Properties of a Pure Substance
What is the difference between saturated vapor and
superheated vapor?
A vapor that is about to condense is saturated vapor; otherwise it is superheated vapor.
Is an isothermal process necessarily internally reversible?
Explain your answer with an example.
No. An isothermal process can be irreversible. Example: A system that involves paddle-wheel work while losing an equivalent amount of heat.
How are heat, internal energy, and thermal energy
related to each other?
Thermal energy is the sensible and latent forms of internal energy, and it is referred to as heat in daily life.
Define mass and volume flow rates. How are they
related to each other?
Mass flow rate is the amount of mass flowing through a cross-section per unit time whereas the volume flow rate is the amount of volume flowing through a cross-section per unit time.
You are trying to understand how a reciprocating air
compressor (a piston-cylinder device) works. What system would you use? What type of system is this?
The system is taken as the air contained in the piston-cylinder device. This system is a closed or fixed mass system since no mass enters or leaves it.
An ideal gas at a given state expands to a fixed final
volume first at constant pressure and then at constant temperature.
For which case is the work done greater?
The area under the process curve, and thus the boundary work done, is greater in the constant pressure case.
Is it possible for a heat engine to operate without
rejecting any waste heat to a low-temperature reservoir?
Explain.
No. Such an engine violates the Kelvin-Planck statement of the second law of thermodynamics.
In what kind of pot will a given volume of water boil
at a higher temperature: a tall and narrow one or a short and wide one? Explain.
A given volume of water will boil at a higher temperature in a tall and narrow pot since the pressure at the bottom (and thus the corresponding saturation pressure) will be higher in that case.
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 since there are no offsetting entropy changes associated with reservoirs exchanging
heat with the system.
When is the energy crossing the boundaries of a
closed system heat and when is it work?
The form of energy that crosses the boundary of a closed system because of a temperature difference is heat; all other forms are work.
Consider a device with one inlet and one outlet. If the volume flow rates at the inlet and at the outlet are the same, is the flow through this device necessarily steady? Why?
No, a flow with the same volume flow rate at the inlet and the exit is not necessarily steady (unless the density is constant). To be steady, the mass flow rate through the device must remain constant.
A can of soft drink at room temperature is put into
the refrigerator so that it will cool. Would you model the can of soft drink as a closed system or as an open system? Explain.
A can of soft drink should be analyzed as a closed system since no mass is crossing the boundaries of the system.
Is the relation delta(u) =m*c_v*delta(T) restricted to constant volume processes only, or can it be used for any kind of process of an ideal gas?
It can be used for any kind of process of an ideal gas.
Does a heat engine that has a thermal efficiency of
100 percent necessarily violate (a) the first law and (b) the second law of thermodynamics? Explain.
(a) No, (b) Yes. According to the second law, no heat engine can have and efficiency of 100%.
In the absence of compressed liquid tables, how is
the specific volume of a compressed liquid at a given P and T determined?
The compressed liquid can be approximated as a saturated liquid at the given temperature.
What three different mechanisms can cause the
entropy of a control volume to change?
They are heat transfer, irreversibilities, and entropy transport with mass.
A room is heated by an iron that is left plugged
in. Is this a heat or work interaction? Take the entire room, including the iron, as the system.
It is a work interaction since the electrons are crossing the system boundary, thus doing electrical work.
What is flow energy? Do fluids at rest possess any
flow energy?
Flow energy or flow work is the energy needed to push a fluid into or out of a control volume. Fluids at rest do not possess any flow energy.
Define the isothermal, isobaric, and isochoric processes.
A process during which the temperature remains constant is called isothermal; a process during which the pressure remains constant is called isobaric; and a process during which the volume remains constant is called isochoric.
Is the relation delta(h) =m*c_p*delta(T) restricted to constant-pressure processes only, or can it be used for any kind of process of an ideal gas?
It can be used for any kind of process of an ideal gas.
What is the difference between a refrigerator and a
heat pump?
The difference between the two devices is one of purpose. The purpose of a refrigerator is to remove heat from a cold medium whereas the purpose of a heat pump is to supply heat to a warm medium.
Under what conditions is the ideal-gas assumption
suitable for real gases?
A gas can be treated as an ideal gas when it is at a high temperature or low pressure relative to its critical temperature
and pressure.
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. The entropy of an ideal gas depends on the pressure as well as the temperature.
Lifting a weight to a height of 20 m takes 20 s for
one crane and 10 s for another. Is there any difference in the amount of work done on the weight by each crane?
The work done is the same, but the power is different.
The kinetic energy of a fluid increases as it is accelerated in an adiabatic nozzle. Where does this energy come from?
The kinetic energy of a fluid increases at the expense of the internal energy as evidenced by a decrease in the fluid temperature.
Is the state of the air in an isolated room completely
specified by the temperature and the pressure? Explain.
Yes, because temperature and pressure are two independent properties and the air in an isolated room is a simple compressible system.
A fixed mass of an ideal gas is heated from 50 to
808C (a) at constant volume and (b) at constant pressure. For which case do you think the energy required will be greater?
Why?
For the constant pressure case. This is because the heat transfer to an ideal gas is m*cp*delta(T) at constant pressure, m*cv*delta(T) at constant volume, and cp is always greater than cv.
Define the coefficient of performance of a heat
pump in words. Can it be greater than unity?
The coefficient of performance of a heat pump represents the amount of heat supplied to the heated space for each unit of work supplied. It can be greater than unity.
Does hfg change with pressure? How?
Yes. It decreases with increasing pressure and becomes zero at the critical pressure.
An ideal gas undergoes a process between two
specified temperatures, first at constant pressure and then at constant volume. For which case will the ideal gas experience a larger entropy change? Explain.
The entropy change relations of an ideal gas simplify to delta(s) = cp ln(T2/T1) for a constant pressure process and delta(s) = cv ln(T2/T1) for a constant volume process. Noting that cp > cv, the entropy change will be larger for a constant pressure process.
On a hot summer day, a student turns his fan on
when he leaves his room in the morning. When he returns in the evening, will the room be warmer or cooler than the neighboring rooms? Why? Assume all the doors and windows are kept closed.
Warmer. Because energy is added to the room air in the form of electrical work.
Consider an air compressor operating steadily. How
would you compare the volume flow rates of the air at the compressor inlet and exit?
The volume flow rate at the compressor inlet will be greater than that at the compressor exit.
Consider two identical fans, one at sea level and the
other on top of a high mountain, running at identical speeds. How would you compare (a) the volume flow rates and (b) the mass flow rates of these two fans?
The density of air at sea level is higher than the density of air on top of a high mountain. Therefore, the volume flow rates of the two fans running at identical speeds will be the same, but the mass flow rate of the fan at sea level will be higher.
Is it possible to compress an ideal gas isothermally
in an adiabatic piston–cylinder device? Explain.
No, it isn't. This is because the first law relation Q - W = delta(U) reduces to W = 0 in this case since the system is adiabatic (Q = 0) and delta(U) = 0 for the isothermal processes of ideal gases. Therefore, this adiabatic system cannot receive any net work at constant temperature.
Is there any way to increase the efficiency of a
Carnot heat engine other than by increasing TH or decreasing TL?
No.