Closed System - GCSE Physics Definition

What is a closed system?

In GCSE Physics, a closed system is defined as one in which the total energy does not change. Due to energy conservation, the total amount of energy within a closed system must remain constant.

A thermodynamic system can be isolated, closed, or open.  A closed system can exchange energy but not matter to or from its surroundings. Most energy transfer scenarios are modelled in a closed system. An open system allows the exchange of energy and matter to or from its surroundings. In contrast, an isolated system does not allow the transfer of energy to or from its surroundings.

An example of a closed system is a pot of boiling water that is covered by a lid. Heat energy is transferred to the pot and the water, turning the liquid water into gaseous steam. Since the lid prevents the gas molecules from leaving the pot, energy is exchanged, but matter is not.

Another example of a closed system, which is almost an isolated system, is a sealed thermos containing water and ice cubes. Heat energy is transferred to the ice cubes by the water, causing the ice to slowly melt from a solid to a liquid state. A thermos is designed to limit the exchange of energy between the contents and the surroundings, meaning neither energy nor matter is exchanged. However, it is impossible to prevent energy exchanges entirely, which is why it is almost an isolated system.