In thermodynamics, what is the definition of a closed system?

A closed system is defined as one that can exchange energy in the form of heat or work with its surroundings but does not allow the transfer of matter. This distinction is crucial in thermodynamics because it helps to delineate how systems interact with their environments.

In a closed system, while energy can cross the boundaries (for example, heat can enter or leave the system), the mass within the system remains constant since no material can enter or exit. This is important for many thermodynamic analyses, such as in calculating changes in internal energy, where only the energy exchange needs to be considered without the complicating factor of changing mass.

Understanding this definition aids in various applications, such as in engines or refrigerators, where energy exchange is fundamental to their operation while keeping the materials within the devices contained throughout the process.