Energy Definition

    Energy is actually rather hard to define---all textbooks seem to admit this.

    No short definition is adequate. But a common one-sentence definition that is useful is:

      "Energy is the quantified capacity for change/transformation."

      This is reasonable since loosely speaking, how much energy is available sets a limit on how much change/transformation is possible

    Another one-sentence definition that yours truly made up is:

      "Energy is the transformable and conserved universal essence of structure."

      This definition is also useful.

      Energy has many forms and all are transformable into other forms---NOT necessarily easily---and energy is a conserved quantity. The transformation between energy forms is done by forces: e.g., the electromagnetic force and the gravitational force.

      Calling energy the "essence of structure" also makes sense since a description of a physical structure can be given in terms of the various amounts of energy it has.

    Two other points need to be made:
    1. Individual forms of the energy can ONLY be calculated from formulae using direct observables (i.e., direct in some sense) as far as yours truly knows---except total energy in a volume can be calculated from total mass in that volume using E=mc2 (see Relativity file: e_mc2_4.html).
    2. Energy conservation in a simple sense does NOT always hold. This difficult refinement is also discussed in Relativity file: e_mc2_4.html. But in everyday life and almost all astrophysics, energy conservation does hold.

    As the discussion above suggests, energy is a somewhat abstract thing---but we're used to abstract things like money.

    A fullish explication of energy is given in Mechanics file: energy_explication_4.html which may be this file itself.

    Local file: local link: energy_explication.html.
    Extended file: Physics file: energy_explication_4.html.
    File: Physics file: energy_explication.html.