The Relevant Physics

The following definitions and problems (but not solutions!) were taken from Harris Benson's University Physics, 1996 revised edition.

*From Chapter 19, "The First Law of Thermodynamics," a few definitions:

1) Heat Capacity=, where is a quantity of heat transferred to a body, and is the corresponding change in temperature induced in the body. ("Heat is energy transferred between two bodies as a consequence of a temperature difference between them" (p. 382).) The SI unit for heat capacity is Joules/Kelvin (as it is as well for entropy, touched on below). in turn can be defined in relation to a particular mass and substance as , where c is heat capacity per unit mass (specific heat) for a particular material. For example, for water J/(K.kg).

2) One calorie once was defined as the quantity of heat needed to raise one gallon of water from 14.5 ºC to 15.5 ºC The modern definition of a calorie is 4.186 Joules. This definition is the result of the work of James Joule, who in 1842 formally established, through experimentation with weights and electrical generators, the precise "mechanical equivalent to heat." A Joule is an energy (work and heat) measure, ForceDistance, or NewtonsMeters in SI units.

3) Thermodynamics is concerned with certain macroscopic variables of systems, in particular, with Temperature, Pressure, Volume, Internal Energy, and Entropy. In fact, thermodynamics focuses explicitly on the work done by or to systems as related to these variables. Work, unlike the others, is not a state function--its measure does depend on the thermodynamic path between two points in time, that is, on the path as represented on a Pressure-Volume diagram. In thermodynamic analysis, a series of instantaneous equilibria is assumed (quite unrealistically). In this way, work done by or to a fluid (liquid or gas) is defined .

4) The first law of thermodynamics: , where is the change in internal energy of the system under examination, is the quantity of heat transferred, and W is the work done by the system. Thus, in words, the change in internal energy of a system is equal to the quantity of heat transferred minus the work done in the process. Heat (energy transferred as a result of a temperature differential) is included in the universal conservation of energy (and matter). Here, work is seen to be extracted from heat transfer. U does not include the potential and kinetic energies of the center of mass of the system. It does include all other forms of energy, including the potential and kinetic energies of the constituent particles of the system (the thermal energy), in addition to electrical, magnetic, chemical, nuclear, etc. The first law, stated this way (final state compared to initial), is what establishes internal energy as a state function. It relies on the existence of the mechanical equivalent to heat.