Thermal Equilibrium| Definition and Daily Life Examples

Thermal equilibrium is a state in which two or more connected systems have the same temperature, and there is no net flow of heat between them. In other words, it is a condition where the thermal energy exchange between systems is balanced, resulting in a stable temperature distribution.
Mathematically, thermal equilibrium can be expressed using the zeroth law of thermodynamics:
If system A is in thermal equilibrium with system B, and system B is in thermal equilibrium with system C, then system A is in thermal equilibrium with system C.
This law is often stated as:

T_1​=T_2​

Where T₁ and T₂ are the temperatures of the two systems in thermal equilibrium.

Daily Life Examples:

1. Coffee Cup Example: A hot cup of coffee left on a table will eventually reach thermal equilibrium with the room temperature.
2. Room Temperature: When entering a room, you quickly feel the air’s temperature matching your body’s, indicating thermal equilibrium between you and the room.

Important Points:

1. Zero Net Heat Flow: In thermal equilibrium, there’s no net heat transfer between systems. Although heat exchange occurs, absorption and release rates are equal.
2. Temperature Equality: Thermal equilibrium is marked by equal temperatures in connected systems.
3. Macroscopic and Microscopic Equilibrium: Systems achieve thermal equilibrium on macroscopic or microscopic scales. For macroscopic equilibrium, entire systems share the same temperature, while in microscopic equilibrium, average temperatures across different regions are equal.

Simple Case Study

• Consider two metal blocks, A and B, initially at different temperatures.
  • Placed in thermal contact for heat exchange.
• Heat flows from the hotter block to the colder one over time.
• Eventually, A and B reach thermal equilibrium.
  • Temperatures of A and B become equal.
  • No further net heat transfer occurs.
• Illustrates thermal equilibrium.
  • Heat transfer continues until both systems satisfy the zeroth law of thermodynamics.

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