---
title: "Closed system vs open system"
subject: "Fundamental of Thermodynamics & Heat Transfer"
chapter: 1
key: "semester_2/fundamental_of_thermodynamics_and_heat_transfer:closed-vs-open-system"
source: https://www.bctengineer.com/q/semester_2/fundamental_of_thermodynamics_and_heat_transfer/closed-vs-open-system
site: BCT Engineering
---

# Closed system vs open system

Fundamental of Thermodynamics & Heat Transfer · Chapter 1: Introduction

- Asked in: 2069 Bhadra, 5 marks; 2069 Poush, 5 marks (asked 2 times)
- Video: [Open, CLosed & Isolated Systems](https://www.youtube.com/watch?v=7RawwstN-Pw) — Jason Hitsman
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/fundamental_of_thermodynamics_and_heat_transfer/closed-vs-open-system
- Whole chapter: https://www.bctengineer.com/md/semester_2/fundamental_of_thermodynamics_and_heat_transfer/chapter-1

## Question

Explain different types of thermodynamic systems and differentiate between them with examples.

## Short answer

A closed system (control mass) allows only energy transfer across its boundary, e.g., sealed piston-cylinder. An open system (control volume) allows both mass and energy transfer, e.g., turbine. An isolated system permits neither, e.g., an idealized thermos flask.

## Answer

A thermodynamic system is a definite quantity of matter or a region in space chosen for study. Everything outside it is the surroundings, and the real or imaginary surface between them is the boundary. Systems are of three types.

#### 1. Closed system (control mass)
No mass crosses the boundary, but energy (heat and work) can. The mass is fixed.
Example: gas in a piston-cylinder device with the valve closed; a sealed pressure cooker before venting.

#### 2. Open system (control volume)
Both mass and energy can cross the boundary. The boundary is the control surface and the region is fixed in space.
Example: turbine, compressor, nozzle, boiler, a water pump.

#### 3. Isolated system
Neither mass nor energy crosses the boundary.
Example: a perfectly insulated and sealed thermos flask (ideal); the universe as a whole.

```text
 Closed          Open              Isolated
 +--------+   +----------+      +----------+
 | fixed  |   | mass in->|      |  no mass |
 | mass   |   |   ->out  |      |  no heat |
 +--------+   +----------+      +----------+
 Q,W cross    m, Q, W cross     nothing crosses
```

| Basis | Closed system | Open system |
|---|---|---|
| Mass transfer | Not allowed | Allowed |
| Energy transfer | Allowed (Q, W) | Allowed (Q, W, flow energy) |
| Also called | Control mass | Control volume |
| Boundary | Moving or fixed | Usually fixed (control surface) |
| Examples | Gas in closed cylinder | Turbine, compressor, nozzle |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What can cross the boundary of a closed system?
   A. Mass and energy
   B. Energy (heat and work) only, not mass
   C. Mass only
   D. Nothing
   Answer: B. A closed system (control mass) has fixed mass, but heat and work may cross its boundary. (Key point: Closed system)

2. Which device is treated as an open system (control volume)?
   A. Gas in a cylinder with the valve closed
   B. A sealed pressure cooker before venting
   C. A turbine
   D. An ideal sealed thermos flask
   Answer: C. Turbines, compressors, nozzles and pumps have mass flowing through a fixed control surface. (Key point: Open system)

3. Which statement defines an isolated system?
   A. Mass can enter but energy cannot
   B. Heat can cross but mass cannot
   C. Work can cross but heat cannot
   D. Neither mass nor energy crosses the boundary
   Answer: D. An isolated system exchanges neither mass nor energy with its surroundings, e.g. the universe. (Key point: Isolated system)

4. Control mass and control volume refer to which systems respectively?
   A. Open and closed
   B. Isolated and open
   C. Closed and open
   D. Closed and isolated
   Answer: C. A closed system is a control mass; an open system is a control volume bounded by a control surface. (Key point: System terminology)

## Related in Fundamental of Thermodynamics & Heat Transfer

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