So far, we have only observed the thermal equilibrium experienced by two objects in contact.
To understand the concept of thermal equilibrium in more depth, let’s review three objects (say objects A, object B, and objects C). For example, object B and object C do not touch each other, but object A in contact with object B and object A in contact with object C. Observe the image below. Because in contact with each apart from object A and object B are in thermal equilibrium, so also object A and object C are in thermal equilibrium. Are objects B and C that do not touch each other also in thermal equilibrium? If we only use logic, we can say that object B and object C are also in thermal equilibrium, even though they are not in contact. Object A and object B are in thermal equilibrium, meaning the temperature of the object A = the temperature of object B.
Objects A and object C are also in thermal equilibrium, the temperature of the object A = temperature of the object C. Because of TA = TB and TA = TC, then TB = TC. Since physics does not only rely on logic, it needs to be proven through experiments. Based on the experimental results, it turns out that object B and object C are also in thermal equilibrium. In this case, the temperature of the object B = the temperature of the object C.
So even though object B and object C are not touching each other, but because they are in contact with object A, then object B and object C are also in thermal equilibrium.
So, if two objects are in thermal equilibrium with a third object, then the three objects are in thermal equilibrium with each other. This is the Zeroth Law of Thermodynamics. The Zeroth Law of Thermodynamics explains the working principle of a thermometer, a temperature measuring device. Review a mercury thermometer or an alcohol thermometer. Alcohol or mercury in contact with glass and glass in contact with objects whose temperature is measured, such as air, water or the human body. Although mercury is not in contact with air or water or the human body, because of mercury in contact with the glass,
when glass and air or water or the human body are in thermal equilibrium, mercury, and air or water or the human body, is also in thermal equilibrium.
The laws of physics usually start from 1, rarely beginning from zero. Called the thermodynamic 0 law because, after the 1st law of thermodynamics, the second law of thermodynamics and the third law of thermodynamics were formulated, scientists realized that there was a more fundamental law that had not been formulated. Therefore, scientists call this the Zeroth Law of Thermodynamics.
