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Calories are units of energy. Various definitions exist but fall into two broad categories. The first, the small calorie, or gram calorie (symbol: cal), is defined as the approximate amount of energy needed to raise the temperature of one gram of water by one degree Celsius at a pressure of one atmosphere.
In physics, the electronvolt (symbol eV, also written electron-volt and electron volt) is a unit of energy equal to approximately 1.6×10−19 joules (symbol J).