Thermal Equilibrium: Class 11 Physics Notes, Definition, Characteristics & Real-Life Applications

Thermal equilibrium happens when two or more systems, in thermal contact, have no net heat flow between them, as their temperatures are equal. Macroscopic properties (pressure, volume, temperature) remain constant over time. 

Importance of Thermal Equilibrium for Students

If you check the chapter-wise weightage for JEE Mains, thermodynamics has around 6.6%. Meaning, you can expect two questions. Now, previous year question papers for JEE Main happen to include various types of questions, as shown in the table below. 

Even other exams, including IISER Exam or similar, require knowledge of all NCERT-focused concepts outlined in the Thermodynamics chapter. You can explore the IAT Physics syllabus as well.  

For a quick textbook recall, here is how NCERT defines thermal equilibrium. 

"...the state of a system is an equilibrium state if the macroscopic variables that characterize the system do not change in time. For example, a gas inside a closed rigid container, completely insulated from its surroundings, with fixed values of pressure, volume, temperature, mass, and composition that do not change with time, is in a state of thermodynamic equilibrium."

Here are some key terms you might want to remember from the above explanation.  

In thermal equilibrium, there are a few defining characteristics. 

• Constant Macroscopic Variables: In thermal equilibrium, pressure, volume, and temperature remain steady, indicating no net energy transfer. This is a stable state, where no overall change occurs in the system. 
• Temperature Equality: Systems in thermal contact reach the same temperature, halting heat flow. This means that there will come a point when two objects in contact reach thermal equilibrium. 
• Dependence on Surroundings: Equilibrium depends on the wall separating systems (adiabatic vs. diathermic) and external conditions. Meaning, that it’s the type of boundary between two objects that determines whether both systems will reach thermal equilibrium or not.

Based on the definition and characteristics of thermal equilibrium, you will need to know a few additional concepts commonly found in annuals and on entrance exams. 

• Adiabatic vs. Diathermic Walls: An adiabatic wall prevents heat flow, allowing independent equilibrium states. A diathermic (conducting) wall permits heat flow, leading to thermal equilibrium when temperatures equalise.
• Zeroth Law of Thermodynamics: "Two systems in thermal equilibrium with a third system separately are in thermal equilibrium with each other." This establishes temperature as the property equal in thermal equilibrium.
• Temperature as a Marker: Temperature determines the direction of heat flow, and it stops when systems reach thermal equilibrium.

So, how do we achieve thermal equilibrium in theory?

Consider two gases, A and B, in separate containers. 

Adiabatic Separation: If separated by an adiabatic wall, A and B maintain independent pressures $\left(P_A,P_B\right)$ and volumes $\left(V_A,V_B\right)$ , with no heat exchange.

An adiabatic wall completely blocks any heat exchange between the two systems. Their temperatures are constant, while the pressure and volume for each remain constant. The total entropy of both systems remains constant, allowing for reversible processes in a theoretical scenario. 

Diathermic Separation: With a diathermic wall, heat flows until temperatures equalise. The pressures and volumes adjust to new equilibrium values $\left(P_A^{\text{'}},V_A^{\text{'}}\right.$ , $\left.P_B^{\text{'}},V_B^{\text{'}}\right)$ , and no further heat flows.

A diathermic wall allows heat exchange, where the heat from a hotter system flows into the cooler system. During the heat transfer process, the pressure and volume of both systems change until thermal equilibrium is reached.   

Here are some applications of thermal equilibrium. 

Thermometry: The Zeroth Law enables temperature measurement by using a reference system (e.g., a thermometer) in thermal equilibrium with the system being measured.  

System Interactions: In engines or refrigerators, thermal equilibrium concepts guide heat transfer between reservoirs and working substances.

Natural Systems: Bodies of water or atmospheric layers reach thermal equilibrium with their surroundings, stabilizing local climates.

Below is the schematic diagram of systems A and B. They are separated by a diathermic wall, reaching thermal equilibrium when $T_A=T_B$ . Heat flows until temperatures equalize.

Thermal Equilibrium: $T_A=T_B$