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Mathematically, we can simply write the zeroth law of thermodynamics as

If object 1 is in thermal equilibrium with objects 2 and 3, respectively, then objects 2 and 3 must also be in thermal equilibrium. How do we know whether they are in thermal equilibrium? The governing principle here is the zeroth law of thermodynamics: Let us consider three objects at temperatures \(T_1, \, T_2\), and \(T_3\) respectively. Thus, when we say two objects (a thermodynamic system and its environment, for example) are in thermal equilibrium, we mean that they are at the same temperature. Intuitively, such a balance is reached if the temperature becomes the same for different objects or parts of the system in thermal contact, and the net heat transfer over time becomes zero. In a similar way, we should examine the heat transfer between a thermodynamic system and its environment or between the different parts of the system, and its balance should dictate the thermal equilibrium of the system. When we studied a mechanical system, we focused on the forces and torques on the system, and their balances dictated the mechanical equilibrium of the system. However, when we discuss a thermodynamic system in this chapter, we study those that have uniform properties throughout the system.īefore we can carry out any study on a thermodynamic system, we need a fundamental characterization of the system.

You could have, for example, a temperature gradient across the system. Any thermodynamic system is therefore treated as a continuum that has the same behavior everywhere inside. In other words, we concentrate on the macroscopic properties of the system, which are the averages of the microscopic properties of all the molecules or entities in the system. When we examine a thermodynamic system, we ignore the difference in behavior from place to place inside the system for a given moment. (credit a: modification of work by Gina Hamilton) A little steam escapes through the top valve to prevent explosion. (b) A pressure cooker is a good approximation to a closed system. It transfers heat and matter (steam) to its surroundings. \): (a) This boiling tea kettle is an open thermodynamic system.