chapter  29
28 Pages

Energy and Thermodynamics

What is thermodynamics, you ask? Thermodynamics determines and studies the relationships among the various properties of materials, without knowing the internal structure of the materials. Thermodynamics, in particular, deals with energy; its transmission from one body to another and its transformations from one form to another. The laws of thermodynamics were discovered empirically, i.e., through many observations and experiments, before humans understood molecules and atoms and the microscopic structure of matter.1 As the prefix “thermo” suggests, thermodynamics started with studies of heat, steam engines to be precise. In fact, engineers started and led the early study of thermodynamics, not physicists or mathematicians. The caloric theory of heat was dominant in the beginning of the study of thermodynamics. This theory holds that heat is a material substance called caloric, and possession of caloric is what makes hot objects hot. Scientists of the day used the caloric theory to explain thermal expansion and why water boils when heated and many other phenomena. The unit of heat energy in name and concept, the Calorie, is a legacy of the caloric theory. When James Joule began his serious study of energy in 1837, he had adopted

the kinetic theory which holds that what makes a body hot is the kinetic energy (energy due to motion) of invisible molecules that make up the body.2 Since the kinetic theory of heat is taken for granted today (after all, we now have tools that can “see” molecules for us) it is perhaps difficult for today’s students to appreciate how genuinely revolutionary the kinetic theory was when it was introduced-and remained for some time afterward. Work, Potential Energy and Kinetic Energy. Being well-versed in Newton’s

laws, Joule knew that it took work to set a body in motion and that the kinetic energy (energy due to motion) of the body was precisely equal to the work

1Sadi Carnot (1796-1832) and James Joule (1818-1889) are credited with discovering the Second and First Laws of Thermodynamics, respectively. The Second Law of Thermodynamics was discovered before the First! 2Joule’s study of energy was motivated by his desire to find an inexpensive source of power for his family’s brewery!