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In thermodynamics, a closed system is an essential concept where certain exchanges occur with the surroundings. A closed system allows energy transfers, such as in the forms of heat or work, to cross its boundaries. However, it keeps its mass constant by not permitting any mass to leave or enter through its boundary.
A classic example would be a sealed containers or a piston-cylinder device where no gas or fluid can escape. This helps in maintaining the total mass within the system, making it easier to analyze certain thermodynamic processes.

• Mass remains constant within the system.
• Energy, such as heat or work, can cross the boundary.

It is distinct from an open system, which allows both energy and mass to pass through, and from an isolated system, which permits neither.

Thermodynamics is the branch of physics focusing on the study of energy, energy transfer, and how energy affects matter. It examines the interactions between objects or systems and their surroundings, highlighting the laws governing these processes.
The core aspects of thermodynamics include the laws of energy conservation (known as the First Law of Thermodynamics), entropy (the Second Law), and thermodynamic equilibrium. These principles help predict how systems will respond to various changes in external factors like temperature and pressure.
Understanding thermodynamics is essential for fields like engineering, chemistry, and environmental science. It provides the underlying principles for many natural phenomena, from engine operations to climate change, and is crucial in designing systems that efficiently manage energy.

Thermodynamic properties are characteristics of a system that help describe its physical and chemical condition. These properties can be categorized into intensive and extensive properties.

• Intensive properties: These do not depend on the amount of matter present. Examples include temperature, pressure, and density.
• Extensive properties: These directly depend on the system's size or the amount of matter in it. Examples include volume, mass, and total energy.

By analyzing these properties, scientists and engineers can deduce how a system behaves and predict its responses to various changes.
These properties are often used in equations of state, which relate them to each other and help understand the thermodynamic equilibrium of systems.

Energy transfer is a fundamental concept in thermodynamics referring to the movement of energy from one part of a system to another, or between the system and its surroundings. This transfer can occur in two primary forms: heat and work.

• Heat: Thermal energy that flows due to a temperature difference between system and surroundings.
• Work: Energy transferred when a force is applied over a distance.

In closed systems, while mass remains constant within the boundaries, these energy exchanges allow the system to undergo various processes and reach different states.
Understanding energy transfer helps in analyzing how systems evolve over time. It is pivotal in designing efficient machines, engines, and thermal systems, and plays a critical role in various industrial applications.