Introduction to Thermodynamics
2. Systems and Surroundings
Learning outcomes
- I can distinguish between a system and its surroundings.
- I can identify open, closed, and isolated systems.
- I can describe system boundaries.
- I can explain how energy crosses system boundaries.
- I can classify real-world systems.
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.
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.
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.
Figure 1. The system is the part being studied, while everything else forms the surroundings.
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.
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.
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.
Figure 2. Heat and work are the two main ways energy crosses a system boundary.
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.
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.
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.
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.
Figure 3. Open, closed, and isolated systems differ in how they exchange energy and matter.
Comparing the Three Types of Systems
| System Type. | Energy Exchange. | Matter Exchange. | Example |
|---|---|---|---|
| Open | Yes | Yes | Human body, boiling pot |
| Closed | Yes | No | Sealed bottle, piston |
| Isolated | No | No | Ideal thermos (approximate) |
This classification helps scientists analyse energy transfers in different situations.
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.
Sealed Soft Drink Can
- Heat may enter or leave.
- No matter enters or leaves while sealed.
Classification: Closed system.
Thermos Flask
- Designed to reduce heat transfer.
- Matter remains inside.
Classification: Approximately isolated.
Boiling Pot Without a Lid
- Heat enters from the stove.
- Steam escapes.
Classification: Open system.
Figure 4. Many everyday objects can be classified as open, closed, or approximately isolated systems.
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.
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.
Figure 5. Defining systems helps scientists understand how energy is transferred in everyday technologies and natural processes.
Worked Example
Question
Classify each system.
| Example | Type of System |
|---|---|
| Boiling kettle with steam escaping. | ? |
| Sealed bottle of water | ? |
| Ideal insulated container | ? |
Solution
| Example | Type of System |
|---|---|
| Boiling kettle with steam escaping | Open system |
| Sealed bottle of water | Closed system |
| Ideal insulated container | Isolated system |
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.
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.
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.
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.