Introduction

A thermodynamic process occurs whenever a system changes from one state to another. During these changes, properties such as pressure, volume, temperature, and internal energy may change as energy is transferred as heat or work.

Scientists classify thermodynamic processes according to which state variable remains constant. Four important processes are isothermal, isobaric, isochoric, and adiabatic. These processes help us understand how engines, refrigerators, air conditioners, power stations, and many natural systems operate.

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What Is a Thermodynamic Process?

A thermodynamic process is a change in the state of a system.

During a process:

• Pressure may change.
• Volume may change.
• Temperature may change.
• Internal energy may change.
• Heat and work may be transferred.

The beginning and ending conditions are called the initial state and final state.

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Figure 1. A thermodynamic process changes the state of a system by transferring heat and/or work.

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Isothermal Process

An isothermal process occurs at constant temperature.

During an isothermal process:

• Temperature remains constant.
• Pressure and volume may change.
• Heat is transferred into or out of the system to keep the temperature constant.
• For an ideal gas, the internal energy remains constant because the temperature does not change.

Example:

• A gas expanding slowly inside a cylinder that is kept at a constant temperature.

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Isobaric Process

An isobaric process occurs at constant pressure.

During an isobaric process:

• Pressure remains constant.
• Volume usually changes.
• Temperature usually changes.
• Heat is often transferred into or out of the system.

Example:

• Heating water in an open saucepan at atmospheric pressure.

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Isochoric Process

An isochoric process (also called an isovolumetric process) occurs at constant volume.

During an isochoric process:

• Volume remains constant.
• Pressure changes.
• Temperature changes.
• No work is done because the volume does not change.

Example:

• Heating a gas inside a rigid sealed container.

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Adiabatic Process

An adiabatic process occurs without any heat transfer between the system and its surroundings.

During an adiabatic process:

• No heat enters or leaves the system.
• Temperature usually changes.
• Pressure and volume usually change.
• Internal energy changes because work is done.

Examples:

• Rapid compression of air in a bicycle pump.
• Expansion of gases in some engines.

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Figure 2. The four common thermodynamic processes differ in which quantity remains constant.

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Comparing the Four Processes

Each process has unique characteristics that help scientists analyse energy transfers.

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Heat and Work in Different Processes

Heat and work behave differently in each process.

Isothermal

• Heat is transferred to balance the work done.
• For an ideal gas, internal energy remains constant.

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Isobaric

• Heat may increase both the internal energy and the work done as the gas expands.

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Isochoric

• Heat changes the internal energy.
• No work is done because the volume remains constant.

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Adiabatic

• No heat is transferred.
• Changes in internal energy are caused entirely by work done on or by the system.

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Identifying Processes on a P–V Diagram

A Pressure–Volume (P–V) diagram shows how pressure changes with volume during a process.

Isobaric Process

A horizontal line.

Pressure remains constant while volume changes.

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Isochoric Process

A vertical line.

Volume remains constant while pressure changes.

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Isothermal Process

A smooth downward-curving line (hyperbola).

As volume increases, pressure decreases while temperature remains constant.

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Adiabatic Process

Also a downward-curving line, but it is steeper than an isothermal curve because no heat enters or leaves the system.

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Figure 3. Different thermodynamic processes produce characteristic shapes on a pressure–volume diagram.

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Comparing the Processes

These comparisons make it easier to identify each process.

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Real-World Examples

Isothermal

• Slow expansion of a gas kept in contact with a large heat reservoir.

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Isobaric

• Heating food in an open pot.
• Air inside an open hot-air balloon.

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Isochoric

• Heating gas inside a rigid aerosol can.
• A sealed metal gas cylinder.

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Adiabatic

• Compressing air in a bicycle pump.
• Rapid expansion of gases in a diesel engine.
• Rising air that cools as it expands in the atmosphere.

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Figure 4. Many everyday technologies involve one or more thermodynamic processes.

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Why These Processes Matter

Understanding thermodynamic processes allows scientists and engineers to:

• Design efficient engines.
• Improve refrigerators and air conditioners.
• Develop power stations.
• Predict atmospheric changes.
• Improve renewable energy technologies.

These idealised processes form the basis for analysing many real systems.

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Figure 5. Thermodynamic processes are fundamental to many technologies and natural phenomena.

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Worked Example

Question

A gas is heated inside a rigid steel container.

Identify the type of thermodynamic process.

Solution

Because the container is rigid, the volume cannot change.

Therefore:

• Volume remains constant.
• Pressure increases.
• Temperature increases.
• No work is done.

This is an isochoric process.

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Real-World Connection

A bicycle pump demonstrates an adiabatic process. When you rapidly compress the air inside the pump, there is very little time for heat to escape. Instead, the work you do compressing the gas increases its internal energy, causing the air inside the pump to become noticeably warmer. This same principle is used in diesel engines, where rapid compression heats the air enough to ignite the fuel without a spark plug.

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Did You Know?

The cooling of rising air that forms clouds is largely an adiabatic process. As warm air rises through the atmosphere, the surrounding air pressure decreases. The air expands, does work on its surroundings, and cools. If it cools enough, water vapour condenses into tiny droplets, forming clouds.

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Key Terms

Adiabatic process – A thermodynamic process in which no heat is transferred between the system and its surroundings.

Isobaric process – A process that occurs at constant pressure.

Isochoric process – A process that occurs at constant volume.

Isothermal process – A process that occurs at constant temperature.

P–V diagram – A graph showing the relationship between pressure and volume during a thermodynamic process.

Process – A change that takes a system from one thermodynamic state to another.

Work – Energy transferred when a force causes movement or when a gas expands or is compressed.

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Key Takeaways

• A thermodynamic process changes the state of a system through transfers of heat and/or work.
• Isothermal processes occur at constant temperature, isobaric processes at constant pressure, isochoric processes at constant volume, and adiabatic processes with no heat transfer.
• Heat and work contribute differently to changes in internal energy depending on the type of process.
• On a P–V diagram, isobaric processes are horizontal lines, isochoric processes are vertical lines, while isothermal and adiabatic processes are curved lines, with the adiabatic curve being steeper.
• These idealised processes help scientists and engineers analyse engines, refrigerators, weather systems, and many other real-world applications of thermodynamics.