Thermodynamics

Thermodynamic Systems

Question

Explain the following terms in thermodynamics:

System, boundary and surroundings
Closed system
Open system
Isolated system
Adiabatic system
Phase
Macroscopic approach

Given

In thermodynamics, we select a particular quantity of matter or a prescribed region of space for analysis. This selected part is called the system.

Everything outside the system is called the surroundings, while the actual or hypothetical envelope separating the system from its surroundings is called the boundary.

Depending on whether matter and energy can cross the boundary, systems are classified as closed, open, isolated and adiabatic systems.

Basic Concept

Before solving a thermodynamics problem, the first step is to decide what part of the physical situation is being studied. That selected part is the system.

The system is separated from everything else by a boundary. The region outside the boundary is the surroundings.

\boxed{ \text{Universe} = \text{System} + \text{Surroundings} }

The important question is then: What can cross the boundary?

Matter may cross the boundary in an open system, while it cannot cross the boundary of a closed system. An isolated system exchanges neither matter nor energy with its surroundings.

Detailed Answer

1. System

A system is a finite quantity of matter or a prescribed region of space selected for study.

In simple words, the system is the part of the physical situation that we are interested in analysing.

For example, if we are studying the gas inside a cylinder, the gas may be selected as the thermodynamic system.

\boxed{ \text{System} = \text{Selected matter or region of space under study} }

2. Surroundings

The surroundings are everything outside the selected system that can interact with the system.

For example, if the gas inside a cylinder is the system, the cylinder, piston and the external environment can be considered part of the surroundings, depending on how the system is defined.

\boxed{ \text{Surroundings} = \text{Everything outside the system} }

3. Boundary

The boundary is the actual or hypothetical envelope enclosing the system.

The boundary separates the system from its surroundings. It may be fixed or moving.

It may also be real or imaginary.

Fixed boundary

A fixed boundary does not change its position. The walls of a rigid container are an example.

Moving boundary

A moving boundary changes its position. For example, the boundary of a gas contained inside a cylinder with a movable piston changes as the gas is compressed or expanded.

Real boundary

A real boundary is physically present, such as the walls of a cylinder or vessel.

Imaginary boundary

An imaginary boundary is an assumed boundary drawn around a region for the purpose of analysis. It does not have to be a physical wall.

\boxed{ \text{Boundary separates the system from its surroundings} }

4. Closed System

If the boundary of a system is impervious to the flow of matter, the system is called a closed system.

Therefore, matter cannot enter or leave a closed system. However, energy may cross the boundary in the form of heat or work.

\boxed{ \text{Closed system} \Rightarrow \text{No mass crosses the boundary} }

The mass of the system therefore remains constant, even though the system may exchange energy with its surroundings.

Example

Gas contained in an engine cylinder with its boundary formed by the cylinder walls, cylinder head and piston crown can be treated as a closed system when no mass enters or leaves.

\boxed{ m=\text{constant} }

5. Open System

An open system is a system in which matter flows into or out of the system.

Since mass can cross the boundary, the mass contained inside an open system is not necessarily constant.

\boxed{ \text{Open system} \Rightarrow \text{Mass can cross the boundary} }

Most engineering systems are open systems because many engineering devices involve continuous flow of matter.

Simple example

Consider a cylinder through which gas enters and leaves. The gas entering the system represents mass entering through the boundary, while the gas leaving represents mass leaving the system.

\boxed{ \text{Mass in} \neq \text{necessarily Mass out} }

6. Isolated System

An isolated system is a system which exchanges neither energy nor matter with any other system or with its environment.

Thus, neither mass nor energy crosses the boundary of an isolated system.

\boxed{ \text{Isolated system} \Rightarrow \text{No mass exchange and no energy exchange} }

This makes an isolated system more restrictive than a closed system. A closed system prevents mass transfer, but it can still exchange energy. An isolated system prevents both.

7. Adiabatic System

An adiabatic system is one which is thermally insulated from its surroundings.

Therefore, heat transfer through the boundary is prevented. However, the system can still exchange work with its surroundings.

\boxed{ Q=0 }

If an adiabatic system also cannot exchange work with its surroundings, it becomes an isolated system.

\boxed{ \text{Adiabatic + No work exchange} \Rightarrow \text{Isolated system} }

8. Phase

A phase is a quantity of matter which is homogeneous throughout in chemical composition and physical structure.

In simple words, a phase is a region of matter that has uniform chemical composition and physical structure throughout.

\boxed{ \text{Phase} = \text{Homogeneous chemical composition and physical structure} }

9. Macroscopic Approach

In the macroscopic approach, a certain quantity of matter is considered without taking into account the events occurring at molecular level.

In other words, thermodynamics is concerned with the gross or overall behaviour of the system rather than the motion of individual molecules.

This approach is also known as classical thermodynamics.

\boxed{ \text{Macroscopic approach} = \text{Study of overall behaviour without molecular details} }

Use of mathematical formulas

The analysis of a macroscopic system generally requires relatively simple mathematical formulas involving measurable properties such as pressure, temperature, volume and energy.

Average property values

The values of the properties of a system are treated as their average values. For example, the pressure of a gas in a closed container represents the average effect of the pressure exerted by the enormous number of individual molecules.

10. Comparison of Different Systems

The easiest way to distinguish the different types of systems is to check whether mass, heat and work can cross the boundary.

\boxed{ \begin{array}{c|c|c} \text{System} & \text{Mass transfer} & \text{Energy transfer} \\ \hline \text{Closed} & \text{No} & \text{Possible} \\ \text{Open} & \text{Yes} & \text{Possible} \\ \text{Isolated} & \text{No} & \text{No} \end{array} }

An adiabatic system is identified specifically by the absence of heat transfer:

\boxed{ Q=0 }

11. Closed System vs Open System

The main difference is the movement of mass across the boundary.

\boxed{ \begin{aligned} \text{Closed system:}&\quad \text{Mass cannot cross boundary}\\ \text{Open system:}&\quad \text{Mass can cross boundary} \end{aligned} }

Do not confuse a closed system with an isolated system. A closed system can exchange energy, while an isolated system cannot exchange either matter or energy.

12. Adiabatic System vs Isolated System

These two terms are related but they are not identical.

An adiabatic system only means that there is no heat transfer through the boundary. Work may still be exchanged.

An isolated system has no energy transfer and no matter transfer. Therefore, it cannot exchange heat or work with the surroundings.

\boxed{ \text{Adiabatic} \Rightarrow Q=0 } \] \[ \boxed{ \text{Isolated} \Rightarrow Q=0,\quad W=0,\quad \text{no mass transfer} }

13. How to Identify a System in a Problem

When solving a thermodynamics problem, follow these simple steps:

Decide exactly what matter or region of space you want to study.
Mark or imagine its boundary.
Identify everything outside the boundary as the surroundings.
Check whether mass can cross the boundary.
Check whether heat or work can cross the boundary.

This immediately helps you decide whether the system is open, closed, isolated or adiabatic.

Final Answer

A thermodynamic system is a finite quantity of matter or a prescribed region of space selected for study. It is separated from the surroundings by a boundary.

The important classifications are:

Closed system: Mass cannot cross the boundary, but energy may cross.
Open system: Mass can flow into or out of the system, and energy may also be transferred.
Isolated system: Neither matter nor energy is exchanged with the surroundings.
Adiabatic system: No heat is transferred through the boundary, although work exchange may still occur.
Phase: A quantity of matter homogeneous in chemical composition and physical structure.
Macroscopic approach: Study of the overall behaviour of matter without considering individual molecular events.

Quick Revision

System

\boxed{ \text{Selected matter or region of space} }

Boundary

\boxed{ \text{Separates system from surroundings} }

Closed system

\boxed{ \text{No mass transfer} }

Open system

\boxed{ \text{Mass can enter or leave} }

Isolated system

\boxed{ \text{No mass or energy transfer} }

Adiabatic system

\boxed{ Q=0 }

Phase

\boxed{ \text{Homogeneous chemical composition and physical structure} }

Macroscopic approach

\boxed{ \text{Overall behaviour without molecular-level details} }

Exam Tip

The fastest way to identify a thermodynamic system is to ask: Can mass cross the boundary?

\boxed{ \begin{aligned} \text{Mass cannot cross} &\Rightarrow \text{Closed system}\\ \text{Mass can cross} &\Rightarrow \text{Open system}\\ \text{Neither mass nor energy can cross} &\Rightarrow \text{Isolated system} \end{aligned} }

For an adiabatic system, remember the specific condition:

\boxed{Q=0}

The common mistake is to assume that every adiabatic system is isolated. That is not correct. An adiabatic system may still exchange work with its surroundings.

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