A thermodynamic system (or simply ‘system’) is a definite macroscopic region or space in the universe, in which one or more thermodynamic processes take place.
Everything external to a thermodynamic system is called surroundings.
System and surroundings are separated by a definite border called boundary. System, surroundings and boundary constitute the universe.
See the image on the right for better understanding.
Types of Thermodynamic System:
Thermodynamic systems can be broadly classified into three types. They are:
An open system is a thermodynamic system which allows both mass and energy to flow in and out of it, across its boundary. The image below illustrates open system.
Example of open system: Water heated in an open container – Here, heat is the energy transferred, water is the mass transferred and container is the thermodynamic system. Both heat and water can pass in and out of the container.
A closed system allows only energy (heat and work) to pass in and out of it. It does not allow mass transfer across its boundary. The following image shows a closed system:
Example of closed system: Water heated in a closed vessel – Here only heat energy can pass in and out of the vessel
An isolated system does not interact with its surroundings. It does not allow both mass and energy transfer across its boundary. It is more restrictive.
In reality, complete isolated systems do not exist. However, some systems behave like an isolated system for a finite period of time. The following image illustrates an isolated system: