December 28, 2015



Heat is a sensation that we feel, but what is that sensation and how is it caused? Heat in the earlier times was thought to be a kind of fluid that goes from one body to another. But later they understood that heat is nothing but the jiggling and wiggling of atoms and molecules. Matter is not all one piece. Inside it is made up of crores and crores of atoms. These atoms move randomly and more the movement of the atoms, more the heat. Our skins have evolved to precise heat with a sensation of hot and cold! In a cold night, why do we feel cold? It is because the molecules of air move about at less average energy( called kinetic energy). When we rub our hands - hard lot of heat is automatically produced. Why? Again it is because when you rub your hands together, the atoms inside jiggle more and more and that gives us a sensation of heat! So when pressure is increased then heat is produced. The pressure can be due to friction, electricity. Then there is another thing that heat does , it can change the state from solid to (picture)

Change of State

liquid and liquid to gas! Why does this happen? It happens because in a solid the atoms are fixed in their places. They cannot move about and when heat is given to such a solid, the bonds etween the atoms that keep them fixed weaken and they move about though the link between them does not break. This is a liquid. On further heating, the links that is the bonds are broken completely and that is a gas obviously!

Heat also expands solids, liquids and gases. This is because the bonds between the atoms inside get loosened and the body as a whole expands. (Picture)

Expansion of Solid

Heat is also transmitted in 3 ways - conduction, convection, radiation. When one part of a solid body is heated, the atoms inside that part starts jiggling and wiggling. These in turn moves the atoms beside them and so on. Thus heat is transmitted from one end to the other. This is conduction(see Picture)

Convection is very interesting. Suppose we take a bucket full of water and heat it for sometime. The lower part which is closer to the heat gets heated up. The bonds inside loosen up and the distance between the atoms become greater. When the distance becomes greater it is like the weight is reduced as less is now in the space occupied earlier. The gravitational attraction hence reduces and the water goes up. The colder heavier water pushes into its place (the lower part of the water). Thus heat gets transmitted by actual movement of water. This is called convection. Exactly to the same thing happens causing winds. The air closer to the earth is heated up, becomes lighter due to the atoms going further apart and hence rising due to less gravitational attraction. The colder heavier gas pushes into its place and winds are caused!

There is a third way in which heat is transmitted. They are through waves. There are waves called heat waves or infrared waves. They are part of the electromagnetic spectrum (to be covered in a later section). These waves do not need any medium. The sun’s heat is actually heat waves which travel through space and come to earth !