A system that can exchange energy with the surroundings but not mass is called a closed system. When tea is placed in a closed vessel, only energy is exchanged with the surrounding so it is an example of a closed system.
Comparison of tea for understanding system and surrounding
The tea has a lot of energy compared to the room where the tea is kept. As time passes the heat in the tea will diffuse into the room. The tea will turn colder. This is because the energy (heat) in the tea moves to the surrounding area. Once the tea became cold, there is no more heat that can be diffuse. The tea has reached total entropy.
There are two types of these ‘rooms’: An open system and a closed system.
- An open system implies that energy (like heat) can freely flow in and out of the room.
- A closed system signifies that the room is closed off from the outside; no energy can go in or out.
In the case of the tea, the room was a closed system as no energy could access it. But we can also present it as an open system by setting a heater into the room. If we turn on the heater, we can utilise the heat from it to reheat the cup of tea. The new energy has been brought into the room. The entropy has thus reduced. The heat that moved from the heater into the tea can then move into the room again until total entropy has arrived.