything outside the system.
The boundary separates the system from the surroundings.

Why defining the system matters

The same physical situation can be analyzed in different ways depending on what we call the system. For example, in a piston with gas inside, we might choose only the gas as the system. Or we might choose the gas and the piston together. Each choice changes what counts as energy transfer or matter transfer across the boundary.

This is why the first step in many thermodynamics problems is to state the system clearly. A badly chosen system can make a simple problem confusing.

Types of thermodynamic systems

Thermodynamic systems are usually divided into three main types.

Open systems

An open system can exchange both matter and energy with its surroundings. A pot of boiling water without a lid is an open system. Water vapor leaves the pot, and heat enters from the stove.

Closed systems

A closed system can exchange energy with its surroundings, but not matter. A sealed bottle of gas is a closed system if no gas escapes, even though heat may still pass through the walls.

Isolated systems

An isolated system exchanges neither matter nor energy with its surroundings. A perfectly isolated system is an ideal model. In practice, real systems can only approximate isolation.

Type of system	Matter exchange	Energy exchange	Example
Open	Yes	Yes	Boiling water in an open pot
Closed	No	Yes	Gas in a sealed piston
Isolated	No	No	Ideal thermos, approximately

Open system, matter and energy can cross the boundary.
Closed system, energy can cross but matter cannot.
Isolated system, neither energy nor matter crosses the boundary.

Examples from everyday physics

Consider a cup of hot coffee left on a table. If we choose the coffee as the system, heat flows from the coffee to the air, so energy crosses the boundary. Also, water molecules may evaporate, so matter can leave. This makes it an open system.

Now consider gas trapped in a cylinder with a movable piston. If the gas cannot escape, then matter does not cross the boundary. But the gas may be heated, cooled, compressed, or expanded, so energy can cross. This is a closed system.

A well-insulated sealed container is often treated as isolated for calculations, even though perfect isolation is impossible in the real world.

Macroscopic description of a system

Thermodynamics usually describes systems using large-scale measurable properties, rather than tracking each particle one by one. These measurable properties are called state variables or thermodynamic variables. Common examples include pressure, volume, temperature, and amount of substance.

A system is described by its state, meaning the set of values of these variables at a given time. If the state changes, the system undergoes a process. The detailed types of thermodynamic processes, such as isothermal or adiabatic processes, belong to later chapters. Here, the key idea is that a system has a definable state that can be measured macroscopically.

Homogeneous and heterogeneous systems

A homogeneous system has the same composition and properties throughout every part of the system, at least on the scale of interest. A uniform gas in a container is a common example.

A heterogeneous system contains distinct parts or phases with different properties. Ice floating in water is a heterogeneous system because solid water and liquid water coexist.

The different uniform parts of a heterogeneous system are called phases. A phase is a region where the physical properties are uniform.

System	Homogeneous or heterogeneous	Phases
Dry air in a room	Homogeneous	1 gas phase
Ice and liquid water	Heterogeneous	2 phases
Oil and water	Heterogeneous	2 liquid phases

Simple compressible systems

Many beginner thermodynamics problems involve a simple compressible system, such as a gas in a piston. In such systems, the main mechanical effect is a change in volume due to compression or expansion. This makes them especially useful for learning the basic ideas of thermodynamics.

For a gas in a cylinder, the boundary may move when the gas expands or contracts. Even though the amount of gas stays fixed in a closed system, the volume can change because the boundary moves.

Rendering scientific sketch…

Gas in a piston-cylinder system

Real and imaginary boundaries

Sometimes the boundary is a physical wall, such as glass, steel, or rubber. Sometimes it is imaginary. For example, if we study the air inside a room, we can imagine an invisible boundary surrounding a chosen volume of air. Thermodynamics allows either kind of boundary, as long as it is defined clearly.

This is especially useful when dealing with flowing fluids, where the system may be a region in space rather than a fixed object.

Equilibrium and system definition

A thermodynamic system is often easiest to describe when it is in equilibrium. In equilibrium, its macroscopic properties do not change with time, and there are no unbalanced driving effects inside it. For example, the temperature is uniform throughout a system in thermal equilibrium.

A system not in equilibrium can still be studied, but its description becomes more complicated because different parts may have different temperatures, pressures, or compositions.

State of a system

The thermodynamic state of a system is specified by measurable macroscopic variables. For many simple systems, common variables are pressure $P$, volume $V$, and temperature $T$. If enough independent variables are known, the state is determined.

For an ideal gas, one important relation between state variables is

$$PV = nRT$$

where $n$ is the amount of substance and $R$ is the gas constant. This equation belongs more fully to the study of gases, but it is a good example of how a system can be described by state variables.

The state of a thermodynamic system is described by macroscopic variables such as $P$, $V$, and $T$.
A change in these variables means the system changes state.

Choosing the system in problem solving

In thermodynamics, choosing the system well is a practical skill. A good system boundary makes it easy to identify what enters, what leaves, and what changes inside. For example, if mass flow is important, an open system may be the best choice. If the amount of matter stays fixed, a closed system is often simpler.

When solving problems, always ask three questions. What is the system. Where is the boundary. What can cross it.

Summary ideas

Thermodynamic systems are the foundation of all thermodynamics. Before studying heat, work, energy, or thermodynamic processes, we must decide what part of the physical world we are analyzing. The system is the chosen part, the surroundings are everything else, and the boundary separates them. Systems may be open, closed, or isolated depending on whether matter and energy can cross the boundary. Once the system is defined, we can describe its state using macroscopic variables such as pressure, volume, and temperature.

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