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containing water, the water becomes hot and the red hot iron piece cools down fill the temperatures of the two become equal. It appears that 'something' has flown from red hot iron piece to water. This 'something' which flows from a body at higher temperature to another body at lower temperature, when the two are in contact, is called 'Heat'.
However, caloric theory failed to explain the production of heat on account of friction. For example, when we rub our hands against each other, we feel warm. Similarly, when two pieces of metal are rubbed together, heat is produced. The famous paddle wheel experiment performed by Joule also led to the production of heat by friction between the rotating paddle and water.
These observations led to the Dynamic theory of heat, according to which heat is a form of energy called thermal energy.
Every body is made up of large number of tiny particles, called molecules. Depending on the nature of the substance (solid, liquid or gas) and temperature of the substance, the molecules may possess:
(i)
Translatory motion i.e. motion along straight lines.
(ii)
Vibratory motion i.e. to and for motion about the mean position of the molecules.
(iii) Rotatory motion i.e. rotation of the molecules about their axis." 102. Gomber and Gupta, op. cit., p. 8/101
"Thermal Radiations
Thermal radiations are those, which produce in us the sensation of warmth.. They are emitted by a body on account of its temperature. It was established later that every body whose temperature is above OK emits thermal radiations. The energy emitted depends on : (i) the temperature of the body.
(ii)
nature of radiating surface of the body.
The wavelength of thermal radiations ranges from 8 x 10 m to 4 x 104 m. They belong to infra-red region of the electromagnetic spectrum. That is why thermal radiations are also called infra-red radiations.
Some of the basic characteristics of thermal radiations are :
They travel along straight lines with the speed of light.
They require no medium for their propagation i.e. they can pass through vacuum too.
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