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| Introduction |
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| Heat is a form of energy. Heat energy is also called thermal energy. When heat is given to a body, its temperature increases and when heat is removed from a body, its temperature decreases. |
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| Heat is a measurable quantity. The measurement of the quantity of heat is called calorimetry. |
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| Every substance is made up of molecules. The molecules of a substance are in continuous random motion. According to the state of the substance, they possess kinetic energy
due to translatory motion or vibratory
motion or rotatory motion, together with potential energy. The sum of their kinetic and potential energies is generally referred to as the internal energy of a body. Thus, heat is a form of energy produced by random motion of its molecules. |
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| When heat is supplied to a body, the internal molecular motion increases, which results in increase in temperature of the body. If some quantity of heat is removed from a body, its molecular motion decreases and the temperature of the body decreases. |
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| When two bodies at different temperatures, are kept in contact, heat flows from the body at a higher temperature to the body at a lower temperature. |
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 SI unit of heat is the joule (J) |
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 CGS unit of heat is erg and 1 joule = 107 erg |
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 The most commonly used unit of heat is the calorie. |
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| One calorie of heat is the quantity of heat required to raise the temperature of 1 g of water through 10C. |
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| In this definition of calorie, it has been assumed that the heat required to raise 1 g of water through 1OC at any temperature is the same. However, this is not true. The reason is that the thermal expansion of water is not uniform near 4OC. Hence, the correct definition of calorie is given as follows: |
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| One calorie of heat is the quantity of heat required to raise the temperature of 1 g of water from 14.5OC to 15.5OC. |
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| The unit calorie is related to joule as follows: |
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| 1 calorie (1 cal) = 4.2 J. |
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| Calorie is small unit of heat, so we use another unit called the kilocalorie. |
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| One kilocalorie of heat is the quantity of heat required to raise the temperature of 1 kg of water from 14.5OC to 15.5OC. |
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| 1 kilocalorie = 1000 calorie |
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| 1 kilocalorie = 4200 J |
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