Specific energy is defined as the energy per unit mass. Common metric units are J/kg. It is an intensive property. Contrast this with energy, which is an extensive property. There are two main types of specific energy: potential energy and specific kinetic energy. Others are the gray and sievert, measures for the absorption of radiation. The concept of specific energy applies to a particular or theoretical way of extracting useful energy from the material considered that is usually implied by context.
Thermodynamic properties related to specific energy include specific internal energy, specific enthalpy, specific Gibbs free energy, and specific Helmholtz free energy, all of which use units of energy per mass such as J/kg. These intensive properties are each symbolized by using the lower case letter of the symbol for the corresponding extensive property, which is symbolized by a capital letter. For example, the extensive thermodynamic property enthalpy is symbolized by H; specific enthalpy is symbolized by h.
If a defined chemical compound is used which has a definite molar mass, such intensive thermodynamic properties can be expressed on a per mole basis instead of a per mass basis. Such quantities can be described as molar quantities; for example, molar enthalpy meaning enthalpy per mole. These intensive quantities will use units of energy per mole, such as J/mol (mol is short for mole) or in older chemistry literature kcal/mol.
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.
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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 in kilojoules or calories) by the serving size (usually in grams, milliliters or fluid ounces). Energy density is thus expressed in cal/g, kcal/g, J/g, kJ/g, cal/mL, kcal/mL, J/mL, or kJ/mL. The "calorie" commonly used in nutritional contexts is the kilogram-calorie (abbreviated "Cal" and sometimes called the "dietary calorie", "food calorie" or "Calorie" with a capital "C"). This is equivalent to a thousand gram-calories (abbreviated "cal") or one kilocalorie (kcal). Because food energy is commonly measured in calories, the energy density of food is commonly called "caloric density".[1]
Energy density measures the energy released when the food is metabolized 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. Besides alcohol the only sources of food energy are carbohydrates, fats and proteins, which make up ninety percent of the dry weight of food.[2] Therefore, water content is the most important factor in energy density. Carbohydrates and proteins provide four calories per gram (17 kJ/g), whereas fat provides nine calories per gram (38 kJ/g), 2 1⁄4 times as much energy. 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 as fiber or sugar 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. High-energy foods would have more than three calories per gram and include crackers, cheese, dark chocolate, and peanuts.[3]
Specific energy, rather than simply energy, is often used in astrodynamics, because gravity changes the kinetic and potential specific energies of a vehicle in obvious ways that are independent of the mass of the vehicle, consistent with the conservation of energy in a Newtonian gravitational system.