Heat is energy transferred from one object to another because of a difference in temperature. In SI units, the unit for heat, as for any form of energy , is the joule. Nonetheless, calories of kcal(Calories) are still sometimes used. The calorie was originally defined as the amount of heat required at a pressure of 1 standard atmosphere to raise the temperature of 1 gram of water 1° Celsius. [To be precise, the particular temperature range from 14.5°C to 15.5°C is specified because the heat required is very slightly different at different temperatures.] Joule determined that a give amount of work done was always equivalent to a particular amount of heat input.