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Thermodynamic Equilibrium

Thermodynamic Equilibrium

Vishal kumarUpdated on 02 Jul 2025, 07:49 PM IST

Thermodynamic equilibrium is a fundamental concept in physics, describing a system where all macroscopic flows of matter and energy cease, and the system's properties remain uniform and stable over time. It occurs when a system's temperature, pressure, and chemical potential are balanced, ensuring no net exchange of energy or matter with its surroundings. This equilibrium can be divided into three types: thermal, mechanical, and chemical.

This Story also Contains

  1. Thermodynamic Equilibrium
  2. Quasi-Static Process
  3. Zeroth Law of Thermodynamics
  4. Solved Examples Based on Thermodynamic Equilibrium
  5. Summary

In real life, thermodynamic equilibrium is seen in everyday phenomena, such as a room reaching a stable temperature after being heated or cooled. When you leave a cup of coffee on the table, it eventually reaches the same temperature as the surrounding air, illustrating thermal equilibrium. Similarly, the pressure inside a sealed balloon equalizing with atmospheric pressure demonstrates mechanical equilibrium. These examples show how thermodynamic principles govern the balance of energy in natural and artificial systems.

Thermodynamic Equilibrium

When all the thermodynamic variables attain a steady value i.e. they do not change with respect to time, the system is said to be in the state of thermodynamic equilibrium. For the system to attain thermodynamic equilibrium, the following equilibrium must be attained

(i) Mechanical equilibrium: There is no unbalanced force between the system and its surroundings. There is no pressure gradient.

(ii) Thermal equilibrium: There is a uniform temperature in all parts of the system and is the same as that of the surroundings. There is no temperature gradient.

(iii) Chemical equilibrium: There is a uniform chemical composition throughout the system and the surroundings. There is no concentration gradient.

Quasi-Static Process

A quasi-static process is a thermodynamic process which happens slowly enough for the system such that each state will remain in internal equilibrium.

Example of quasi-static compression - when the volume of a system changes at enough rate to allow the pressure to remain constant throughout the system

Zeroth Law of Thermodynamics

If systems A and B are each in thermal equilibrium and B and C are in thermal equilibrium with each other, then A and C are in thermal equilibrium with each other.



Zeroth law leads to the concept of temperature. All bodies in thermal equilibrium must have a common property. This common property is called temperature.

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Solved Examples Based on Thermodynamic Equilibrium

Example 1: A system can said to be in thermodynamic equilibrium if

1) The mass of the system does not change with time

2) The pressure of the system does not change with time

3) Macroscopic variables (Pressure, volume, temperature, mass and composition) that characterise the system do not change with time

4) None of these

Solution:

Thermodynamics Equilibrium - When Thermodynamics variables attain a steady state i,e, they are independent of time.

e.g. P, V, and T become independent of time.

Hence, the answer is the option (3).

Example 2: The condition for "mechanical equilibrium" in the thermodynamic process is

1) The temperature of the system and surroundings should remain the same

2) There should be no unbalanced force either within the system or between the system and its surroundings.

3) The mass of the system should be conserved.

4) The momentum of the system should be conserved.

Solution:

Mechanical Equilibrium

There is no unbalanced force between the system and its surroundings.

Fresultant =0

Hence, the answer is the option (2).

Example 3: "If the temperature of working substance must not differ appreciably from that of the surrounding of any stage of the cycle of the operations" is the condition of

1) Chemical equilibrium

2) Thermal equilibrium

3) Mechanical equilibrium

4) None of these

Solution:

Thermal Equilibrium

There is a uniform temperature in all parts of the system and is the same as that of the surroundings.

T is independent of time.

All parts of the system and surroundings should remain at the same temperature

Hence, the answer is the option (2).

Example 4: In the case of "chemical equilibrium" which statement is not true?

1) The internal structure of the system should not change due to chemical reactions.

2) No new product should formed in the process.

3) There should be uniform chemical composition throughout the system and surroundings.

4) None of these

Solution:

Chemical Equilibrium

There is a uniform chemical composition throughout the system and the surroundings.

No variation of the chemical composition.

All statements are true for chemical equilibrium

Hence, the answer is the option (4).

Example 5: Zeroth law of thermodynamics is related to the concept of

1) Pressure

2) Heat

3) Temperature

4) Energy

Solution:

Concept of Temperature

Zeroth law leads to the concept of temperature. All bodies in thermal equilibrium must have a common property.

This common property is called temperature.

Hence, the answer is the option (3).

Summary

Thermodynamic equilibrium occurs when a system's temperature, pressure, and chemical composition remain constant over time, with no net exchange of matter or energy with its surroundings. It consists of three types: mechanical, thermal, and chemical equilibrium. The Zeroth Law of Thermodynamics introduces the concept of temperature, stating that if two systems are in thermal equilibrium with a third, they are also in equilibrium with each other. Quasi-static processes allow systems to transition slowly while maintaining equilibrium at each state.

Frequently Asked Questions (FAQs)

Q: How does the concept of thermodynamic equilibrium apply to nuclear reactions?
A:
In nuclear reactions, thermodynamic equilib
Q: How does the concept of thermodynamic equilibrium apply to the Earth's climate system?
A:
The Earth's climate system is not in true thermodynamic equilibrium due to constant energy input from the Sun and internal dynamics. However, it can reach a steady state or "energy balance" where incoming and outgoing energy fluxes are equal. Understanding deviations from this balance is crucial for climate science and predicting climate change.
Q: What is the significance of the Le Chatelier's principle in thermodynamic equilibrium?
A:
Le Chatelier's principle states that when a system at equilibrium is subjected to a change in conditions (temperature, pressure, concentration), the system will respond by shifting the equilibrium to counteract the change. This principle is crucial for predicting how equilibrium systems will respond to perturbations.
Q: How does the concept of thermodynamic equilibrium apply to glasses and amorphous solids?
A:
Glasses and amorphous solids are non-equilibrium systems that are trapped in metastable states. They do not reach true thermodynamic equilibrium on observable timescales but instead undergo extremely slow relaxation processes. Understanding their behavior often requires concepts from non-equilibrium thermodynamics.
Q: What is the role of the canonical ensemble in describing thermodynamic equilibrium?
A:
The canonical ensemble is a statistical ensemble used to represent the possible states of a system in thermal equilibrium with a heat bath at a fixed temperature. It provides a probabilistic description of the system's microstates and allows for the calculation of macroscopic thermodynamic properties, bridging microscopic and macroscopic descriptions of equilibrium.
Q: How does the concept of thermodynamic equilibrium apply to the oceans?
A:
The Earth's oceans are not in true thermodynamic equilibrium due to constant energy input from the Sun, tidal forces, and interactions with the atmosphere. However, local regions can be approximated as being in quasi-equilibrium. Understanding deviations from equilibrium is crucial for studying ocean circulation, heat transport, and the carbon cycle.
Q: What is the role of ergodicity in thermodynamic equilibrium?
A:
Ergodicity is the assumption that, over long periods, the time average of a system's properties equals the ensemble average. This concept is crucial for connecting microscopic dynamics to macroscopic equilibrium properties. In ergodic systems, a single trajectory will eventually explore all accessible microstates, justifying the use of statistical ensembles to describe equilibrium.
Q: How does the concept of thermodynamic equilibrium apply to magnetic systems?
A:
In magnetic systems, thermodynamic equilibrium involves the alignment of magnetic moments. The competition between the tendency of moments to align (minimizing energy) and the tendency towards disorder (maximizing entropy) leads to phenomena like paramagnetism, ferromagnetism, and phase transitions, all of which can be described using equilibrium thermodynamics.
Q: What is the significance of the Onsager reciprocal relations in near-equilibrium thermodynamics?
A:
The Onsager reciprocal relations describe symmetries in the coefficients that relate thermodynamic forces to their corresponding fluxes in systems close to equilibrium. These relations are a consequence of microscopic reversibility and provide important constraints on the behavior of systems slightly perturbed from equilibrium.
Q: What is the role of fluctuation-dissipation theorem in thermodynamic equilibrium?
A:
The fluctuation-dissipation theorem relates the response of a system in thermodynamic equilibrium to a small applied force to the spontaneous fluctuations of the system. It provides a deep connection between microscopic fluctuations and macroscopic dissipative processes, and is fundamental to understanding transport phenomena and linear response theory.
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