## energy, $$E$$

https://doi.org/10.1351/goldbook.E02101
In mechanics the sum of potential energy and kinetic energy. In thermodynamics the thermodynamic energy or thermodynamic energy increase, $$\Delta U$$, is the sum of heat and work brought to the system. Only changes in energy are measurable. For photons $E=h\ \nu$ where $$h$$ is the Planck constant and $$\nu$$ the frequency of radiation. In relativistic physics $E=m\ c^{2}$ where $$c$$ is the speed of light and $$m$$ the mass.
Source:
Green Book,2nd ed., p. 12 (https://www.cintegrado.com.br/site/documentos/green_book_2ed.pdf)