We measure energy in many different ways and for many different purposes. Quantities of energy given in one unit can always be converted to any other unit of energy. For example, 1 gram calorie is equivalent to 4.186 joules, which is used to measure heat energy.
Many people are familiar with kilogram calories, which are often used to measure the energy available in food. One calorie is the amount of energy needed to raise the temperature of one kilogram of water by one degree Celsius. When you see calories on the back of a package, they indicate the total amount of food energy in that item.
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