Etot

Etot is a common notation for the total energy of a system. It is used across disciplines to denote the energy content that must be accounted for in energy balances and simulations. The exact definition of Etot can vary by context, but it generally includes kinetic, potential, and sometimes internal or rest energy.

In classical mechanics, Etot equals the sum of kinetic energy (E_k) and potential energy (E_p): Etot = E_k + E_p. For isolated systems, Etot remains constant over time, reflecting conservation of energy.

In special relativity, Etot includes rest energy: Etot = γ m c^2, where γ is the Lorentz factor and m is the rest mass. The non-relativistic limit reduces to Etot ≈ m c^2 + E_k + E_p, with m c^2 often treated as a large, nearly constant rest energy.

In thermodynamics and statistical mechanics, Etot can be defined as the total energy of a macroscopic body, comprising internal energy U and macroscopic kinetic energy K: Etot = U + K (and in some presentations Etot = U + K + E_p for gravitational systems). The specific decomposition depends on the model.

In computational physics and simulations, Etot is tracked as a diagnostic to assess numerical accuracy and energy conservation. Deviations in Etot over time can indicate integration error or unmodeled interactions.

Variants: Etot is sometimes written as E_tot or Ē_tot in publications, and may appear differently in software inputs. The term is generally not a fixed physical constant; its meaning is defined by the equations or model in use.