---
title: "Thermodynamic equilibrium with illustrations"
subject: "Fundamental of Thermodynamics & Heat Transfer"
chapter: 1
key: "semester_2/fundamental_of_thermodynamics_and_heat_transfer:thermodynamic-equilibrium-illustrations"
source: https://www.bctengineer.com/q/semester_2/fundamental_of_thermodynamics_and_heat_transfer/thermodynamic-equilibrium-illustrations
site: BCT Engineering
---

# Thermodynamic equilibrium with illustrations

Fundamental of Thermodynamics & Heat Transfer · Chapter 1: Introduction

- Asked in: 2070 Magh, 4 marks
- Video: [Thermodynamic Equilibrium](https://www.youtube.com/watch?v=TTjZlU1CSrQ) — BEST MECHANICAL ENGINEERING
- 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/thermodynamic-equilibrium-illustrations
- Whole chapter: https://www.bctengineer.com/md/semester_2/fundamental_of_thermodynamics_and_heat_transfer/chapter-1

## Question

Define thermodynamic equilibrium. Explain with illustrations.

## Short answer

Thermodynamic equilibrium requires simultaneous mechanical, thermal, and chemical equilibrium so properties are uniform and unchanged. Example: a gas at uniform T and P balancing a piston is in equilibrium; a hot metal in water is not until thermal equilibrium is reached.

## Answer

A system is in **thermodynamic equilibrium** when it is simultaneously in mechanical, thermal and chemical equilibrium, so that its properties are uniform and do not change with time, and no spontaneous change occurs if it is isolated from the surroundings.

#### Components
1. **Mechanical equilibrium:** no unbalanced force; pressure is the same everywhere and equal to external pressure.
2. **Thermal equilibrium:** no temperature difference; no heat flow.
3. **Chemical equilibrium:** no net chemical reaction or mass diffusion; composition is constant.

#### Illustrations
```text
 (a) Not in equilibrium       (b) In equilibrium
 +------------------+       +------------------+
 |  hot gas | cold  |       |   gas, uniform   |
 |  400 K   | 300 K |       |   T = 350 K      |
 |  5 bar   | 1 bar |       |   P = 3 bar      |
 +------------------+       +------------------+
 heat and mass flow          no change with time
```

- A gas in a cylinder with a weighted piston: if the gas is at uniform T and its pressure balances the piston weight plus atmospheric pressure, it is in equilibrium. If the weight is suddenly removed, the piston moves and the system is out of equilibrium until it settles at a new equilibrium state.
- Hot metal dropped in cold water: the system is not in equilibrium until both reach the same temperature.
- Water and its vapour at the same T and P in a closed vessel (no net evaporation/condensation) is in phase (chemical) equilibrium.

Thermodynamic properties are defined only for equilibrium states.

## Check yourself

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

1. A system is in thermodynamic equilibrium when it is
   A. in thermal equilibrium only
   B. simultaneously in mechanical, thermal and chemical equilibrium
   C. moving with uniform velocity
   D. at atmospheric pressure
   Answer: B. All three equilibria together make thermodynamic equilibrium, with properties uniform and constant in time. (Key point: Definition)

2. A weight on a gas piston is suddenly removed. What happens?
   A. Nothing; equilibrium continues
   B. The gas temperature falls to zero
   C. The piston moves and the system is out of equilibrium until it settles
   D. The gas becomes isolated
   Answer: C. Removing the weight unbalances forces, so the piston moves until a new equilibrium state is reached. (Key point: Weighted piston)

3. Hot metal is dropped into cold water. When is the system in equilibrium?
   A. Immediately
   B. When the metal is heavier than the water
   C. Never
   D. When both reach the same temperature
   Answer: D. Until temperatures equalise there is heat flow, so thermal equilibrium is not satisfied. (Key point: Hot metal in cold water)

4. Water and vapour in a closed vessel at the same T and P with no net evaporation is an example of
   A. phase (chemical) equilibrium
   B. mechanical imbalance
   C. an open system
   D. non-equilibrium
   Answer: A. No net mass transfer between phases means chemical (phase) equilibrium. (Key point: Phase equilibrium)

## Related in Fundamental of Thermodynamics & Heat Transfer

- [Thermodynamic equilibrium, state and conditions](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Aequilibrium-state-conditions) — Fundamental of Thermodynamics & Heat Transfer, chapter 1