Are the following statements true or false? Explain why. a) If a control mass we

Question

Are the following statements true or false? Explain why.



a) If a control mass were at any state other than the dead state, the system would be able to change its condition spontaneously toward the dead state, these tendency would cease when the dead state was reached. Work accompanying this spontaneous change can be negative, positive or zero.

b) The specific internal energy, enthalpy and entropy of an incompressible liquid are each functions of pressure alone.

c) The specific internal energy, enthalpy and entropy of an ideal gas are each functions of temperature alone.

d) The non-flow exergy of a fixed amount of an ideal gas remains constant in every reversible adiabatic compression process.

e) The entropy of a fixed amount of an incompressible substance decreases in every process in which temperature increases.

Explanation / Answer

(a) TRUE,This is according to quasi static process. If a control mass were at any state other than the dead state, the system would be able to change its condition spontaneously toward the dead state, these tendency would cease when the dead state was reached. Work accompanying this spontaneous change can be negative, positive or zero. (b) FALSE, The specific internal energy of an incompressible substance does not vary much with pressure, therefore the specific heat only depends on temperature.
By definition enthalpy varies both with P and T h(P,T) = u(T) + Pv For an incompressible fluid, the entropy is independent of pressure and depends only on the temperature. (c) TRUE, The specific internal energy, enthalpy and entropy of an ideal gas are each functions of temperature alone. For the definition of enthalpy and the equation of state of an ideal gas, it follows that h=u+pv h=u+RT Since R is an constant and u is a function of temperature only, it follows that the enthalpy, h of an ideal gas is also a  function of temperature only. That is, dh=cp dT h=f(T)
Internal energy does not depend on pressure at all but is a function of temperature only.That is , for an ideal gas, PV=RT du=m cv dT and u=f(T) only Entropy is also a function of Temperature only. S=f(T) S=m cp ln(T2/T1). (d) TRUE, it is constant at constant pressure.The non-flow exergy of a fixed amount of an ideal gas remains constant in every reversible adiabatic compression process.
(e) False. ds = q/T, where q is the heat transferred in a process. In order for the temperature to decrease, heat energy must be removed from a system, so q < 0. T is strictly positive, so ds < 0. The entropy decreases when the temperature decreases. Therefore, the entropy of a fixed amount of an incompressible substance increases in every process in which temperature increases.
Therefore, the entropy of a fixed amount of an incompressible substance increases in every process in which temperature increases.

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