Thermodynamic Equilibrium

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.

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.