| Specific energy | |
|---|---|
Common symbols | |
| SI unit | J/kg |
Other units | kcal/g, W⋅h/kg, kW⋅h/kg, Btu/lb |
| InSI base units | m2⋅s−2 |
| Intensive? | Yes |
Derivations from other quantities | |
| Dimension | |
Specific energy ormassic energy isenergyper unit mass. It is also known asgravimetric energy density, which is not to be confused withenergy density, which is defined as energy per unit volume. It is used to quantify, for example, storedheat and otherthermodynamic properties of substances such asspecific internal energy,specific enthalpy, specificGibbs free energy, and specificHelmholtz free energy. It may also be used for thekinetic energy orpotential energy of a body. Specific energy is anintensive property, whereas energy and mass areextensive properties.
TheSI unit for specific energy is thejoule perkilogram (J/kg). Other units still in use worldwide in some contexts are thekilocalorie per gram (Cal/g or kcal/g), mostly in food-related topics, andwatt-hours per kilogram (W⋅h/kg) in the field of batteries. In some countries theImperial unitBTU perpound (Btu/lb) is used in someengineering and applied technical fields.[1]
Specific energy has the same units asspecific strength, which is related to the maximum specific energy of rotation an object can have without flying apart due tocentrifugal force.The concept of specific energy is related to but distinct from the notion ofmolar energy inchemistry, that is energy permole of a substance, which uses units such as joules per mole, or the older but still widely usedcalories per mole.[2]
The following table shows the factors for conversion to J/kg of some non-SI units:
| Unit | SI equivalent |
|---|---|
| kcal/g[3] | 4.184 MJ/kg |
| Wh/kg | 3.6 kJ/kg |
| kWh/kg | 3.6 MJ/kg |
| Btu/lb[4] | 2.326 kJ/kg |
| Btu/lb[5] | 2.32444 kJ/kg |
For a table giving the specific energy of many different fuels as well as batteries, see the articleEnergy density.
Forionising radiation, thegray is the SI unit of specific energy absorbed by matter known asabsorbed dose, from which the SI unit thesievert is calculated for the stochastic health effect on tissues, known asdose equivalent. TheInternational Committee for Weights and Measures states: "In order to avoid any risk of confusion between the absorbed doseD and thedose equivalentH, the special names for the respective units should be used, that is, the name gray should be used instead of joules per kilogram for the unit of absorbed doseD and the namesievert instead of joules per kilogram for the unit of dose equivalentH."[6]
Energy density is the amount of energy per mass or volume of food. The energy density of a food can be determined from the label by dividing the energy per serving (usually inkilojoules orfood calories) by the serving size (usually in grams, milliliters or fluid ounces). An energy unit commonly used in nutritional contexts within non-metric countries (e.g. the United States) is the "dietary calorie", "food calorie", or "Calorie" with a capital "C" and is commonly abbreviated as "Cal." A nutritional Calorie is equivalent to a thousand chemical or thermodynamic calories (abbreviated "cal" with a lower case "c") or one kilocalorie (kcal). Because food energy is commonly measured in Calories, the energy density of food is commonly called "caloric density".[7] In the metric system, the energy unit commonly used on food labels is the kilojoule (kJ) or megajoule (MJ). Energy density is thus commonly expressed in metric units of cal/g, kcal/g, J/g, kJ/g, MJ/kg, cal/mL, kcal/mL, J/mL, or kJ/mL.
Energy density measures the energy released when the food ismetabolized by a healthy organism when it ingests the food (seefood energy for calculation). In aerobic environments, this typically requires oxygen as an input and generates waste products such ascarbon dioxide and water. Besidesalcohol, the only sources of food energy arecarbohydrates,fats andproteins, which make up ninety percent of the dry weight of food.[8] Therefore,water content is the most important factor in computing energy density. In general, proteins have lower energy densities (≈16 kJ/g) than carbohydrates (≈17 kJ/g), whereas fats provide much higher energy densities (≈38 kJ/g),[8]2+1⁄4 times as much energy. Fats contain more carbon-carbon and carbon-hydrogen bonds than carbohydrates or proteins, yielding higher energy density.[9] Foods that derive most of their energy from fat have a much higher energy density than those that derive most of their energy from carbohydrates or proteins, even if the water content is the same. Nutrients with a lower absorption, such asfiber orsugar alcohols, lower the energy density of foods as well. A moderate energy density would be 1.6 to 3 calories per gram (7–13 kJ/g); salmon, lean meat, and bread would fall in this category. Foods with high energy density have more than 3 calories per gram (13 kJ/g) and include crackers, cheese, chocolate, nuts,[10] and fried foods like potato or tortilla chips.
Energy density is sometimes more useful than specific energy for comparing fuels. For example,liquid hydrogen fuel has a higher specific energy (energy per unit mass) thangasoline does, but a much lower volumetric energy density.[citation needed]
Specific mechanical energy, rather than simply energy, is often used inastrodynamics, because gravity changes the kinetic and potential specific energies of a vehicle in ways that are independent of the mass of the vehicle, consistent with theconservation of energy in aNewtonian gravitational system.
The specific energy of an object such as ameteoroid falling on the Earth from outside the Earth's gravitational well is at least one half the square of theescape velocity of 11.2 km/s. This comes to 63 MJ/kg (15 kcal/g, or 15 tonnesTNT equivalent per tonne).Comets have even more energy, typically moving with respect to the Sun, when in our vicinity, at about the square root of two times the speed of the Earth. This comes to 42 km/s, or a specific energy of 882 MJ/kg. The speed relative to the Earth may be more or less, depending on direction. Since the speed of the Earth around the Sun is about 30 km/s, a comet's speed relative to the Earth can range from 12 to 72 km/s, the latter corresponding to 2592 MJ/kg. If a comet with this speed fell to the Earth it would gain another 63 MJ/kg, yielding a total of 2655 MJ/kg with a speed of 72.9 km/s. Since the equator is moving at about 0.5 km/s, the impact speed has an upper limit of 73.4 km/s, giving an upper limit for the specific energy of a comet hitting the Earth of about 2690 MJ/kg.
If theHale-Bopp comet (50 km in diameter) had hit Earth, it would have vaporized the oceans and sterilized the surface of Earth.[11]
