Basic Physics

# The Zeroth Law of Thermodynamics

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 other than 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.