Heat : Introduction And Classification

Heat is one of the essential forms of energy for the survival of life on earth. It plays a major role in our lives. We use heat energy for cooking, ironing, transportation, recreation etc. This form of energy also plays a very important role in nature. The occurrence of the wind, rain, change of seasons etc. is all dependent on the gradient created due to uneven heating of different regions. Here we will learn about the concept of heat and the classification of objects into hot and cold.

Heat - Cold

Heat: Definition

With the increase in temperature of a body, the vibrations of molecules or atoms increase. These vibrations are then transferred from one part of the body to another. The measure of energy with which the molecules vibrate in a system is termed as heat stored in that object. It flows from a body at higher temperature to a body at a lower temperature. This is because the molecules vibrating faster transfer their energy to the molecules vibrating slower. The vibrational energy is also termed as its heat content. The heat content in the body makes it hot or cold. Greater the heat content, hotter will be the body.

Let us take three beakers, first with cold water, the second with hot and the third with water at normal temperature. If we dip our finger in beaker 1 and then in beaker 3, we will observe that water in beaker 3 is hot as compared to that in beaker one. Whereas, if we dip our fingers in beaker 2 and then in beaker 3, we will observe that the water in beaker 3 is colder as compared to that in beaker 2. This shows that we cannot rely on our sense of touch to judge the hotness or coldness of something and that’s why the degree of hotness or coldness of any object is measured in terms of temperature.


Objects with high heat content are defined as hot (Hotness or coolness of an object being a relative term is always measured with respect to a reference object).

Examples of hot objects around us include the sun, fire, hot pans, air from a hair dryer, lava from volcanic eruptions,etc.


Objects with lower heat content are defined as cold objects. Hotness or coolness of an object being a relative term is always measured with respect to a reference object.

Examples of cold objects around us include ice, air from an air conditioner, cold drinks metal vessels kept in open in cold winter days etc.

Heat as a form of energy can be converted to other forms of energy. For example, in motorized vehicles heat is converted to mechanical energy. In electric bulbs, it is converted to light energy. In thermal power plants, it is finally converted to electrical energy.

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Practise This Question

 Which of the following heat flow situations pertains to free or natural convection?