Calorie
Calories are units of energy. Various definitions exist but fall into two broad categories.
Although these units relate to the metric system all forms of the calorie were deemed obsolete in science after the SI system was adopted in the 1950s. The unit of energy in the International System of Units is the joule. One small calorie is approximately 4.2 joules (so one large calorie is about 4.2 kilojoules). The factor used to convert calories to joules at a given temperature is numerically equivalent to the specific heat capacity of water expressed in joules per kelvin per gram or per kilogram. The precise conversion factor depends on the definition adopted.
In spite of its non-official status, the large calorie is still widely used as a unit of food energy. The small calorie is also often used for measurements in chemistry, although the amounts involved are typically recorded in kilocalories.
The calorie was first defined by Nicolas Clément in 1824 as a unit of heat energy, and entered French and English dictionaries between 1841 and 1867. The word comes from Latin calor meaning "heat".
The energy needed to increase the temperature of a given mass of water by 1 °C depends on the atmospheric pressure and the starting temperature. Accordingly, several different precise definitions of the calorie have been used.
The pressure is usually taken to be the standard atmospheric pressure (101.325 kPa). The temperature increase can be expressed as one kelvin, which means the same as an increment of one degree Celsius.
≈ 0.003964 BTU ≈ 1.162×10−6 kWh ≈ 2.611×1019 eV
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