## Thermal Equilibrium – Class 11 | Chapter – 12 | Physics Short Notes Series PDF for NEET & JEE

Thermal Equilibrium: Thermal equilibrium is a state of a system in which there is no net flow of heat energy between the system and its surroundings. When two or more systems are in thermal contact with each other, heat energy flows from the hotter system to the colder system until both systems reach the same temperature, at which point they are said to be in thermal equilibrium.

In thermal equilibrium, the temperatures of the two systems are equal and there is no temperature gradient between them. This means that there is no spontaneous transfer of heat energy between the systems, as the rate of energy transfer is the same in both directions.

Thermal equilibrium is a fundamental concept in thermodynamics, as it is used to describe the behavior of heat energy in systems and the way in which energy is transferred from one system to another. It is also important in many areas of science and engineering, as it provides a basis for understanding the behavior of materials and processes under different conditions of temperature and heat transfer.

Thermal equilibrium can be achieved through a variety of mechanisms, including conduction, convection, and radiation. When two systems are in thermal equilibrium, they are in a state of balance, with no net transfer of heat energy occurring between them.

## Equation of Thermal Equilibrium

The equation that describes thermal equilibrium is based on the zeroth law of thermodynamics, which states that if two systems are separately in thermal equilibrium with a third system, then they are also in thermal equilibrium with each other.

Mathematically, this can be expressed as:

If system A is in thermal equilibrium with system C, and system B is in thermal equilibrium with system C, then system A is in thermal equilibrium with system B.

This relationship can be expressed using the following equation:

T(A) = T(C) and T(B) = T(C) ==> T(A) = T(B)

where T(A), T(B), and T(C) are the temperatures of systems A, B, and C, respectively.

This equation shows that if two systems are each in thermal equilibrium with a third system, then they must be at the same temperature as each other. This is a fundamental principle of thermodynamics, and it provides a basis for understanding the behavior of heat energy in systems and the way in which energy is transferred from one system to another.

## Examples of Thermal Equilibrium

Thermal equilibrium is a fundamental concept in thermodynamics, and it is observed in many different systems and processes. Here are a few examples of thermal equilibrium in action:

• Heat transfer between two objects: When two objects are in thermal contact with each other, heat energy will flow spontaneously from the hotter object to the colder object, until both objects reach the same temperature and are in thermal equilibrium. For example, if you place a hot cup of coffee in a cool room, the coffee will lose heat energy to the surrounding air until it reaches the same temperature as the air.
• Temperature regulation in the human body: The human body maintains a constant internal temperature of around 37°C (98.6°F), through a process of thermoregulation. When the body is too warm, it releases heat energy through sweating and vasodilation (expansion of blood vessels), and when the body is too cool, it conserves heat energy through shivering and vasoconstriction (contraction of blood vessels).
• Thermal equilibrium in the atmosphere: The Earth’s atmosphere is in a state of thermal equilibrium, with heat energy from the sun absorbed by the Earth’s surface and then radiated back into space. The temperature of the atmosphere varies with altitude, but the overall balance of energy ensures that the atmosphere is in thermal equilibrium.
• Heat exchange in a refrigeration system: In a refrigeration system, a refrigerant fluid is used to transfer heat energy from a cool environment (e.g. inside a refrigerator) to a warm environment (e.g. outside the refrigerator). This process relies on the principles of thermal equilibrium, as heat energy flows from the warmer environment to the cooler environment until both are at the same temperature.

These are just a few examples of how thermal equilibrium is observed in different systems and processes. In general, thermal equilibrium is a fundamental principle of thermodynamics, and it provides a framework for understanding the behavior of heat energy in systems and the way in which energy is transferred between different objects and environments.

 JOIN OUR TELEGRAM CHANNELS Biology Quiz & Notes Physics Quiz & Notes Chemistry Quiz & Notes 