When Feynman wrote, “It is important to realize that in physics today, we have no knowledge of what energy is,”1 he was recognizing that although we have expressions for various forms of energy from kinetic to elastic, we seem to have no idea of what the all-encompassing notion of “energy” is: This paper addresses that issue offering a definition of energy in terms of the more accessible concept of “change.” The various forms of energy (½mv2, mgh, ½kx2, qV, mcΔT, ½Iω2, ½CV2, etc.) are abstractions not directly observable. We see a cannonball rise in the air; we do not see potential energy (PE). We see physical change, not energy.
Topics
Energy forms and sources
REFERENCES
1.
R.P. Feynman, R.B. Leighton, and M. Sands, The Feynman Lectures on Physics (Addison-Wesley, MA, 1963), p. 4–2.
2.
Art
Hobson
, “Energy and work
,” Phys. Teach.
42
, 260
(May 2004
). Those who maintain that “energy is the ability to do work” might as well assert that “energy is the ability to melt snow.”3.
J.C. Maxwell, Matter and Motion (Dover Pub., Mineola, NY, 1991), p. 56.
4.
Albert
Einstein
, “On the relativity principle and the conclusions drawn from it
,” Jahrbuch der Radioaktivität und Elektronik
4
, 411
–462
(1907
).5.
E. Fermi, Thermodynamics (Dover Pub., Mineola, NY, 1936), pp. 12–15.
6.
C.
Swartz
in “Why use the work-energy theorem?
” Am. J. Phys.
72
, 1145
(Sept. 2004
) agreed: “Work is just one way of transferring energy.”C.
Mungan
in “Work and energy
,” Am. J. Phys.
73
, 197
(March 2005
) concurred: “It is correct to define work as an energy transfer… .”See H.B. Callen, Thermodynamics (Wiley, New York, 1962), p. 7.
7.
E. Teller, Energy From Heaven and Earth (W.H. Freeman, New York, 1979), p. 9.
8.
Many great physicists have argued that light is matter: “Light is, in short, the most refined form of matter” (de Broglie); “even light has become matter now, due to Einstein's discoveries” (Pauli); “Matter in the widest meaning of the word” includes “light and other forms” of energy (Born); “even in the remotest voids of the universe there is always starlight—and that is matter” (Schrödinger). The term “radiant energy” wrongly suggests light is energy. Yet like other particles of matter, photons have spin, carry momentum and energy, and have wavelengths.
9.
L. Sartori, Understanding Relativity (University of California Press, CA, 1996), pp. 213–219.
10.
According to Noether's principle, conservation laws arise from symmetries. Conservation of momentum exists because the laws and constants of physics can be translated through space without changing—that's spatial displacement symmetry.
11.
Conservation of energy exists because the laws and constants of physics can be translated through time without changing—that's temporal displacement symmetry.
12.
R. Mills (of gauge-theory fame) asserts that “the idea of potential energy is not truly fundamental … .” See Space Time and Quanta (W.H. Freeman, New York, 1994), p. 152.
13.
E. Hecht, Physics in Perspective (Addison-Wesley, Reading, MA, 1980), p. 158. E. Hecht, Physics: Algebra/Trig, 3rd ed. (Brooks Cole, Pacific Grove, CA, 2003), p. 171 (1st ed. 1994, p. 278). For a philosophical discussion of energy as change, see Scientific Realism, Selected Essays of Mario Bunge (Prometheus Books, Amherst, NY, 2001), p. 51.
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© 2007 American Association of Physics Teachers.
2007
American Association of Physics Teachers
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