1. What Is Thermodynamics?

2. Systems and Surroundings

Introduction

When scientists study energy transfers, they first decide what part of the universe they want to investigate. This chosen part is called the system, while everything outside it is called the surroundings. Separating a situation into a system and its surroundings allows scientists to carefully track how energy and matter move.

For example, when studying a hot cup of coffee, the coffee itself may be chosen as the system, while the cup, table, and surrounding air become the surroundings. This simple idea forms the foundation of thermodynamics and helps explain how engines, refrigerators, living organisms, and even planets exchange energy.

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What Is a System?

A system is the part of the universe selected for study.

The system can be:

• An object.
• A substance.
• A machine.
• A living organism.
• A group of objects.

Examples include:

• A cup of hot tea.
• The air inside a balloon.
• A car engine.
• A human body.
• A steam turbine.

Scientists choose the system depending on the question they want to investigate.

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What Are the Surroundings?

The surroundings are everything outside the system.

The surroundings may include:

• The surrounding air.
• Nearby objects.
• The room.
• The Earth.
• Any source or receiver of energy.

Energy and sometimes matter may move between the system and its surroundings.

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Figure 1. The system is the part being studied, while everything else forms the surroundings.

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System Boundaries

A system boundary is the real or imaginary surface that separates the system from its surroundings.

The boundary determines:

• What belongs to the system.
• What belongs to the surroundings.
• Whether energy or matter can cross.

Boundaries may be:

• Physical (such as the wall of a container).
• Imaginary (used for scientific analysis).

Choosing appropriate boundaries helps scientists study energy transfers accurately.

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Energy Crossing System Boundaries

Energy can cross system boundaries in two main ways:

Heat

Energy transferred because of a temperature difference.

Example:

• A hot drink transfers heat to the surrounding air.

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Work

Energy transferred when forces cause movement.

Example:

• A piston compressing a gas.
• A motor lifting an object.

Energy can move either:

• Into the system.
• Out of the system.

These transfers change the system's internal energy.

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Figure 2. Heat and work are the two main ways energy crosses a system boundary.

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Types of Thermodynamic Systems

There are three main types of systems.

• Open systems.
• Closed systems.
• Isolated systems.

They differ in whether energy and matter can cross the boundary.

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Open Systems

An open system exchanges both energy and matter with its surroundings.

Examples:

• A boiling pot without a lid.
• A human body.
• A campfire.
• A running car engine.

Matter enters and leaves, and energy is also transferred.

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Closed Systems

A closed system exchanges energy but not matter with its surroundings.

Examples:

• A sealed bottle warming in the sun.
• A pressure cooker with the lid locked.
• Gas inside a sealed piston.

Heat and work may cross the boundary, but matter remains inside.

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Isolated Systems

An isolated system exchanges neither energy nor matter with its surroundings.

Perfect isolated systems do not exist in nature, but some systems come close.

Examples include:

• A well-insulated thermos (approximately).
• Scientific models used in thermodynamics.

An isolated system keeps both matter and energy contained.

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Figure 3. Open, closed, and isolated systems differ in how they exchange energy and matter.

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Comparing the Three Types of Systems

This classification helps scientists analyse energy transfers in different situations.

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Classifying Real-World Systems

Many everyday situations can be classified.

Human Body

• Energy enters as food.
• Energy leaves as heat and movement.
• Matter enters as food, water, and oxygen.
• Matter leaves as carbon dioxide and wastes.

Classification: Open system.

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Sealed Soft Drink Can

• Heat may enter or leave.
• No matter enters or leaves while sealed.

Classification: Closed system.

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Thermos Flask

• Designed to reduce heat transfer.
• Matter remains inside.

Classification: Approximately isolated.

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Boiling Pot Without a Lid

• Heat enters from the stove.
• Steam escapes.

Classification: Open system.

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Figure 4. Many everyday objects can be classified as open, closed, or approximately isolated systems.

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Why Systems Matter

Choosing an appropriate system allows scientists to:

• Track energy transfers.
• Study heat flow.
• Analyse engines.
• Improve energy efficiency.
• Predict how systems behave.

The concept of systems is fundamental throughout science, not only in thermodynamics but also in biology, chemistry, and environmental science.

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Systems in Everyday Life

Examples include:

• Refrigerators transferring heat.
• Car engines converting fuel into work.
• Air conditioners cooling buildings.
• Human metabolism.
• Solar panels converting sunlight into electrical energy.

Each example can be analysed by defining a system and examining how energy crosses its boundary.

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Figure 5. Defining systems helps scientists understand how energy is transferred in everyday technologies and natural processes.

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Worked Example

Question

Classify each system.

 

 

Solution

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Real-World Connection

Engineers designing spacecraft pay close attention to system boundaries. Although a spacecraft is mostly closed to matter, it still exchanges energy with space by absorbing sunlight and radiating heat. Careful thermal design keeps astronauts and onboard equipment at safe operating temperatures despite the extreme conditions of space.

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Did You Know?

A vacuum flask (thermos) is designed to reduce heat transfer by using a vacuum between its inner and outer walls. Because a vacuum contains almost no particles, it greatly reduces heat transfer by conduction and convection. However, some energy is still transferred by radiation, so no thermos is a perfectly isolated system.

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Key Terms

Boundary – The real or imaginary surface separating a system from its surroundings.

Closed system – A system that exchanges energy but not matter with its surroundings.

Energy transfer – The movement of energy into or out of a system.

Isolated system – A system that exchanges neither energy nor matter with its surroundings.

Matter – Anything that has mass and occupies space.

Open system – A system that exchanges both energy and matter with its surroundings.

Surroundings – Everything outside the system being studied.

System – The part of the universe selected for study.

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Key Takeaways

• A system is the part of the universe chosen for study, while everything else is the surroundings.
• A system boundary separates the system from its surroundings and determines what may cross between them.
• Energy crosses system boundaries mainly as heat or work.
• Open systems exchange both energy and matter, closed systems exchange energy but not matter, and isolated systems exchange neither.
• Many everyday objects, organisms, and technologies can be classified as open, closed, or approximately isolated systems.
• Defining systems and their boundaries is essential for understanding energy transfers in thermodynamics and many other areas of science.

3. State Variables

Introduction

To understand a thermodynamic system, scientists must be able to describe its condition at any moment. Instead of tracking every individual particle, they use a small set of measurable quantities called state variables. These variables describe the overall condition, or state, of a system.

For example, if you know the pressure, volume, and temperature of the air inside a bicycle tyre, you have a good description of its thermodynamic state. If any of these quantities changes, the state of the system changes as well. State variables help scientists predict how systems behave and form the basis of many important gas laws and thermodynamic equations.

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What Are State Variables?

A state variable is a measurable property that describes the current condition (state) of a thermodynamic system.

State variables describe the system itself, not how it reached that state.

Common state variables include:

• Pressure (P)
• Volume (V)
• Temperature (T)
• Internal energy (U)

Knowing these variables allows scientists to describe the condition of a system.

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Figure 1. State variables describe the condition of a thermodynamic system.

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Pressure

Pressure (P) is the force exerted per unit area.

In gases, pressure is caused by particles colliding with the walls of their container.

Pressure depends on factors such as:

• Temperature.
• Volume.
• Number of gas particles.

Common units include:

• Pascal (Pa)
• Kilopascal (kPa)
• Atmosphere (atm)

Higher pressure means particles collide with the container walls more frequently or more forcefully.

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Volume

Volume (V) is the amount of space occupied by a substance.

For gases:

• Volume usually refers to the size of the container holding the gas.

Common units include:

• Cubic metre (m³)
• Cubic centimetre (cm³)
• Litre (L)

Changing the volume often changes the pressure and temperature of a gas.

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Temperature

Temperature (T) measures the average kinetic energy of the particles in a substance.

Higher temperature means:

• Faster-moving particles.
• More energetic collisions.

The SI unit of temperature is the kelvin (K).

Degrees Celsius (°C) are also commonly used.

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Internal Energy

Internal energy (U) is the total microscopic energy stored within a substance.

It includes:

• The kinetic energy of moving particles.
• The potential energy due to forces between particles.

Internal energy changes when:

• Heat is transferred.
• Work is done on or by the system.

Internal energy is another important state variable.

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Figure 2. Pressure, volume, temperature, and internal energy are all linked through the behaviour of particles.

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State Variables vs Process Variables

State variables and process variables describe different aspects of thermodynamics.

State Variables

Describe the condition of the system.

Examples:

• Pressure
• Volume
• Temperature
• Internal energy

Their values depend only on the current state.

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Process Variables

Describe how energy is transferred during a change.

Examples:

• Heat (Q)
• Work (W)

Process variables depend on the path taken between two states, not just the starting and ending conditions.

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Comparing State and Process Variables

State variables describe the system.

Process variables describe changes occurring to the system.

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How State Variables Describe a System

Suppose a sealed gas container has:

• Pressure = 200 kPa
• Volume = 2.0 L
• Temperature = 300 K

These values completely describe the current state of the gas.

If the gas is heated:

• Temperature increases.
• Pressure may increase.
• Internal energy increases.

The system has entered a new thermodynamic state.

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Figure 3. Changing one state variable often changes the others, resulting in a new thermodynamic state.

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Relationships Between State Variables

The state variables are often connected.

Examples include:

Increase Temperature

If volume remains constant:

• Pressure increases.

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Increase Volume

If temperature remains constant:

• Pressure decreases.

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Compress a Gas

• Volume decreases.
• Pressure increases.
• Internal energy may increase if the gas is compressed rapidly.

These relationships are explained by the kinetic particle model and later by the gas laws.

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Predicting Changes

Understanding state variables allows us to predict what happens when conditions change.

These predictions are based on particle behaviour.

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State Diagrams

A state diagram is a graph showing how one state variable changes with another.

Common examples include:

• Pressure–Volume (P–V) diagrams.
• Pressure–Temperature (P–T) diagrams.
• Volume–Temperature (V–T) diagrams.

Each point on a state diagram represents one particular state of the system.

Moving from one point to another represents a thermodynamic process.

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Figure 4. A pressure–volume (P–V) diagram shows how the state of a system changes during a thermodynamic process.

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Reading a Simple State Diagram

Consider a simple Pressure–Volume (P–V) graph.

If the graph moves:

• Upwards → Pressure increases.
• Downwards → Pressure decreases.
• Right → Volume increases.
• Left → Volume decreases.

Each point represents a different state.

The path between points shows how the system changed.

Later in thermodynamics, these diagrams will be used to calculate work done by gases.

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Why State Variables Matter

State variables allow scientists and engineers to:

• Describe systems accurately.
• Predict system behaviour.
• Analyse engines.
• Study gases.
• Design refrigeration systems.
• Improve energy efficiency.

They are fundamental to every area of thermodynamics.

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Figure 5. State variables are used to analyse engines, refrigerators, power plants, and many other thermodynamic systems.

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Worked Example

Question

A sealed container of gas is heated while its volume remains constant.

Predict what happens to:

• Temperature
• Pressure
• Internal energy

Solution

• Temperature increases because the particles move faster.
• Pressure increases because faster-moving particles collide more frequently and with greater force against the container walls.
• Internal energy increases because the particles have more kinetic energy.

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Real-World Connection

A pressure cooker is a practical example of changing state variables. As the cooker is heated, the temperature of the water and steam increases. Because the lid is sealed, the volume remains almost constant, causing the pressure inside the cooker to rise. The higher pressure allows water to boil at a higher temperature, helping food cook more quickly.

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Did You Know?

Meteorologists measure atmospheric pressure, temperature, and sometimes humidity at thousands of locations around the world every day. These measurements describe the state of the atmosphere and are used by computer models to predict weather patterns and storms.

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Key Terms

Internal energy (U) – The total microscopic energy stored within a substance.

Pressure (P) – The force exerted per unit area by particles colliding with a surface.

Process variable – A quantity, such as heat or work, that describes energy transferred during a process.

State – The current condition of a thermodynamic system.

State diagram – A graph showing the relationship between state variables during a thermodynamic process.

State variable – A measurable property that describes the current condition of a thermodynamic system.

Temperature (T) – A measure of the average kinetic energy of the particles in a substance.

Volume (V) – The amount of space occupied by a substance.

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Key Takeaways

• Pressure, volume, temperature, and internal energy are important state variables that describe the condition of a thermodynamic system.
• State variables describe the current state of a system, while process variables such as heat and work describe energy transfers during changes.
• Changing one state variable often affects the others because they are interconnected.
• State variables allow scientists to predict how thermodynamic systems behave under different conditions.
• State diagrams, such as pressure–volume (P–V) graphs, visually represent changes in the state of a system.
• Understanding state variables provides the foundation for studying gas laws, heat engines, and many other topics in thermodynamics.

4. Thermodynamic Processes

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.

5. Thermal Equilibrium and the Zeroth Law

Introduction

When a hot object touches a cold object, energy is transferred from the hotter object to the cooler one. This transfer continues until both objects reach the same temperature. At that point, thermal equilibrium has been reached, and there is no further net transfer of heat between them.

The concept of thermal equilibrium is so fundamental that it forms the basis of the Zeroth Law of Thermodynamics. Although it was named after the First and Second Laws, scientists realised later that this principle was even more basic. The Zeroth Law explains what temperature really means and why thermometers can accurately measure it.

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What Is Thermal Equilibrium?

Thermal equilibrium is the state in which two objects in thermal contact have the same temperature.

When thermal equilibrium is reached:

• No net heat flows between the objects.
• Both objects remain at the same temperature.
• Their temperatures stay constant unless conditions change.

Heat transfer stops because there is no longer a temperature difference.

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Figure 1. Heat flows from the hotter object to the cooler object until thermal equilibrium is reached.

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Heat Flow and Thermal Equilibrium

Heat always flows:

• From a hotter object.
• To a cooler object.

As heat is transferred:

• The hotter object cools.
• The cooler object warms.

Eventually:

• Both objects reach the same temperature.
• Net heat transfer stops.

This state is called thermal equilibrium.

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The Zeroth Law of Thermodynamics

The Zeroth Law of Thermodynamics states:

If object A is in thermal equilibrium with object B, and object B is in thermal equilibrium with object C, then object A is also in thermal equilibrium with object C.

In simple terms:

If two objects have the same temperature as a third object, they must also have the same temperature as each other.

This law allows us to compare temperatures using a thermometer.

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Figure 2. The Zeroth Law allows temperature to be compared using a third object, such as a thermometer.

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Why Is the Zeroth Law Important?

The Zeroth Law provides the scientific basis for measuring temperature.

It allows us to:

• Compare temperatures.
• Use thermometers.
• Define temperature scales.
• Determine when systems are in thermal equilibrium.

Without the Zeroth Law, temperature measurement would not be reliable.

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What Is Temperature?

Temperature is a measure of the average kinetic energy of the particles in a substance.

Higher temperature means:

• Particles move faster.
• Particles have greater average kinetic energy.

Lower temperature means:

• Particles move more slowly.
• Particles have lower average kinetic energy.

Temperature tells us how hot or cold an object is.

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Temperature and Heat

Although closely related, temperature and heat are different.

A hot cup of coffee and a hot swimming pool may have the same temperature, but the swimming pool contains much more thermal energy because it has far more particles.

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Figure 3. Temperature measures particle motion, while heat is energy transferred between objects.

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How Thermometers Work

A thermometer measures temperature by reaching thermal equilibrium with the object being measured.

The process is:

1. The thermometer is placed in contact with the object.
2. Heat flows between the object and the thermometer.
3. They reach thermal equilibrium.
4. The thermometer displays the common temperature.

Because of the Zeroth Law, the thermometer now has the same temperature as the object.

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Types of Thermometers

Several types of thermometers are used in science and everyday life.

Liquid-in-Glass Thermometers

• Use expanding liquids such as coloured alcohol.
• The liquid rises as temperature increases.

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Digital Thermometers

• Use electronic sensors.
• Provide fast, accurate readings.

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Infrared Thermometers

• Detect infrared radiation emitted by objects.
• Measure temperature without direct contact.

Each type relies on reaching or relating to thermal equilibrium.

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Figure 4. Different types of thermometers measure temperature using different physical principles.

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Applying the Zeroth Law

The Zeroth Law is used in many everyday situations.

Measuring Body Temperature

A thermometer reaches thermal equilibrium with the body.

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Cooking

An oven thermometer measures the oven temperature after reaching thermal equilibrium with the air inside.

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Weather Stations

Thermometers measure air temperature by reaching equilibrium with the surrounding air.

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Scientific Experiments

Researchers ensure systems reach thermal equilibrium before taking accurate measurements.

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Thermal Equilibrium in Everyday Life

Examples include:

• A hot drink cooling to room temperature.
• Ice melting in warm water.
• Metal left outdoors reaching the air temperature.
• Food cooling before being refrigerated.

In each case:

• Heat flows.
• Temperatures become equal.
• Thermal equilibrium is reached.

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Figure 5. Thermal equilibrium occurs in many everyday situations involving heat transfer.

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Worked Example

Question

A metal spoon is placed in a cup of hot tea.

Describe what happens.

Solution

Initially:

• The tea is hotter than the spoon.

Therefore:

• Heat flows from the tea to the spoon.
• The spoon's temperature increases.
• The tea cools slightly.

Eventually:

• The spoon and tea reach the same temperature.
• No net heat transfer occurs.

They are now in thermal equilibrium.

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Real-World Connection

Doctors use digital thermometers to measure body temperature because they quickly reach thermal equilibrium with the tissues they are measuring. Once the thermometer and the body are at the same temperature, the device displays an accurate reading. This simple procedure depends directly on the Zeroth Law of Thermodynamics, which makes reliable temperature measurement possible.

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Did You Know?

The Zeroth Law of Thermodynamics was named after the First and Second Laws had already been established. Scientists later realised that this principle was even more fundamental because the other laws rely on the concept of temperature. Rather than renumber the existing laws, they called it the Zeroth Law.

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Key Terms

Heat – Energy transferred between objects because of a temperature difference.

Temperature – A measure of the average kinetic energy of the particles in a substance.

Thermal contact – A situation in which two objects can exchange heat.

Thermal equilibrium – A state in which two objects have the same temperature and no net heat transfer occurs between them.

Thermometer – An instrument used to measure temperature.

Zeroth Law of Thermodynamics – The law stating that if two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other.

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Key Takeaways

• Thermal equilibrium occurs when two objects have the same temperature and no net heat flows between them.
• The Zeroth Law of Thermodynamics provides the basis for defining temperature and comparing temperatures.
• Temperature measures the average kinetic energy of particles, while heat is energy transferred because of a temperature difference.
• Thermometers work by reaching thermal equilibrium with the object being measured.
• The Zeroth Law makes accurate temperature measurement possible in medicine, science, engineering, and everyday life.
• Understanding thermal equilibrium is fundamental to the study of thermodynamics and heat transfer.