System, Surroundings, and State

Thermodynamics studies the relationship between heat, work, temperature, and energy. A key concept in this field is a "system," the macroscopic part of the universe under observation. Systems can interact with their surroundings, leading to three types: open, closed, and isolated systems.

Open systems permit the exchange of both matter and energy with their surroundings, like a boiling pot of water.

In contrast, closed systems only allow the transfer of energy, restricting the movement of matter between the system and its surroundings. An example of a closed system would be a sealed container of gas.

Isolated systems ideally do not exchange either matter or energy with their surroundings; a thermos flask is often used as an approximate example of such a system.

The separation between a system and its surroundings is often defined by a "wall." Walls can be classified as rigid or nonrigid, permeable or impermeable, and adiabatic or nonadiabatic.

A rigid wall is fixed and cannot move, whereas a nonrigid wall, like a movable piston, can change position. An impermeable wall does not allow matter to pass through it, while a permeable wall allows matter transfer. An adiabatic wall does not conduct heat, meaning no heat exchange occurs between the system and its surroundings. In contrast, a nonadiabatic wall allows heat transfer.

This knowledge enables us to describe the state of a system accurately and predict how it might respond to changes in its surroundings, informing everything from engine design to climate modeling.