Introduction:

We are familiar with the different forms of energy that can be transferred from one form to another. Heat is produced by rubbing the palms of hands together. By rubbing two sticks of wood together, a five may be started. When an object is released from a certain height, heat is developed as the weight strikes the ground. These are some examples which show that mechanical energy can be converted into heat energy.

In the reverse way, heat can also be converted into mechanical energy. The steam engine, diesel engine and jet propulsion engine converts heat into mechanical energy. In all these engines, fuel is burned to produce heat and this heat in turn is used to expand the volume of gas and hence mechanical work is done.

This relationship between heat and mechanical energy is dealt in thermodynamics. But we know that energy can be converted from one form into another. Hence, Thermodynamic deals with heat and interrelationship with other forms of energy like mechanical, chemical, electrical, magnetic etc.

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Kelvin, a British Physicist, was born in Belfast in 1824. He worked in the field of thermodynamics. He restated the second law of thermodynamics in 1850, and introduced the absolute scale of temperature. Jule-Kelvin effect was developed by him in collaboration with Joule.

The words "Temperature" and "Heat" are not new
in our daily life. We use the words cold, cool, warm, hot etc. many times which are related with heat and temperature. When we touch ice we may say "Oh cold!" and when we stand outside our home in the summer day we may say "Oh! its very hot today." The cause of these sensations is nothing but heat. Previously it was thought that heat was a fluid which when transferred in cold objects makes them hot. But this concept was unable to explain many experimental phenomena. The quantity of heat has a meaning only when we define it in terms of energy. Transfer of energy takes place from a hot to a cold body by conduction, convection or radiation. Heat, unlike other forms of energy, arises due to the molecular motions of the matter. Temperature measure the degree of hotness of a body. It also determines the direction of flow of heat when two bodies are placed in thermal contact. Temperature of a body is associated with the average kinetic energy of the molecules of the matter. The sense was use when we touch an object is temperature which determines body may produce a sense of cold for one person and a sense of hot for another person, at the same time. A familiar example is, if we place our one hand in cold water and another hand in hot water then we place bother hands in water at room temperature, we will find that the hand which was previously placed in cold water feels hot and the other hand feels cold.

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According to Prevost, a body emits heat radiations at all temperatures which is independent of surroundings but the quantity of heat radiated per second depends on the temperature of the body. The rate of radiation is greater at a higher temperature than at a lower one. At the same time, the body absorbs thermal radiation from the surrounding. If the rate of emission by the body is less than the rate of absorption from surroundings, there is a net heat gained by the body and its temperature rises. Conversely, if the body is emitting at the rate higher than the absorption from the surroundings, there is a net loss of heat, therefore, there is a fall of temperature of the body. The rate of emission will be greater if the temperature of the body is higher than that of the surrounding and vice-versa.

If the body temperature is equal to the temperature of surroundings, the rate of emission and the rate of absorption will be equal and temperature of the body remains constant. Thus, thermal equilibrium between the objects is not static equilibrium but a dynamic equilibrium since they are exchanging heat at the same rate resulting constant temperature of the body.

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