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A calorie is the amount of energy required to raise the temperature of 1g of pure water by 34°F (1°C) under standard conditions. These conditions include an atmospheric pressure of one atmosphere, and a temperature change from 60° to 62°F (15.5 to 16.5°C).

The calorie is also sometimes designated as a gram-calorie or small calorie (abbreviated: cal), to distinguish it from the calorie of dieticians (abbreviated: Cal), also known as a large calorie, or kilocalorie (kcal), which is equal to 1,000 (small) calories.

One calorie is equivalent to 3.968 British thermal units (btu), a non-metric measure of energy content. A calorie is also equivalent to 4.187 joules (also known as an International Table calorie), which is now the unit of energy that is most commonly used in science.

Scientists are often interested in the energy contents of organic materials. These data are usually obtained by completely oxidizing (burning) a known quantity of a substance by igniting it in an oxygen-rich atmosphere inside of a device known as a bomb-calorimeter. The quantity of energy released is determined by measuring the increase in temperature of a known quantity of water contained within the bomb.

Dieticians are interested in the calorie contents of foods of various sorts. The potential energy of food is utilized metabolically by animals to drive their physiological processes, and to achieve growth and reproduction. Foods vary tremendously in their energy contents, so careful planning of food intake requires an understanding of the balance of the nutrients, such as vitamins and amino acids.

On average, pure carbohydrates have a calorific content of about 4,600 cal/g (or 4.6 Cal/g), while proteins contain about 4,800 cal/g, and fats or lipids about 6,000-9,000 cal/g. Because fats are so energy-dense, they are commonly used by organisms as a compact material in which to store potential energy for future use. Of course, some of us store more of this potential energy of fat than others.

Engineers are often concerned with the energy contents of petroleum, coal, and natural gas, and of distillates or synthetic materials refined from any of these fossil fuels. Knowledge of the amounts of energy that are liberated through the complete oxidation of these materials is important in the design of engines, fossil-fueled generating stations, and other machines that we use to achieve mechanical work. In order to maximize the amount of useful work that is achieved per unit of fuel consumed, that is, the energy-conversion efficiency, engineers are constantly re-designing machines of these sorts, and tuning their operating parameters, such as fuel-oxygen ratios.

Ecologists are also interested in the energy contents of organic materials, and how these change over time. Although ecologists commonly measure biomass and productivity in terms of weight, these are often converted into energy units, in order to account for the greatly varying calorific contents of different sorts of biomass, as was described above for carbohydrates, proteins, and fats. In parallel with the interests of engineers, ecologists are concerned with the efficiency of ecosystems in converting solar energy into plant productivity, as well as the transfers of the energy of plants to herbivores and carnivores. These efficiencies are best determined through knowledge of the amounts and transfers of energy, as expressed in calorific units.

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