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The greatest energy source by far is matter itself, according to the mass–energy equivalence. This energy is described by E = mc 2, where c is the speed of light. In terms of density, m = ρV, where ρ is the volumetric mass density, V is the volume occupied by the mass.
Energy (from Ancient Greek ἐνέργεια (enérgeia) 'activity') is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat and light. Energy is a conserved quantity—the law of conservation of energy states that energy can be converted in form, but ...
If it is at a higher energy level, it is said to be excited, or any electrons that have higher energy than the ground state are excited. Such a species can be excited to a higher energy level by absorbing a photon whose energy is equal to the energy difference between the levels. Conversely, an excited species can go to a lower energy level by ...
Radiant energy per unit volume. Radiant flux: Φ e [nb 2] watt: W = J/s M⋅L 2 ⋅T −3: Radiant energy emitted, reflected, transmitted or received, per unit time. This is sometimes also called "radiant power", and called luminosity in Astronomy. Spectral flux: Φ e,ν [nb 3] watt per hertz: W/Hz: M⋅L 2 ⋅T −2: Radiant flux per unit ...
This low level of light has a much weaker intensity than the visible light produced by bioluminescence, but biophotons are detectable above the background of thermal radiation that is emitted by tissues at their normal temperature. [2]
As a photon is absorbed by an atom, it excites the atom, elevating an electron to a higher energy level (one that is on average farther from the nucleus). When an electron in an excited molecule or atom descends to a lower energy level, it emits a photon of light at a frequency corresponding to the energy difference.
The thermodynamics of a black-body photon gas may be derived using quantum statistical mechanical arguments, with the radiation field being in equilibrium with the atoms in the wall. The derivation yields the spectral energy density u, which is the energy of the radiation field per unit volume per unit frequency interval, given by: [3]
The internal energy of a body can change in a process in which chemical potential energy is converted into non-chemical energy. In such a process, the thermodynamic system can change its internal energy by doing work on its surroundings, or by gaining or losing energy as heat.