Open vs Closed Systems in Thermodynamics
Open System
A system whose boundary lets both matter and energy cross — steam escaping a boiling pot, a car engine taking in air and fuel and expelling exhaust.
Unit: joule (J)
Example: A boiling kettle is open: water leaves as steam carrying energy away, which is why the water never exceeds 100 °C no matter how hard it boils.
Closed System
A system whose boundary lets energy cross but not matter — a sealed bottle of gas being heated, or the refrigerant loop inside a fridge (an isolated system exchanges neither).
Unit: joule (J)
Example: A sealed pressure cooker is (nearly) closed: heat enters, but the water and steam stay inside, so pressure — and the boiling point — climbs.
Key differences
| Aspect | Open System | Closed System |
|---|---|---|
| Matter crosses boundary? | Yes | No |
| Energy crosses boundary? | Yes | Yes (heat and work) |
| Analysis needs | Track mass flow as well as heat/work | First law in its simplest form |
| Example | Boiling pot, jet engine, human body | Sealed gas syringe, closed water bottle |
| Stricter cousin | — | Isolated system: neither matter nor energy crosses (an ideal thermos) |
'Closed' doesn't mean sealed against everything — heat and work still cross a closed boundary. Students who treat a heated sealed can as isolated miss the heat entering, and their energy accounting breaks.