Thermo Chapter 7

A piston-cylinder device contains helium gas. During a reversible, isothermal process, the entropy of the helium will sometimes increase.

A piston-cylinder device contains helium gas. During a reversible, isothermal process, the entropy of the helium will sometimes increase.

A piston-cylinder device contains nitrogen gas. During a reversible, adiabatic process, the entropy of the nitrogen will always increase.

A piston-cylinder device contains nitrogen gas. During a reversible, adiabatic process, the entropy of the nitrogen will never increase. Solution The given statement is false.

A piston-cylinder device contains superheated steam. During an actual adiabatic process, the entropy of the steam will never increase. Group startsTrue or False

A piston-cylinder device contains superheated steam. During an actual adiabatic process, the entropy of the steam will always increase. Solution The given statement is false.

Is the isentropic process a suitable model for compressors that are cooled intentionally?

Compressors that are cooled intentionally are no longer adiabatic devices. Hence, the isentropic process is not the ideal process for compressors that are cooled intentionally. Solution No, the isentropic process is not a suitable model for compressors that are cooled intentionally.

During a heat transfer process, the entropy of a system sometimes increases.

During a heat transfer process, the entropy of a system sometimes increases. Solution The given statement is true.

When a system is adiabatic, what can be said about the entropy change of the substance in the system?

If the system undergoes a reversible process, the entropy of the system cannot change without a heat transfer.Otherwise, the entropy must increase.

Which of the following statements is true?

It is possible to create entropy but impossible to destroy it

Which of the following statements is true?

It is possible to create entropy but impossible to destroy it.

It is well known that the power consumed by a compressor can be reduced by cooling the gas during compression. Inspired by this, somebody proposes to cool the liquid as it flows through a pump, in order to reduce the power consumption of the pump. Is this a good proposal?

No This is not a good proposal since the steady-flow work input to the pump is proportional to the specific volume of the liquid, and cooling will not affect the specific volume of a liquid significantly. Solution No, this is not a good proposal.

Does a cycle for which ∮δQ>0∮��>0 violate the Clausius inequality?

No, a cycle for which ∮δQ>0∮��>0 does not violate the Clausius inequality.

Does the cyclic integral of heat have to be zero (i.e., does a system have to reject as much heat as it receives to complete a cycle)?

No, it does not. A system may reject more (or less) heat than it receives during a cycle. The steam in a steam power plant, for example, receives more heat than it rejects during a cycle. Solution No, the cyclic integral of heat does not have to be zero.

The entropy of a hot baked potato decreases as it cools. Is this a violation of the increase of entropy principle?

No, this is not a violation of the increase of entropy principle because the entropy of the surrounding air increases even more during the given process, making the total entropy change positive.

Some properties of ideal gases such as internal energy and enthalpy vary with temperature only (that is, u = u(T) and h = h(T)). Is this also the case for entropy?

No, this is not the case for entropy. The entropy of an ideal gas depends on the pressure as well as the temperature. Solution No, this is not the case for entropy.

What are Pr and vr called? Is their use limited to isentropic processes?

Pr is relative pressure and vr is relative specific volume. Their use is limited to isentropic processes only.

Steam is accelerated as it flows through an actual adiabatic nozzle. The entropy of the steam at the nozzle exit will be less than the entropy at the nozzle inlet. Group startsTrue or False

Steam is accelerated as it flows through an actual adiabatic nozzle. The entropy of the steam at the nozzle exit will be greater than the entropy at the nozzle inlet. Solution The given statement is false.

An ideal gas undergoes a process between two specified temperatures, first at constant pressure and then at constant volume. The entropy change for the ideal gas will be larger for the constant pressure process.

The entropy change relations of an ideal gas simplify to Δs=cpln(T2T1)∆�=��ln�2�1 for a constant pressure process m˙ = 6.886 kg/s�˙ = 6.886 kg/s for a constant volume process Noting that cp > cv, the entropy change will be larger for the constant pressure process. Solution The given statement is true.

An ideal gas undergoes a process between two specified temperatures, first at constant pressure and then at constant volume. The entropy change for the ideal gas will be larger for the constant pressure process. Group startsTrue or False

The entropy change relations of an ideal gas simplify to Δs=cpln(T2T1)∆�=��ln�2�1 for a constant pressure process m˙ = 6.886 kg/s�˙ = 6.886 kg/s for a constant volume process Noting that cp > cv, the entropy change will be larger for the constant pressure process. Solution The given statement is true.

Can the entropy of an ideal gas change during an isothermal process?

The entropy of a gas can change during an isothermal process since entropy of an ideal gas depends on the pressure as well as the temperature. Solution Yes, the entropy of an ideal gas can change during an isothermal process.

The ideal process for an adiabatic nozzle is the reversible adiabatic process, and its isentropic efficiency is given by ηN=actual exit kinetic energyisentropic exit kinetic energy��=actual exit kinetic energyisentropic exit kinetic energy .

The ideal process for an adiabatic nozzle is the reversible adiabatic (isentropic) process. Its adiabatic efficiency is given by ηN=actual exit kinetic energyisentropic exit kinetic energy��=actual exit kinetic energyisentropic exit kinetic energy Solution The given statement is true.

Compare the values of the integral ∫21δQ/T∫12��/� for a reversible and an irreversible process between the same end states.

The value of this integral is always larger for a reversible process.

In large compressors, the gas is often cooled while being compressed. Does cooling the gas during a compression process reduce the power consumption?

The work associated with steady-flow devices is proportional to the specific volume of the gas. Cooling a gas during compression will reduce its specific volume, and thus the power consumed by the compressor. Solution Yes, cooling the gas during a compression process reduces the power consumption.

The entropy of steam will increase as it flows through an actual adiabatic turbine.

True The entropy of steam will increase as it flows through an actual adiabatic turbine.

Is a process that is internally reversible and adiabatic necessarily isentropic?

Yes, a process that is internally reversible and adiabatic is necessarily isentropic. This is because an internally reversible, adiabatic process involves no irreversibilities or heat transfer. Solution Yes, a process that is internally reversible and adiabatic is necessarily isentropic.

Is it possible for the entropy change of a closed system to be zero during an irreversible process?

Yes, it is possible. This will happen when the system is losing heat, and the decrease in entropy as a result of this heat loss is equal to the increase in entropy as a result of irreversibilities.

Is the value of the integral ∫21δQ/T∫12��/� the same for all reversible processes between states 1 and 2?

Yes, the value of the integral ∫21δQ/T∫12��/� is the same for all reversible processes between states 1 and 2.