We already know that heat is a form of energy. It is a common observation that when we come out of the blanket on a cold morning we feel cold for sometime but after that it becomes normal to some extent. So what exactly is happening? Initially heat begins from us to the surrounding and thus we feel cold. So till when heat will flow from us to surrounding? To answer this we introduce something known as temperature. It is basically the measure of hotness or coldness of a body. Heat exchange takes place between two bodies till their temperatures become equal. Also it flows from body at higher temperature to one at a lower temperature.
So now that we know about temperature we are in a position to define thermal equilibrium. It is defined as a situation in which no heat flows from one body to another when brought in contact with each another. In a broader sense when the parameters of the system like volume, pressure, temperature, or composition does not change. As heat does not flow we can also say both body acquire same temperature at equilibrium.
One of the laws related to the thermal equilibrium is Zeroth law of Thermodynamics. Suppose two bodies are separated by a heat insulator. Now a third body is brought in contact with both the bodies simultaneously. When the bodies attain thermal equilibrium we remove the third body. Now we can also remove the heat insulator and find no heat is transferred from one body to another.
According to Zeroth Law if two bodies I and II are in equilibrium with a body III separately then it follows that body I and body II are also in thermal equilibrium. As no heat flows from I to III hence, TI = TIII similarly TII = TIII. It follows that TI = TII, hence no heat flows.
The zeroth law of thermodynamics can be applied to compare temperatures of more than two objects. If the heat can pass through two objects without any external heating source, the two objects are said to be in thermal equilibrium. For instance, if the swimming pool you are in and your temperatures are similar, no heat flows from either sides apart from minute possibility. While, if you get into the pool cracking through the ice layer, you are not in the state of thermal equilibrium with the water. Please don’t try this experiment. To check for thermal equilibrium mainly in case of frozen pools, you should use a thermometer.
The zeroth law frames an idea of temperature as an indicator of thermal equilibrium. The two given objects are in a state of equilibrium with the third object that gives you the required reading on a scale.
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