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Second Law Of Thermodynamics

Updated on November 26, 2009


FIRST LAW OF THERMODYNAMICS: This law states that " The heat of mechanical work are inter convertible" According to this law a definite amount of mechanical work is needed to produce a definite amount of heat and vice verse.

If W is the amount of mechanical work converted from heat energy Q, then W∞Q or W=Q

Where J is constant and is called Joule's mechanical equivalent of heat. It is defined as the amount of work done to produce unit quanjtity of heat.

Form the above equation J=W/Q

Thus J=1400ft/BTU



1Kcal= 427kgf=m= 4.1868*103 N-m


This law states there is a definite limit of the amount of mechanical energy which can be obtained from given quantity of hat energy

This law of thermodynamics has been enunciated by Clausius in a lightly different form, as "it is impossible a self acting machine working in a cyclic process, to transfer heat from a body at a lower temperature to a body at a higher temperature without the aid of on external agency. Or in other words the heat can not flow itself from cold body to hot body without the help of an external agency.

This law has also been stated by Kelvin Planck as ; it is impossible to construct on engine working on a cyclic process, whose sole purpose s to convert heat energy into work" in other words m no actual heat engine, working on a cyclic process, can convert the heat energy supplied to it into mechanical work. It means that there is a degradation of energy in the process of producing mechanical work. from heat. According to this statement the second law of thermodynamics is sometimes called a law of degradation of energy.

In MKS system the unit of heat is Kilo calorie (Kcal) which is defined as the quantity of heat required to raise the temperature of one Kg of water through one degree centigrade.

LAW OF CONSERVATION OF ENERGY: This law states that energy can neither be cerated nor be destroyed. This means that the total energy possesed by a body remains constant.

This law when applied to gases means that the total heat supplied or rejected in a system must be equal to the work done plus the change in internal energy:

Thus Q= W+▲U

Where Q= total heat supplied or rejected

W= Work done in heat unit

▲U= Changes in internal energy.

in its differential form the above equation may be written as

dQ= dW+dU

dQ= small amount of heat added or rejected

dW= small amountof work done

dU= small change in internal energy.

it must be remembered that the above equation is derived from an application of the first law of thermodynamics to a close system.The application of first law to such a system shows that out of the energy entering, some is utilized in change in stored energy whilst some appears as work.


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