![]() The concept of temperature is fundamental to any discussion of thermodynamics, but its precise definition is not a simple matter. Many of the results of thermodynamics are derived from the properties of reversible processes. ![]() It represents an idealized limiting case that is very useful in discussing the properties of real systems. The concept of reversible processes is something like motion without friction in mechanics. The first is irreversible (the balloon bursts), and the second is reversible. This example illustrates how two different paths can connect the same initial and final states. Such a process is said to be reversible because the system is at (or near) equilibrium at each step along its path, and the direction of change could be reversed at any point. However, the same final state could be achieved by placing the same compressed gas in a cylinder with a movable piston and applying a sequence of many small increments in volume (and temperature), with the system being given time to come to equilibrium after each small increment. For example, when a balloon bursts, the compressed gas inside is suddenly far from equilibrium, and it rapidly expands until it reaches a new equilibrium state. In general, a system is not in equilibrium as it adjusts to an abrupt change in its environment. A sequence of one or more such steps connecting different states of the system is called a process. The system can then be made to change to a new state only by an externally imposed change in one of the state functions, such as the temperature by adding heat or the volume by moving the piston. For example, the gas in a cylinder with a movable piston will be at equilibrium if the temperature and pressure inside are uniform and if the restraining force on the piston is just sufficient to keep it from moving.
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