A calorie is a unit of energy that's used currently only for measuring the energy in food. The amount of energy in a calorie isn't rigorously defined, but it is equal to about 4,190 joules. The joule is the standard unit of energy and is defined with scientific rigor.
History
The gram-calorie originally was defined as a unit of heat by Nicolas Clement in 1824; the definition later was modified to be a unit of energy. The calorie essentially was defined as the energy needed to heat a gram of water by 1 degree Celsius. The joule gradually replaced the gram-calorie as a unit of energy, and the gram-calorie became officially obsolete in 1960. The kilogram-calorie is 1,000 times the energy of a gram-calorie; it still is used for measuring food energy. The term "calorie" today refers to the kilogram-calorie unless otherwise specified.
Test Conditions
The amount of energy in a calorie can't be established with scientific precision because it depends upon an experimental test. One factor that has a significant effect on the amount of energy in a calorie is the water's initial temperature.
Calorimeter
The test for measuring the energy in a gram-calorie requires a device called a calorimeter. A gram of water that doesn't have any dissolved air is placed in the calorimeter's chamber. The temperature of the water is adjusted to the initial temperature required by the test. The water is heated by 1 degree Celsius, and the energy that was expended is measured by the calorimeter. This energy may then be multiplied by 1,000 to obtain the energy in a kilogram-calorie.
Range of Values
A calorie is equal to about 4,204 joules of energy if the water's initial temperature is 3.5 degrees Celsius. If the initial temperature of the water is 19.5 degrees Celsius, a calorie will have about 4,182 joules.
Differing Definitions
The lack of standardization has led to several slightly different definitions of the calorie. One of the most common definitions of the calorie is known as the 15-degree Celsius calorie. The initial temperature of the water is 14.5 degrees Celsius, so the final temperature is 15.5 degrees Celsius. This definition yields a value for the calorie that ranges from 4,185.2 joules to 4,185.8 joules.
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