It is customary to say that energy exists in different forms which are transformed or converted into one another during physical processes. However, a careful analysis shows that thinking in and speaking of energy forms is inappropriate and conceptually even misleading. Since most textbooks use the term ‘‘energy form’’ without spelling out a clear procedure by which different ‘‘forms’’ of energy can be categorized, rigorous criteria for categorizing flowing and stored energy are discussed in this paper. These criteria show that the term ‘‘energy form’’ for the respective categories is unsatisfactory because it easily leads to the misinterpretation that there are different kinds of energy, rather than emphasizing the simpler and physically more correct picture of energy as an unalterable substance. Taking into account the well-known but little recognized natural law that energy always flows simultaneously with at least one other physical quantity, the concept of energy carrier is introduced. This concept provides a clear picture of how energy is transported, exchanged, and stored. This picture is scientifically accurate, yet simple and easy to present even at an elementary level.

The dissipative transport of energy is described in the momentum current picture. This picture provides a local-causes approach to mechanics whereby forces are considered as momentum currents. In this approach, friction, i.e. mechanical heat production, appears when a momentum current flows between two bodies of different velocities. The treatment of the transport and dissipation of energy follows the same rules in mechanics as in electricity. An 'Ohm's Law of momentum currents' is introduced in analogy to Ohm's Law in electricity. Newton's Third Law reduces to a simple statement about momentum conservation.

A unified approach to science teaching based upon a certain class of quantities which play fundamental roles in classical and modern physics is introduced. These quantities share the property of being substance-like, that is, each can be pictured to be contained in bodies and to flow from one body to another like a kind of ‘‘stuff.’’ Such quantities include, for example, energy (=mass), momentum, angular momentum, electric charge, particle number (=amount of substance), and entropy. When emphasizing substance-like quantities, the breakup of physics into sub-branches is nothing more than a classification of natural processes according to the substance-like quantity playing the dominant role in each case. The method of presentation, however, remains the same from one sub-branch to another: different natural processes can be simply visualized and quantitatively described according to the same formal rules in terms of the increasing, decreasing, and flowing of the respective substance-like quantities in each case. Thus knowledge of a single branch of physics already provides an analogy for the ways and means by which processes are described in other branches (including chemistry and biology) as well. These claims are illustrated with the help of a few simple examples.

Reports on the first course of a new physics curriculum developed at the Karlsruhe Institute for the Didactics of Physics (Falk and Herrmann 1977, 1978, 1979, 1981). The entire curriculum begins at the elementary school level with children aged 10-12 and is intended to extend beyond high school and through university studies (Falk and Ruppel 1975, 1976). Energy is introduced as the primary quantity at the very beginning of the course. It is not 'derived' from other seemingly more fundamental quantities such as mass, displacement, velocity and force. However, the course is not an ad hoc construction simply to explain the concept of energy. The essential features of many natural and technological processes can be understood by considering the flow of energy. This is the basic idea underlying the course, and can be restated more completely in terms of the following rule: 'something is happening whenever energy is flowing and a flow of energy is always accompanied by the flow of at least one other substance-like quantity'. The course strategy is designed to make this simple rule obvious by way of numerous examples taken from everyday life. Selected topics are highlighted and they introduce concepts unique to the authors approach. These concepts are presented in the same chronological order as they appear in the course.

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