What Is Specific Energy?

M.J. Casey

Put simply, specific energy is a way of measuring the exact amount of energy within a specific unit, normally expressed in terms of weight. It’s essentially the amount of energy per unit mass — per joule, for instance, or in less exact circumstances per calorie or British Thermal Unit (BTU). Understanding the measured and quantifiable energy of a substance or compound is often really useful to researchers and engineers. It can help determine things like combustion times in engines and other large mechanics, and also helps food researchers draw conclusions about nutritive compositions and energy profiles of various meals and food items. It’s usually possible to calculate the specific energy of any material that has a mass, or a weight. Things that are weightless, like light, often require a slightly different calculation and measuring rubric.

Scientist with beakers
Scientist with beakers

Ways of Thinking About Energy More Broadly

To a scientist or engineer, the term energy is the potential of a system to perform work or to produce heat. Energy more generally is an important and fairly ubiquitous concept, and measuring it makes it more easily quantifiable for research and other calculation-based purposes. Concentration by mass is the measurement of a quantity per unit mass, and to this extent specific energy may be thought of as the concentration of energy per unit mass.

Primary Advantages of Specific Calculations

There are usually a couple of reasons why measuring the energy present per mass unit is important. It is often used as a point of comparison, as similar products, elements, or compounds can often be distinguished based on the density of the energy they’re capable of putting out. This is often particularly important where fuel gradations are concerned. The concept is also usually important to aerodynamics and the automotive industry, as well as food scientists and researchers.

Measuring Output

A joule is the metric, or more accurately, the International System of Units (SI) measurement of energy. It is derived from the formula for kinetic energy: Ek = ½ mv2, where m is the mass of the object in motion and v is its velocity. A joule is the energy required to accelerate a mass of one kilogram at the rate of one meter per second squared over a distance of one meter. Measured energy is often expressed in SI units as joules per gram (J/g), kilojoules per kilogram (kJ/kg), or joules per kilogram (J/kg), depending on the system under study.

In a Caloric Context

An earlier attempt by chemists to define energy was the calorie, the energy required to raise the temperature of one gram of water by 1°C. This is not an exact measurement, however, as the energy required depends slightly on the starting temperature of the water. The SI definition of calorie is now 1 calorie equals 4.184 joules. In specific terms, the energy would typically be measured as calories/gram (cal/g), kilocalories/kg (kcal/kg), or kilocalories per gram (kcal/g).

The British thermal unit (BTU) is the analog to the calorie and is an inexact measurement. A BTU is the energy required to raise the temperature of one pound of water by 1°F. A BTU is slightly less energy than 1 kilojoule. Specifically measured energy in this system is usually expressed as BTUs per pound (BTU/lb).

Relationship to Light

Light, another form of energy, has no mass. Therefore, no term exists for the specific energy of light. In discussing chemical energy, it is often more useful to be concerned with specific measurements on a molecular basis instead of a mass basis. A chemist may use joules per mole (J/mol) instead of joules per kilogram, as it is the number of molecules that react that is useful in this application.

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