Every change in internal energy impacts entropy changes and brings irreversibility in the process. ![]() Cp/Cv is a measure of how much internal energy is getting converted to work in any compression/expansion process. Hence Cp > Cvįor ideal gases PV = constant. At constant volume since delta-V =0 heat goes to increase only internal energy. These indicate the quantity of heat that can increase the temperature of unit mass by 1☌.Īccording to the 1st law of thermodynamics, delta H = delta U + delta W [ H is enthalpy, U is internal energy and W is work.Īt constant pressure, the heat absorbed is not only increasing internal energy but also goes for doing work. The Cp and Cv are the specific heats of an ideal gas at constant pressure and at constant volume. In isentropic expansion and compression n = Y = Cp/Cv In each case, the polytropic coefficient must be determined experimentally by measurement of the heat and work transfer and the initial and final states. Word of caution: Many gas compression and expansion processes may be approximated by a polytropic process. The polytropic index is a measure of the work done by the system. The value of n is different in different thermodynamic processes. There are an infinite number of reversible polytropic paths between two given states the most commonly used polytropic path is TdS/dT = C, which is a constant and is equal to zero for an adiabatic process. In such a process, the expression relating the properties of the system throughout the process is called the polytropic path. The term "polytropic" was originally created to describe any reversible process on any open or closed system of gas or vapor which involves both heat and work transfer, such that a specified combination of properties were maintained constant throughout the process. The polytropic process equation can describe multiple expansion and compression processes which include heat transfer. Where P is the pressure, V is volume, n is the polytropic index, and C is a constant. ![]() ![]() A polytropic process is a thermodynamic process that obeys the relation:
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