A system, as used here, is basically any object (or set of objects) that we wish to consider in our application of thermodynamics. It could be a cell, an organism, a car engine, our planet, or even the entire universe. The surroundings would be whatever surrounds the system - i.e. the rest of the universe. (If the system is the entire universe, naturally there can be no surroundings.)
Now, clearly the only two ways for the system to interact with its surroundings would be in the exchange of either energy (in the form of heat or work) or matter. Thus there are four possible options for various types of systems.
- A system could be capable of exchanging both energy and matter
- A system could be capable of exchanging energy but not matter.
- A system could be capable of exchanging matter but not energy.
- A system could be capable of exchanging neither energy nor matter.
In practice, though, if a system is capable of exchanging matter, it can exchange energy too. Thus, we are down to three main types of thermodynamics systems:
- An OPEN system is capable of exchanging both energy and matter with its surroundings. An example would be us - humans.
- A CLOSED system is only capable of exchanging energy with its surroundings, but not matter. An example here would be a greenhouse.
- An ISOLATED system is not capable of exchanging either energy or matter with its surroundings. In reality, this idealisation is impossible to completely obtain, but it can be approximated with, for instance, a gas housed in a rigid, heavily-insulated cannister.