Question

What is the difference between an open and closed system? Explain.

Answer 1

Short answer
- Open system: exchanges both mass and energy with its surroundings.
- Closed system: exchanges energy (heat and/or work) but not mass with its surroundings.
- (For completeness) An isolated system exchanges neither mass nor energy.

Longer explanation with examples and key points

1) Definitions
- Closed system (control mass): A fixed amount of matter is chosen and followed through time. Mass inside the system does not change — no mass crosses the system boundary — but energy (heat and/or work) may cross the boundary.
- Open system (control volume): A region of space through which mass can flow in and out. Both mass and energy (carried by that mass and by heat/work) may cross the boundary.

2) Typical equations (first law)
- Closed system (simple form): ΔE = Q − W
- E usually includes internal energy (U) plus kinetic and potential energy if relevant.
- Open system (control-volume, steady-flow form): Q̇ − Ẇ + Σṁ_in h_in + Σṁ_in(ke+pe)_in − Σṁ_out h_out − Σṁ_out(ke+pe)_out = dE_cv/dt
- Here mass flow brings enthalpy (h) and energy into/out of the control volume; at steady state dE_cv/dt = 0.

3) Key differences (bullet list)
- Mass transfer: closed = none; open = permitted.
- Energy transfer: both can exchange energy, but in open systems energy is also carried by mass flow (enthalpy).
- Analysis method: closed systems are analyzed as control-mass problems; open systems are treated as control-volume problems with mass flow terms.
- Conservation statements: closed-system mass is constant; open-system mass in the control volume can change (mass balance includes flow terms).
- Common uses: closed-system analysis for rigid tanks and isolated packages; open-system analysis for pipes, turbines, pumps, nozzles, heat exchangers.

4) Examples
- Closed: a sealed, insulated piston-sleeve if no mass leaks; a sealed soda can (ignoring carbonation loss).
- Open: a boiling kettle with steam escaping; flowing water in a pipe; a car engine (intake and exhaust flow).
- Isolated: an ideal perfectly insulated, sealed container; the universe approximates an isolated system.

5) Practical note
Real systems may be approximated as closed, open, or isolated depending on the timescale and what exchanges are negligible. For example, a closed insulated tank might be treated as isolated if heat transfer is negligible on the timescale of interest.

If you want, I can give a worked example (energy balance) for either a closed piston-cylinder or for a steady-flow device like a turbine. Which would you prefer?