The Concept of Energy

In elementary physics energy is often defined as "the capacity to produce work". At a descriptive level the idea expressed is correct, but for thermodynamics which is to be applied quantitatively this definition is not a good one because the term "work" itself requires a more precise definition than the general idea it ordinarily conveys. A better definition of energy from the viewpoint of thermodynamics would be "the capacity to induce a change in that which inherently resists change". This capacity represents a combination of an effort, expended in overcoming resistance to a particular type of change, with the change it produces. The combination is called energy.

The effort involved is measured quantitatively by what is defined as a "driving force" in thermodynamics. A driving force is a property which both causes and also controls the direction of change in another property. The quantitative value of this change is called a "displacement". The product of a driving force and its associated displacement always represents a quantity of energy, but in thermodynamics this quantity has meaning only in relation to a specifically defined system. Relative to a particular system there are generally two ways of locating a driving force and the displacement it produces.

In one way both the driving force and the displacement are properties of the system and are located entirely within it, so that the energy calculated from their product represents a change in the internal energy of the system. Similarly, both the driving force and its displacement could be located entirely within the surroundings so that the calculated energy is then a change in the total energy of the surroundings. In another way, however, the displacement occurs within the system but the driving force producing it is a property of the surroundings and is applied externally at the system boundary. By definition, the boundary of a system is a region of zero thickness containing no matter at all so that the energy calculated in this way is not a property of matter either in the system or in its surroundings but represents a quantity of energy in transition between the two. In any quantitative application of thermodynamics it is always important to make a careful distinction between energy changes within a system or within its surroundings and energy in transition between them.

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