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Energy density is the amount of energy stored in a given system or region of space per unit volume, or per unit mass, 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 effectiveness of 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 volume.

Energy density in energy storage and in fuel In energy storage applications, the energy density relates the mass 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 fuel selection, that 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 inefficiency and thermodynamics considerations—hence the specific fuel consumption of an engine will be greater than the reciprocal of the specific energy of the fuel. And in general, specific energy and energy density are at odds due to charge screening.

Gravimetric and volumetric energy density of some fuels and storage technologies (modified from the Gasoline article): (Notes: Some values may not be precise because of isomers or other irregularities. See Heating value for a comprehensive table of specific energies of important fuels. The symbol ** indicates the item is an energy carrier, not an energy source.)

{| class="wikitable"|-! rowspan=3 align=center | storage type !! colspan=2 align=center | energy density !! colspan=2 align=center | recovery efficiency|-! colspan=1 align=center | by mass !! colspan=1 align=center | by volume !! colspan=1 align=center | peak !! colspan=1 align=center | practical|-! align=right width=70 | MJ/kg !! align=right width=70 | MJ/Liter !! align=right width=70 | % !! align=right width=70 | %|-|**mass-energy equivalence of [helium nucleus] of hydrogen (energy from the sun)] (of U-235) (Used in Nuclear Power Plants)] 50%|-|**liquid hydrogen at 700 bar||143||4.7|| |||-|**[hydrogen (toxic) (burned in air)||67.6||125.1|| |||-|[lithium borohydride (burned in air)] (burned in air)||58.9||137.8|| |||-|compressed natural gas at 200 bar]||46.9||34.6|| |||-|diesel fuel/residential heating oil plastic||46.3||42.6|| |||-|[polypropylene plastic] (10% ethanol 90% gasoline)||43.54||28.06|| |||-|lithium (burned in air)] aviation fuel oil (vegetable oil)||42.20||30.53|| |||-|[DMF (2,5-dimethylfuran) ] (according to the definition of ton of oil equivalent)] plastic||41.4||43.5|| |||-|fatty acid metabolism||38||35||22-26%|||-|butanol fuel||36.6||29.2|| |||-|liquified petroleum gas||34.39||22.16|| |||-|**specific orbital energy of Low Earth orbit (burned in air)||32.7||72.9|| |||-|[anthracite coal|-|[silicon (burned in air)] (burned in air)||31.0||83.8|| |||-|ethanol plastic||26.0||35.6|| |||-|[magnesium (burned in air)] coal ] plastic||? 23.5 impure||?|| |||-|methanol (toxic) combusted to N2+H2O||19.5||19.3|| |||-|**liquid [ammonia (combusted to N2+H2O)] plastic (Polyvinyl chloride#Dioxins)||18.0||25.2|| |||-|fatty acid metabolism||17||26.2(dextrose)] + CH4 - computed||17.4|| || |||-|lignite coal] (burned in air)||15.9||24.6|| |||-|dry cowdung and Manure#Uses of manure||15.5|| || |||-|wood + [oxygen (as oxidizer) (1:8 (w/w), 14.1:7.0 (v/v))] (burned to wet sodium hydroxide)] decomposition - computed||12.2|| || |||-|nitromethane||8-11|| || |||-|[sodium (burned to dry sodium oxide)] (burned to iron(III) oxide)] explosive - computed||7.4|| || |||-|ammonal (Al+ammonium nitrate oxidizer)] + hydrazine explosive - computed] explosive - computed||6.5|| || |||-|zinc (burned in air)] plastic (combustion toxic, but flame retardant)||5.1||11.2|| |||-|iron (burned to iron(II) oxide)]||4.184||6.92|| |||-|Copper Thermite (Al + copper(II) oxide as oxidizer)] (powder Al + iron(III) oxide as oxidizer)] at 300 bar||4||0.14||?|||-|ANFO decomposition (as [monopropellant)]||1.62|| || |||-|**hydrazine(toxic) decomposition (as monopropellant)] decomposition (as monopropellant)]||~1|| || |||-|**sodium-sulfur battery||0.77C. Knowlen, A.T. Mattick, A.P. Bruckner and A. Hertzberg, "High Efficiency Conversion Systems for Liquid Nitrogen Automobiles", Society of Automotive Engineers Inc, 1988.||0.62|| |||-|**[lithium ion battery||0.54-1.44||?|| |||-|[kinetic energy penetrator bullet||0.4-0.8||3.2-6.4|| |||-|**[Zn-air batteries||0.5||?||?||81-94%|-|[latent heat of fusion||0.335||0.335|| |||-|**zinc-bromine flow battery at 20 bar||0.27|| ||?||64%|-|**[nickel metal hydride battery||0.22||0.36||?||60% |-|**NiCd Battery||0.09–0.11||0.14–0.17||?||75-85%|-|**commercial lead acid battery pack||0.072-0.079||?||?||?|-|**[vanadium redox battery||.18||.252||?||81%|-|**[ultracapacitor by EEStor (claimed capacity)||1.0 ||?||?||?|-|**[supercapacitor||0.002 ||?||?||?|-|[hydroelectricity||0.001||0.001||?||85-90%|-|**spring power (clock spring), torsion spring||0||0|||||}Conclusion: the highest density sources of energy are [nuclear fusion and fission. Fusion includes energy from the sun which will be available for billions of years (in the form of sunlight) but humans have not learned to make our own sustained fusion power sources. Fission of U-235 in nuclear power plants will be available for thousands of years because of the vast supply of the element on earth. Coal and petroleum are the current primary energy sources in the U.S. but have a much lower energy density. Burning local biomass fuels supplies household energy needs (Cook stove, oil lamps, etc.) worldwide.

Energy density (how much energy you can carry) does not tell you about energy conversion efficiency (net output per input) or embodied energy (what the energy output costs to provide, as energy industry, refinery, distributing, and dealing with pollution all use energy). Like any process occurring on a large scale, intensive energy use creates environmental impacts: for example, global warming, nuclear waste storage, and deforestation are a few of the consequences of supplying our growing energy demands from fossil fuels, nuclear fission, or biomass.

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, as explained by the laws of thermodynamics. No single energy storage method boasts the best in specific power, specific energy, and energy density. 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==

Electric field and magnetic fields store energy. In a vacuum, the (volumetric) energy density (in SI units) is given by

U = \frac{\varepsilon_0}{2} \mathbf{E}^2 + \frac{1}{2\mu_0} \mathbf{B}^2 ,

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 kinetic theory of gas of a plasma (physics).

In normal (linear) substances, the energy density (in SI units) is

U = \frac{1}{2} ( \mathbf{E} \cdot \mathbf{D} + \mathbf{H} \cdot \mathbf{B} ) ,

where D is the electric displacement and H is the magnetic field.

Energy density of empty space In physics, "vacuum 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 theories: 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.

In 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 field (physics). 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 force.

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 gravity (or 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 or 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.

See also

External references 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


  • Density data

    Energy storage

    Books

    References Energy density is the amount of energy stored in a given system or region of space per unit volume, or per unit mass, 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 effectiveness of 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 volume.

    Energy density in energy storage and in fuel In energy storage applications, the energy density relates the mass 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 fuel selection, that 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 inefficiency and thermodynamics considerations—hence the specific fuel consumption of an engine will be greater than the reciprocal of the specific energy of the fuel. And in general, specific energy and energy density are at odds due to charge screening.

    Gravimetric and volumetric energy density of some fuels and storage technologies (modified from the Gasoline article): (Notes: Some values may not be precise because of isomers or other irregularities. See Heating value for a comprehensive table of specific energies of important fuels. The symbol ** indicates the item is an energy carrier, not an energy source.)

    {| class="wikitable"|-! rowspan=3 align=center | storage type !! colspan=2 align=center | energy density !! colspan=2 align=center | recovery efficiency|-! colspan=1 align=center | by mass !! colspan=1 align=center | by volume !! colspan=1 align=center | peak !! colspan=1 align=center | practical|-! align=right width=70 | MJ/kg !! align=right width=70 | MJ/Liter !! align=right width=70 | % !! align=right width=70 | %|-|**mass-energy equivalence of [helium nucleus] of hydrogen (energy from the sun)] (of U-235) (Used in Nuclear Power Plants)] 50%|-|**liquid hydrogen at 700 bar||143||4.7|| |||-|**[hydrogen (toxic) (burned in air)||67.6||125.1|| |||-|[lithium borohydride (burned in air)] (burned in air)||58.9||137.8|| |||-|compressed natural gas at 200 bar]||46.9||34.6|| |||-|diesel fuel/residential heating oil plastic||46.3||42.6|| |||-|[polypropylene plastic] (10% ethanol 90% gasoline)||43.54||28.06|| |||-|lithium (burned in air)] aviation fuel oil (vegetable oil)||42.20||30.53|| |||-|[DMF (2,5-dimethylfuran) ] (according to the definition of ton of oil equivalent)] plastic||41.4||43.5|| |||-|fatty acid metabolism||38||35||22-26%|||-|butanol fuel||36.6||29.2|| |||-|liquified petroleum gas||34.39||22.16|| |||-|**specific orbital energy of Low Earth orbit (burned in air)||32.7||72.9|| |||-|[anthracite coal|-|[silicon (burned in air)] (burned in air)||31.0||83.8|| |||-|ethanol plastic||26.0||35.6|| |||-|[magnesium (burned in air)] coal ] plastic||? 23.5 impure||?|| |||-|methanol (toxic) combusted to N2+H2O||19.5||19.3|| |||-|**liquid [ammonia (combusted to N2+H2O)] plastic (Polyvinyl chloride#Dioxins)||18.0||25.2|| |||-|fatty acid metabolism||17||26.2(dextrose)] + CH4 - computed||17.4|| || |||-|lignite coal] (burned in air)||15.9||24.6|| |||-|dry cowdung and Manure#Uses of manure||15.5|| || |||-|wood + [oxygen (as oxidizer) (1:8 (w/w), 14.1:7.0 (v/v))] (burned to wet sodium hydroxide)] decomposition - computed||12.2|| || |||-|nitromethane||8-11|| || |||-|[sodium (burned to dry sodium oxide)] (burned to iron(III) oxide)] explosive - computed||7.4|| || |||-|ammonal (Al+ammonium nitrate oxidizer)] + hydrazine explosive - computed] explosive - computed||6.5|| || |||-|zinc (burned in air)] plastic (combustion toxic, but flame retardant)||5.1||11.2|| |||-|iron (burned to iron(II) oxide)]||4.184||6.92|| |||-|Copper Thermite (Al + copper(II) oxide as oxidizer)] (powder Al + iron(III) oxide as oxidizer)] at 300 bar||4||0.14||?|||-|ANFO decomposition (as [monopropellant)]||1.62|| || |||-|**hydrazine(toxic) decomposition (as monopropellant)] decomposition (as monopropellant)]||~1|| || |||-|**sodium-sulfur battery||0.77C. Knowlen, A.T. Mattick, A.P. Bruckner and A. Hertzberg, "High Efficiency Conversion Systems for Liquid Nitrogen Automobiles", Society of Automotive Engineers Inc, 1988.||0.62|| |||-|**[lithium ion battery||0.54-1.44||?|| |||-|[kinetic energy penetrator bullet||0.4-0.8||3.2-6.4|| |||-|**[Zn-air batteries||0.5||?||?||81-94%|-|[latent heat of fusion||0.335||0.335|| |||-|**zinc-bromine flow battery at 20 bar||0.27|| ||?||64%|-|**[nickel metal hydride battery||0.22||0.36||?||60% |-|**NiCd Battery||0.09–0.11||0.14–0.17||?||75-85%|-|**commercial lead acid battery pack||0.072-0.079||?||?||?|-|**[vanadium redox battery||.18||.252||?||81%|-|**[ultracapacitor by EEStor (claimed capacity)||1.0 ||?||?||?|-|**[supercapacitor||0.002 ||?||?||?|-|[hydroelectricity||0.001||0.001||?||85-90%|-|**spring power (clock spring), torsion spring||0||0|||||}Conclusion: the highest density sources of energy are [nuclear fusion and fission. Fusion includes energy from the sun which will be available for billions of years (in the form of sunlight) but humans have not learned to make our own sustained fusion power sources. Fission of U-235 in nuclear power plants will be available for thousands of years because of the vast supply of the element on earth. Coal and petroleum are the current primary energy sources in the U.S. but have a much lower energy density. Burning local biomass fuels supplies household energy needs (Cook stove, oil lamps, etc.) worldwide.

    Energy density (how much energy you can carry) does not tell you about energy conversion efficiency (net output per input) or embodied energy (what the energy output costs to provide, as energy industry, refinery, distributing, and dealing with pollution all use energy). Like any process occurring on a large scale, intensive energy use creates environmental impacts: for example, global warming, nuclear waste storage, and deforestation are a few of the consequences of supplying our growing energy demands from fossil fuels, nuclear fission, or biomass.

    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, as explained by the laws of thermodynamics. No single energy storage method boasts the best in specific power, specific energy, and energy density. 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==

    Electric field and magnetic fields store energy. In a vacuum, the (volumetric) energy density (in SI units) is given by

    U = \frac{\varepsilon_0}{2} \mathbf{E}^2 + \frac{1}{2\mu_0} \mathbf{B}^2 ,

    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 kinetic theory of gas of a plasma (physics).

    In normal (linear) substances, the energy density (in SI units) is

    U = \frac{1}{2} ( \mathbf{E} \cdot \mathbf{D} + \mathbf{H} \cdot \mathbf{B} ) ,

    where D is the electric displacement and H is the magnetic field.

    Energy density of empty space In physics, "vacuum 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 theories: 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.

    In 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 field (physics). 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 force.

    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 gravity (or 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 or 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.

    See also

    External references 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


  • Density data

    Energy storage

    Books

    References

    Energy density - 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 mass, depending on the context. In some cases it is obvious from ...

    Energy Density Field Theory
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    Simple explanation of energy density in the framework of the history of the Universe ... Physical Environment > Energy Density This site tells the story of the history of the ...

    Energy Density
    Simple explanation of energy density in the framework of the history of the Universe ... www.historyoftheuniverse.com: Web

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    Energy Density ... and is calculated in the same way as . The resulting form for in terms of reciprocal lattice vectors is therefore,

    High Energy Density Physics - Elsevier
    Editors-in-Chief: Richard W. Lee, Steven J. Rose See editorial board for all editors information Call for Papers: High Energy Density Physics Description

    energy density - definition of energy density by the Free Online ...
    The energy per unit volume of a region of space.

    Topic710 Cohesive Energy Density
    Microsoft Word - Topic710.doc

     

    Energy Density



     
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