From Wikipedia, the free encyclopedia
Energy density is the amount of
energy stored in a given system or region of space per unit
volume or per unit
depending on the context. In some cases it is obvious from
context which quantity is most useful: for example, in
rocketry, energy per unit mass is the most important
parameter, but when studying pressurized gas or
magnetohydrodynamics the energy per unit volume is more
appropriate. In a few applications (comparing, for example, the
hydrogen fuel to
gasoline) both figures are appropriate and should be called
out explicitly (hydrogen has a higher energy density per unit
mass than does gasoline, but a much lower energy density per
unit volume in most applications).
Energy density per unit volume has the same physical units as
pressure, and in many circumstances is an exact
synonym: for example, the energy density of the magnetic
field may be expressed as (and behaves as) a physical pressure,
and the energy required to compress a gas may be determined by
multiplying the pressure of the compressed gas times its final
Note however, that energy density should not be confused with
conservation of energy. The energies gained or lost from a
system are still conserved, because an equal and opposite energy
is gained or lost in another system. In other words, energy has
changed form but was never lost or gained.
Energy density in energy storage and
energy storage applications, the energy density relates the
of an energy store to its stored energy. The higher the energy
density, the more energy may be stored or transported for the
same amount of mass. In the context of
selection, the energy density of a fuel is also called the
specific energy of that fuel, though in general an
engine using that fuel will yield less energy due to
thermodynamic considerations -- hence the
specific fuel consumption of an engine will be greater than
reciprocal of the specific energy of the fuel.
Gravimetric and volumetric energy density of some fuels and
storage technologies (modified from the
Gasoline article): (Note: some values may not be
accurate because of
isomers or other irregularities)
By dividing by 3.6 the figures for megajoules per kilogram
can be converted to kilowatt-hours per kilogram.
Unfortunately, the useful energy available by extraction from
an energy store is always less than the energy put into the
energy store, due to the
laws of thermodynamics.
No single energy storage method boasts the best in
specific power, energy density, and
energies per unit mass.
Peukert's Law describes how the amount of energy we get out
depends how quickly we pull it out.
Energy density of electric and
magnetic fields store energy. In a vacuum, the (volumetric)
energy density (in SI units) is given by:
where E is the electric field and B is the
magnetic induction. In the context of
magnetohydrodynamics, the physics of conductive fluids, the
magnetic energy density behaves like an additional
pressure that adds to the
gas pressure of a
In normal (linear) substances, the energy density (in SI
where D is the electric displacement and H is
Energy density of empty space
energy" or "zero-point
energy" is the volumetric energy density of empty space.
More recent developments have expounded on the concept of energy
in empty space.
Modern physics is commonly classified into two fundamental
quantum field theory and
general relativity. Quantum field theory takes
quantum mechanics and
special relativity into account, and it's a theory of all
the forces and particles except
gravity. General relativity is a theory of gravity, but it
is incompatible with quantum mechanics. Currently these two
theories have not yet been reconciled into one unified
description, though research into "quantum
gravity" seeks to bridge this divide.
general relativity, the
cosmological constant is proportional to the energy density
of empty space, and can be measured by the curvature of space.
It is subsequently related to the age of the universe, as energy
expands outwards with time its density changes.
Quantum field theory considers the vacuum ground state not to
be completely empty, but to consist of a seething mass of
virtual particles and
fields. These fields are quantified as probabilities - that
is, the likelihood of manifestation based on conditions. Since
these fields do not have a permanent existence , they are called
vacuum fluctuations. In the
Casimir effect, two metal plates can cause a change in the
vacuum energy density between them which generates a measurable
Some believe that vacuum energy might be the "dark
energy" (also called
quintessence) associated with the cosmological constant in
general relativity, thought to be similar to a negative force of
antigravity). Observations that the expanding universe
appears to be accelerating seem to support the
cosmic inflation theory — first proposed by
Alan Guth in 1981 — in which the nascent universe passed
through a phase of exponential expansion driven by a negative
vacuum energy density (positive vacuum pressure).
Energy density of food
Energy density is the amount of energy (kilojoules
calories) per amount of food, with food amount being
measured in grams or milliliters of food. Energy density is thus
expressed in cal/g, kcal/g, J/g, kJ/g, cal/mL, kcal/mL, J/mL, or
kJ/mL. This is the energy released when the food is metabolised
by a healthy organism when it ingests the food (see
food energy for calculation) and the food is
metabolized with oxygen, into waste products such as
carbon dioxide and water. Typical values of food energy
density for high energy-density foods, such as a hamburger,
would be 2.5 kcal/g. Purified fats and oils contain the highest
energy densities-- about 9 kcal/g.
Heat of combustion
Zero-point energy -
C. Knowlen, A.T. Mattick, A.P. Bruckner and A. Hertzberg,
"High Efficiency Conversion Systems for Liquid Nitrogen
Automobiles", Society of Automotive Engineers Inc, 1988.
Zero point energy
- Eric Weisstein's world of physics - energy density
- Baez physics - Is there a nonzero cosmological constant?
What's the Energy Density of the Vacuum?.
- Introductory review of cosmic inflation
- An exposition to inflationary cosmology
Fuels." Energy, Technology and the Environment Ed.
Attilio Bisio. Vol. 1. New York: John Wiley and Sons, Inc.,
table of energy density
Energy Density Field Theory
Energy density of foods
- The Inflationary Universe: The Quest for a New Theory
of Cosmic Origins by Alan H. Guth (1998)
- Cosmological Inflation and Large-Scale Structure
by Andrew R. Liddle, David H. Lyth (2000)
- Richard Becker, "Electromagnetic Fields and
Interactions", Dover Publications Inc., 1964
Cleanup from March 2007 |
All pages needing cleanup |
Fundamental physics concepts |
Energy storage |