12/26/2023 0 Comments Ideal gas thermodynamics calculatorSo how do you determine the change in entropy between these two end states that are connected in practice by an irreversible path? ![]() Neither equation describes the intermediate changes along an irreversible path between two widely separated thermodynamic equilibrium states. The 2nd equation you wrote applies only to two closely neighboring thermodynamic equilibrium states of a system, and describes the relationship between the differential changes in U, S, and V between these adjacent states. The ideal gas equation only applies to an ideal gas in a thermodynamic equilibrium state, and not to one that is rapidly deforming. So how does equation 2 make sense in the context of irreversible processes? (Such as free expansion) However, dQ is equal to TdS only for reversible processes. So how can one consider equation 1 to hold true during the transient phase, and then use it to calculate the change in entropy?Įdit: I have another question related to equation 2 the textbooks have substituted TdS for dQ in the first law of thermodynamics. The words 'Pressure' and 'Volume' have no meaning when the gas is still expanding into the volume. ![]() My confusion pertains to how one calculates this entropy change. The textbooks also state that the temperature, and consequently internal energy, do not change for such an expansion. The textbooks calculate the change in entropy for this process using the following equations.Įquation 1: PV = nRT Equation 2: dU = TdS - PdV Now, suddenly the door vanishes, the ideal gas expands and re-establishes equilibrium in the new volume V1+V2. The amount of resistance that the instrument has to put up is what we call pressure.Ĭonsider the scenario where an ideal gas is kept from expanding into another adjacent volume V2 through a door (The second chamber is vacuous). ![]() I am having trouble understanding the transient phase of an ideal gas expanding into vacuum.įirstly, the pressure of any gas is defined only when there is an instrument (barometer/ wall/ piston) equally resisting the expansion of the gas.
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