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The first thermodynamics law. Its application for Isoprocess. Gas work at constant pressure and constant temperature.
The first explicit statement of the first law of thermodynamics, by Rudolf Clausius in 1850, referred to cyclic thermodynamic processes.
"In all cases in which work is produced by the agency of heat, a quantity of heat is consumed which is proportional to the work done; and conversely, by the expenditure of an equal quantity of work an equal quantity of heat is produced."[1]
Clausius stated the law also in another form, this time referring to the existence of a function of state of the system called the internal energy, and expressing himself in terms of a differential equation for the increments of a thermodynamic process. This equation may be translated into words as follows:
In a thermodynamic process of a closed system, the increment in the internal energy is equal to the difference between the increment of heat accumulated by the system and the increment of work done by it.[2]
The first law of thermodynamics may be stated thus:
Increase in internal energy of a body = heat supplied to the body - work done by the body. U = Q - W
For a thermodynamic cycle, the net heat supplied to the system equals the net work done by the system.
More specifically, the First Law encompasses several principles:
· The law of conservation of energy.
This states that energy can be neither created nor destroyed. However, energy can change forms, and energy can flow from one place to another. The total energy of anisolated system remains the same.
· The concept of internal energy and its relationship to temperature.
If a system, for example a rock, has a definite temperature, then its total energy has three distinguishable components. If the rock is flying through the air, it has kinetic energy. If it is high above the ground, it has gravitational potential energy. In addition to these, it has internal energy which is the sum of the kinetic energy of vibrations of the atoms in the rock, and other sorts of microscopic motion, and of the potential energy of interactions between the atoms within the rock. Other things being equal, the internal energy increases as the rock's temperature increases. The concept of internal energy is the characteristic distinguishing feature of the first law of thermodynamics.
· The flow of heat is a form of energy transfer.
In other words, a quantity of heat that flows from a hot body to a cold one can be expressed as an amount of energy being transferred from the hot body to the cold one.
· Performing work is a form of energy transfer.
For example, when a machine lifts a heavy object upwards, some energy is transferred from the machine to the object. The object acquires its energy in the form ofgravitational potential energy
in this example.Combining these principles leads to one traditional statement of the first law of thermodynamics: it is not possible to construct a perpetual motion machine which will continuously do work without consuming energy.
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