Thermo final spring 2021
What is the weight, in N, of an object with a mass of 160 kg at a location where g = 9.6 m/s2?
1536
At sea level, the weight of 1 kg mass in SI units is 9.81 N. The weight of 2 lbm mass in English units is _____.
2Ibf
What is the specific volume of oxygen at 27.5 psia and 80°F? The gas constant of oxygen is R = 0.3353 psia·ft3/lbm·R.
6.58ft^3/Ibm
If the mass of an object is 7 lbm, what is its weight, in lbf, at a location where g = 32.0 ft/s2?
6.96Ibf
A 100-L container is filled with 1 kg of air at a temperature of 24.7°C. What is the pressure in the container? The gas constant of air is R = 0.287 kJ/kg·K.
854.4kpa
What is the COP of a Carnot refrigerator?
COPr=1/th+tl-1
A steady-flow system involves changes with time anywhere within the system or at the system boundaries.
F
Would you expect the temperature of air to drop as it undergoes a steady-flow throttling process?
Hehe no
How does the ideal gas refrigeration cycle differ from the Carnot refrigeration cycle?
In the ideal gas refrigeration cycle, the heat-absorption and the heat-rejection processes occur at constant pressure instead of at constant temperature.
Why is the Carnot cycle not a realistic model for steam power plants? (Check all that apply.)
Limiting the heat transfer processes to two-phase systems to maintain isothermal conditions severely limits the maximum temperature that can be used in the cycle, The turbine will have to handle steam with a high moisture content, which causes erosion and The turbine will have to handle steam with a low moisture content, which causes erosion.
Which of the following physical quantities are conserved during a process? (Check all that apply.)
Mass, energy, temp and pressure
How to achieve very low temperatures with gas refrigeration cycles?
Regeneration
An office worker claims that a cup of cold coffee on his table warmed up to 80°C by picking up energy from the surrounding air, which is at 25°C. Which thermodynamic law does this process violate?
Second Law of thermo
An office worker claims that a cup of cold coffee on his table warmed up to 80°C by picking up energy from the surrounding air, which is at 25°C. Which thermodynamic law does this process violate?
Second law of thermo
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.
T
Compressibility factor Z represents the deviation from ideal-gas behavior.
T
A fixed mass of an ideal gas is heated from 50 to 80°C at a constant volume and again at a constant pressure. For which case do you think the energy required will be greater?
The energy required will be greater for the case with a constant pressure.
A fixed mass of an ideal gas is heated from 50 to 80°C at a constant volume of (a) 1 m3 and (b) 3 m3. For which case do you think the energy required will be greater?
The energy required will be the same for both the cases.
A fixed mass of an ideal gas is heated from 50 to 80°C at a constant pressure of 1 atm and again at a constant pressure of 3 atm. For which case do you think the energy required will be greater?
The energy required will be the same for both the cases.Correct
The COP of vapor-compression refrigeration cycles improves when the refrigerant is subcooled at the condenser exit. To maximize this effect, which of the following can be done?
The refrigerant can be subcooled until the temperature of the sink.
A heat pump system is more cost effective in Miami than in New York, because of the low heating loads and high cooling loads at that location.
Tru
Volume flow rate is the volume flowing through a cross section per unit time.
True
How does the COP of a water-source heat pump system compare to that of an air-source system?
Water-source heat pumps have higher COPs than the air-source systems.
Consider an adiabatic turbine operating steadily. Does the work output of the turbine have to be equal to the decrease in the energy of the steam flowing through it?
Yah
In aircraft cooling, the atmospheric air is compressed by a compressor, cooled by the surrounding air, and expanded in a turbine.
aha true again
Choose the forms of energy that contribute to the internal energy of a system.
chemical energy, magnetic energy and nuclear energy.
What is the effect of lowering the condenser pressure on heat rejected?
decrease
Choose the forms of mechanical energy of a fluid stream.
energy flow
If the pressure of a substance is increased during a boiling process, the temperature decreases.
false
In electric heaters, electrical energy is converted to potential energy.
false
One pound-force is the force required to accelerate a mass of 32.174 lbm by 9.807 ft/s2.
false
The form of energy that crosses the boundary of a closed system because of a temperature difference is work; all other forms are because of heat.
false
The light-year unit has the dimension of mass.
false
The macroscopic forms of energy are those related to the molecular structure of a system and the degree of the molecular activity, and they are independent of outside reference frames.
false
The microscopic forms of energy are those a system possesses as a whole with respect to some outside reference frame.
false
Choose the forms in which energy can cross the boundaries of a closed system.
heat, work and mass
Which of the following equations can be obtained from hfg, hf, and hg?
hfg=hg-hf
What is the effect of lowering the condenser pressure on cycle efficiency?
increase
What is the effect of lowering the condenser pressure on heat supplied?
increase
What is the effect of lowering the condenser pressure on moisture content at turbine exit?
increase
What is the effect of lowering the condenser pressure on pump work input?
increase
What is the effect of lowering the condenser pressure on turbine work output?
increase
How can you verify if a road is pitched uphill or downhill?
level meter
What is the relationship between m and M?
m=N*M
Is it possible for a heat engine to operate without rejecting any waste heat to a low-temperature reservoir?
nada
Is the relation Δu = mcv, avgΔT restricted to constant-volume processes only?
nah
A heat pump is a device that absorbs energy from the cold outdoor air and transfers it to the warmer indoors. Is this a violation of the second law of thermodynamics?
no
A heat pump that is used to heat a house has a COP of 2.5. That is, the heat pump delivers 2.5 kWh of energy to the house for each 1 kWh of electricity it consumes. Is this a violation of the first law of thermodynamics?
no
A refrigerator has a COP of 1.5. That is, the refrigerator removes 1.5 kWh of energy from the refrigerated space for each 1 kWh of electricity it consumes. Is this a violation of the first law of thermodynamics?
no
Can we transfer 5 kWh of heat to an electric resistance wire and produce 6 kWh of electricity?
no
Does a heat engine that has a thermal efficiency of 100 percent necessarily violate the first law.
no
Does quality have any meaning in the superheated vapor region?
no
In a refrigerator, heat is transferred from a lower-temperature medium (the refrigerated space) to a higher-temperature one (the kitchen air). Is this a violation of the second law of thermodynamics?
no
In the absence of any friction and other irreversibilities, can a heat engine have an efficiency of 100 percent?
no
In the gas refrigeration cycle, can the turbine be replaced with an expansion valve, similar to the vapor-compression refrigeration cycle?
no
Is it possible for an electric resistance heater to consume 5 kWh of electricity and supply 6 kWh of heat to a room?
no
Is it possible to transfer 5 kWh of heat to an electric resistance wire and produce 5 kWh of electricity?
no
Is the energy required to heat air from 295 to 305 K the same as the energy required to heat it from 345 to 355 K? Assume the pressure remains constant in both cases.
no
A bicyclist picks up speed on a downhill road even when he is not pedaling. Does this violate the conservation of energy principle?
nope
Is it possible to compress an ideal gas isothermally in an adiabatic piston-cylinder device?
nope
The gas tank of a car is filled with a nozzle that discharges gasoline at a constant flow rate. Based on unit considerations of quantities, choose the correct relation for the filling time in terms of the volume V of the tank (in L) and the discharge rate of gasoline V˙V˙ (in L/s).
t=V/V˙
An adiabatic process is a process during which there is no heat transfer.
true
At supercritical pressures, there is no distinct phase change process, whereas, at subcritical pressures, there is always a distinct surface between the phases.
true
Clausius expression of the second law of thermodynamics states that "No device can transfer heat from a cold medium to a warm medium without requiring a heat or work input from the surroundings."
true
Consider a steady-flow heat exchanger involving two different fluid streams. Under the conditions of no heat and work interactions between the heat exchanger and the surrounding medium, the amount of heat lost by one fluid will be equal to the amount of heat gained by the other.
true
Consider a steady-flow mixing process. Under the conditions of no heat and work interactions between the mixing chamber and the surrounding medium, the energy transported into the control volume by the incoming streams will be equal to the energy transported out of it by the outgoing stream.
true
Heat engines are cyclic devices that receive heat from a source, convert some of it to work, and reject the rest to a sink.
true
In the absence of compressed liquid tables, the compressed liquid can be approximated as a saturated liquid at a given temperature.
true
Kelvin-Planck states that "no heat engine can exchange heat with a single reservoir and produce an equivalent amount of work."
true
Statistical thermodynamics is based on the average behavior of large groups of particles.
true
The coefficient of performance of a heat pump represents the amount of heat supplied to the heated space for each unit of work supplied.
true
The coefficient of performance of a refrigerator represents the amount of heat removed from the refrigerated space for each unit of work supplied.
true
The ideal gas refrigeration cycle is identical to the Brayton cycle, except it operates in the reversed direction.
true
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.
true
The purpose of a refrigerator is to remove heat from a refrigerated space, whereas the purpose of an air conditioner is remove heat from a living space.
true
The term hfg represents the amount of energy needed to vaporize a unit mass of saturated liquid at a specified temperature or pressure.
true
Volume, temperature, and pressure are not conserved during a process.
true
Work-producing devices that operate on reversible processes deliver the most work, and work-consuming devices that operate on reversible processes consume the least work.
true
A water-source heat pump extracts heat from water instead of air.
true dat
Does the area enclosed by the cycle on a T-s diagram represent the net work input for the reversed Carnot cycle?
yah
Will the temperature of air rise as it is compressed by an adiabatic compressor?
yeh
Does a heat engine that has a thermal efficiency of 100 percent necessarily violate the second law of thermodynamics?
yep
Is it true that it takes more energy to vaporize 1 kg of saturated liquid water at 100°C than it would at 120°C?
yep
Is it true that water boils at higher temperature at higher pressure?
yep
A propane tank is filled with a mixture of liquid and vapor propane. Can the contents of this tank be considered a pure substance?
yes
Does the amount of heat absorbed as 1 kg of saturated liquid water boils at 100°C have to be equal to the amount of heat released as 1 kg of saturated water vapor condenses at 100°C?
yes
When you define a system to determine the temperature rise created in a lake when a portion of its water is used to cool a nearby electrical power plant, should we account for all the forms of water entering and leaving the volume selected?
yes
During a throttling process, the temperature of a fluid drops from 30°C to -20°C. Can this process occur adiabatically?
yis