A closed system in a physical system is a system in which the materials contained in the system are not affected by other external agents.
Thermodynamic systems can be closed or open.
Closed systems are sometimes confused with isolated systems. In both cases, energy transfer is possible. However, an isolated system cannot exchange heat, work, or matter with the environment, while a closed system can.
In the case of an adiabatic process in a closed system, even if heat and mass are not transferred, it can do work.
A thermodynamically closed system can be imagined as an airtight container. It is a system whose walls are heat conductive and can be deformable; that is, its volume can vary. If the volume changes without mass exchange, it means that its content has a specific volume constant.
A closed system passes through an infinite number of intermediate states. The temperature and pressure gradients in the system are negligibly small. This is called a quasi-equilibrium process.
The relationship between heat and temperature when the heat is added to a system is the specific heat. Specific heat is the amount of energy that must be added, in the form of heat, to one unit of mass of the substance in order to cause an increase of one unit in temperature.
Frequently considered cases of closed systems are closed isothermal systems at constant pressure.
Examples of Closed Systems
To better understand the concept of this type of system, we list some examples:
Earth is considered a closed system. Heat is added to the Earth as it receives solar energy, while the total mass remains constant, with (almost) no exchange from space.
A pressure cooker that does not allow gas to escape experiences a change in energy but not the mass. The food we cook receives heat energy, but its mass does not vary.
A Stirling engine exchanges energy with the outside without changing its mass. This type of motor receives external thermal energy and supplies mechanical energy. In this case, the work done by the system is positive.
The piston-cylinder arrangement in an internal combustion engine is only a closed system during the compression stroke and the power stroke. In the case studies, the piston-cylinder system is approximated by an ideal gas or an atomic gas.
Laws of Thermodynamics in Closed Systems
According to the first law of thermodynamics for these systems, the change in internal energy is equal to the added work and heat. During an interaction between a system and its surroundings, the amount of energy gained by the system must be precisely equal to the amount of energy lost by the surroundings.
In this type of system, the conservation of masses of classical physics can also be applied, in which the mass remains constant.
In relativistic physics, however, the decrease in the system's energy content automatically reduces the system's mass.