Some Thoughts on Statistical Thermodynamics

For the purpose of thermodynamics, the universe is divided into two parts: a system and its surroundings. The system is that part of the world in which we have a special interest (e.g. a glass of water or a cell); the surroundings are where we make our observations. Work, heat and energy are the basic concepts of thermodynamics. The most fundamental of these concepts is work: all measurements of heat and changes in energy can be expressed in terms of measurements of work. The energy of a system is defined as its capacity to do work. At the molecular level, heat is the transfer of energy that makes use of chaotic molecular motion. The chaotic motion of molecules is called thermal motion. For example, when two bodies at different temperatures are put in contact, the thermal motion of the molecules of the hotter body stimulates the molecules in the colder one to move more vigorously, albeit in a disorganized way. As a result, the energy of the colder body increases. In molecular terms, work is the transfer of energy that makes use of organized motion. For example, when a weight is raised or lowered, its atoms move in an organized way. Notice that the distinction between work and heat is made in the surroundings. When work is done on a system, the energy leaves the surroundings in an organized way, but it does not necessarily arrive in the system that way. Think of heating a glass of water by stirring: because collisions between molecules quickly randomize their directions, the orderly motion of the stirrer is transformed into thermal motion of the water. The first law of thermodynamics states that the change in energy of a system is equal to the energy that passes through its boundary as heat or work: