Question

I have read, Two bodies are said to be in thermal equilibrium if there is no net flow of heat from one to another And also, Heat is the form of energy transferred between two systems or a system and it's surroundings by virtue of temperature difference. According to my understanding, when the temperature difference between two bodies is zero, no heat flows between them and they are said to be in thermal equilibrium. Does that mean, Two bodies, completely insulated from each other as well as their surroundings (both are at the same temperature) are in thermal equilibrium with each other? Or is contact between bodies a prerequisite to define equilibrium?

Answer 1

Two insulated bodies at the same temperature are not necessarily in thermal equilibrium unless they can potentially exchange energy. The zeroth law of thermodynamics implies that if bodies can be in contact and exhibit no net energy transfer, they are in thermal equilibrium. Contact is essential because it allows for the potential of energy transfer.

Step-by-step explanation:

You asked whether two bodies that are completely insulated from each other as well as their surroundings and are at the same temperature, are in thermal equilibrium with each other. The short answer is no; being at the same temperature is a necessary condition but not sufficient on its own for defining thermal equilibrium. According to the Zeroth law of thermodynamics, thermal equilibrium specifically implies the possibility of energy transfer between bodies without any net exchange occurring. For thermal equilibrium to be established, there must be a pathway for heat transfer, which means the bodies in question should have the potential to be in contact. If two insulated bodies, which are at the same temperature, were to be brought into contact, and no heat flow occurs, they can then be considered in thermal equilibrium because they meet the condition of being capable of energy exchange without a net flow of energy.

It is also worth noting that the concept of thermal equilibrium is fundamental to how thermometers work. A thermometer, in contact with an object, will eventually reach thermal equilibrium with it, allowing the thermometer to accurately measure the object's temperature. This implies that contact is necessary for the thermometer and the object to reach thermal equilibrium. The zeroth law helps us understand thermal equilibrium further by stating that if two systems, A and B, are in thermal equilibrium, and system B is also in equilibrium with a third system, C, then A is in thermal equilibrium with C as well. This lays the foundational concept that if multiple systems are each in equilibrium with a common system, they are in equilibrium with each other, provided that a pathway exists for potential energy transfer.