Thermodynamics Comment Questions (HW and Exams) & Equations
6-20) 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.
6-4) When is the flow through a control volume steady?
Flow through a control volume is steady when it involves no changes with time at any specified position.
6-19) What are the different mechanisms for transferring energy to or from a control volume?
For a control volume (open system) the energy can be transferred to or from a system in three forms: 1. Heat 2. work 3. mass flow Heat is the process of energy transfer from one body or system due to thermal contact, Energy transfer by heat can occur between objects by radiation, conduction, and convection.
1-6) What is the driving force for (a) heat transfer, (b) electric current, (c) fluid flow?
(a) The driving force for heat transfer is the temperature difference. (b) The driving force for electric current flow is the electric potential difference (voltage). (c) The driving force for fluid flow is the pressure difference.
Conversions
*Exam 1)* Area of a circle = pi/4 ⋅ d^2 or pi ⋅ r^2 1 m = 100 cm = 1000 mm 1 kPa = 1 kN/m^2 1 bar = 10^5 N/m^2 = 100 kPa g = 9.81 m/s^2 ρliquid water = 1000 kg/m^3 1 kJ = 1000 N⋅m = 1kPa ⋅ m^3 1 kN = 1000 kg ⋅ m/s^2 1 km = 1000 m ρair = 1.25 kg/m^3 1 J = 1 N⋅m 1 bar = 0.1 MPa 1 Pa = 1 N/m^2 1 kJ/kg = 1000 m^2/s^2 1 hr = 3600 s *Exam 2)* 1 m = 100 cm = 1000 mm 1 kPa = 1 kN/m^2 1 bar = 10^5 N/m^2 = 100 kPa g = 9.81 m/s^2 1 kJ/s = 1 kW 1 kJ = 1000 N⋅m = 1kPa ⋅ m^3 1 kN = 1000 kg ⋅ m/s^2 1 km = 1000 m 1 min = 60 s 1 J = 1 N⋅m 1 bar = 0.1 MPa 1 Pa = 1 N/m^2 1 kJ/kg = 1000 m^2/s^2 1 hr = 3600 s *Exam 3)* 1 m = 100 cm = 1000 mm 1 cm^2 = 10^-4 m^2 1 bar = 10^5 N/m^2 = 100 kPa = 0.1 MPa 1 min = 60 s 1 MW = 1000 kJ/s = 1000 kW 1 kJ/s = 1 kW 1 L = 10^-3 m^3 1 hr = 3600 s
Celsius to Kelvin
*T(K)* = T(°C) + 273
Kelvin to Rankine
*T(R)* = 1.8 ⋅ T(K)
Fahrenheit to Rankine
*T(R)* = T(°F) + 460
Fahrenheit to Celsius
*T(°C)* = 5/9 ⋅ (T(°F) - 32)
Celsius to Fahrenheit
*T(°F)* = 1.8 ⋅ T(°C) + 32
4-73) What is the difference between mass and molar mass? How are these two related?
*mass* *m* is simply the amount of matter; *molar mass* *M* is the mass of one mole in grams or the mass of one kmol in kilograms. These two are related to each other by m = N⋅M, where *N* is the number of moles.
4-22) In the absence of compressed liquid tables, how is the specific volume of a compressed liquid at a given *P* and *T* determined?
- Treat the compressed liquid at a given state as a saturated liquid, when the compressible liquid tables are not available - Obtain the value specific volume of the saturated liquid vf from "Properties of saturated water-Temperature table" (Table A-4), at the given temperature. - Thus, the specific volume of compressible liquid at given *P* and *T* (vP,T) is equal to the specific volume of saturated liquid at that temperature (vf @ T) (vP,T = vf @ T)
3-19) In what forms can energy cross the boundaries of a closed system?
- by heat - by work
5-4) Show that 1kPa ⋅ m^3 = 1 kJ.
1 kPa ⋅ m^3 = 1 k(N/m^2) ⋅ m^3 = 1 kN ⋅ m = 1 kJ
4-71) 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.
4-2) What is the difference between saturated liquid and compressed liquid?
A liquid that is about to vaporize is a *saturated liquid*; otherwise it is a *compressed liquid*.
6-26) How is the steady-flow system characterized?
A steady-flow system involves no changes with time anywhere within the system or at the system boundaries.
7-6) What is a thermal energy reservoir? Give some examples.
A thermal-energy reservoir is a body that can supply or absorb finite quantities of heat isothermally. Some examples are the oceans,lakes, and the atmosphere.
4-3) What is the difference between saturated vapor and super-heated vapor?
A vapor that is about to condense is *saturated vapor*; otherwise it is *super-heated vapor*.
3-21) What is an adiabatic process? What is an adiabatic system?
An *adiabatic process* is a process during which there is no heat transfer. A system that does not exchange any heat with its surroundings is an *adiabatic system*.
4-9) A househusband is cooking beef stew for his family in a pan that is (a) uncovered, (b) covered with a light lid, and (c) covered with a heavy lid. For which case will the cooking time be the shortest? Why?
Case (c) when the pan is covered with a heavy lid. Because the heavier the lid, the greater the pressure in the pan, and thus the greater the cooking temperature.
Energy Change of a System, ΔEsystem
Energy change = Energy at final state - Energy at initial state ΔEsystem = Efinal - Einital = E2 - E1 ΔE = ΔU + ΔKE + ΔPE
5-53) In the relation Δu = m⋅*cv*⋅ΔT, what is the correct unit of *cv* - kJ/kg ⋅ °C or kJ/kg ⋅ K?
It can be either. The difference in temperature in both K and °C scales is the same.
The Manometer
It is commonly used to measure small and moderate pressure differences. A manometer contains one or more fluids such as mercury, water, alcohol, or oil. P2 = Patm + ρgh P1 = Patm + ρ1gh1 + ρ2gh2 + ρ3gh3 + ... or P1 = Patm + γs1 ⋅ gh1 + γs2 ⋅ gh2 + γs3 ⋅ gh3 + ...
Kelvin-Planck Statement
It is impossible for any system to operate in a thermodynamic cycle and deliver a net amount of energy by work to its surroundings while receiving energy by heat transfer from a single thermal reservoir. QL ≠ 0
Clausis Statement
It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower-temperature body to a higher-temperature body. Wnet,in ≠ 0
T = Tsat or P = Psat
Mixture Water (Table A-4 or Table A-5) R-134a (Table A-11 or Table A-12)
4-4) Is there any difference between the intensive properties of saturated vapor at a given temperature and the vapor of a saturated mixture at the same temperature?
No.
7-12) Baseboard heaters are basically electric resistance heaters and are frequently used in space heating. A homeowner claims that her 5 year old baseboard heaters have a conversion efficiency of 100 percent. Is this claim in violation of any thermodynamic laws?
No. Because 100 percent of the work can be converted to heat. OR No. This is not the violation of any thermodynamic laws because any of the thermal laws do not say that conversion efficiency cannot be 100% . Therefore the claim made by the homeowner can be absolutely true without any violation to thermodynamic laws.
7-5) An experimentalist claims to have raised the temperature of a small amount of water to 150°C by transferring heat from high-pressure steam at 120°C. Is this a reasonable claim? Why? Assume no refrigerator or heat pump is used in the process.
No. Heat cannot flow from a low-temperature medium to a higher temperature medium.
7-9) 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 thermodynamics.
1-2) Why does a bicyclist pick up speed on a downhill road even when he is not pedaling? Does this violate the conservation of energy principle?
On a downhill road the potential energy of the bicyclist is being converted to kinetic energy, and thus the bicyclist picks up speed. There is no creation of energy, and thus no violation of the conservation of energy principle.
Vacuum Pressures
Pressures below atmospheric pressure Pvac = Patm - Pabs
4-70) Propane and Methane are commonly used for heating in the winter, and the leakage of these fuels, even for short periods, poses a fire danger for homes. Which gas leakage do you think poses a greater risk for fire? Explain.
Propane (molar mass = 44.1 kg/kmol) poses a greater fire danger than Methane (molar mass = 16 kg/kmol) since propane is heavier than air (molar mass = 29 kg/kmol), and it will settle near the floor. Methane, on the other hand, is lighter than air and thus it will rise and leak out.
3-3) What is the difference between the macroscopic and microscopic forms of energy?
The *macroscopic* forms of energy are those a system possesses as a whole with respect to some outside reference frame. The *microscopic* forms of energy, on the other hand, are those related to the molecular structure of a system and the degree of the molecular activity,and are independent of outside reference frames.
Specific Gravity (SG)
The ratio of the density of a substance to the density of some standard substance at a specified temperature (usually water at 4°C). SG = ρ/ρH2O
3-6) What is mechanical energy? How does it differ from thermal energy? What are the forms of mechanical energy of a fluid stream?
The *mechanical energy* is the form of energy that can be converted to mechanical work completely and directly by a mechanical device such as a propeller. It differs from thermal energy in that thermal energy cannot be converted to work directly and completely. The forms of mechanical energy of a fluid stream are kinetic, potential, and flow energies.
Absolute Pressure
The actual pressure at a given position. It is measured relative to absolute vacuum (i.e., absolute zero pressure).
6-3) Does the amount of mass entering a control volume have to be equal to the amount of mass leaving during an unsteady-flow process?
The amount of mass or energy entering a control volume does not have to be equal to the amount of mass or energy leaving during an unsteady-flow process.
Gage Pressure
The difference between the absolute pressure and the local atmospheric pressure. Most pressure-measuring devices are calibrated to read zero in the atmosphere, and so they indicate gage pressure. Pgage = Pabs - Patm
5-54) A fixed mass of an ideal gas is heated from 50 to 80 °C at a constant pressure of (a) 1 atm and (b) 3 atm. For which case do you think energy required will be greater? Why?
The energy required is m⋅*cp*⋅ΔT, which will be the same in both cases. This is because the *cp* of an ideal gas does not vary with pressure.
5-55) A fixed mass of an ideal gas is heated from 50 to 80 °C at a constant volume of (a) 1 m^3 and (b) 3 m^3. For which case do you think energy required will be greater? Why?
The energy required is m⋅*cp*⋅ΔT, which will be the same in both cases. This is because the *cp* of an ideal gas does not vary with volume.
Kinetic and potential energies
The familiar forms of mechanical energy.
Mechanical Energy
The form of energy that can be converted to mechanical work completely and directly by an ideal mechanical device such as an ideal turbine.
3-20) 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.
Energy Balance
The net change (increase or decrease) in the total energy of the system during a process is equal to the difference between the total energy entering and the total energy leaving the system during that process. (Total energy entering the system) - (Total Energy leaving the system) = (Change in the total energy of the system) Ein - Eout = ΔEsystem
7-31) What is the difference between a refrigerator and an air conditioner?
The purpose. the purpose of a refrigerator is to remove heat from a refrigerated space whereas the purpose of an air conditioner is to remove heat from a living space.
3-4) What is total energy? Identify the different forms of energy that constitute the total energy.
The sum of all forms of the energy a system possesses is called *total energy*. In the absence of magnetic, electrical and surface tension effects, the total energy of a system consists of the kinetic, potential, and internal energies.
4-5) If the pressure of a substance is increased during a boiling process, will the temperature also increase or will it remain constant? Why?
The temperature will also increase since the boiling or saturation temperature of a pure substance depends on pressure.
Specific Weight (γ)
The weight of a unit volume of a substance. γs = ρg
3-30) A car is accelerated from rest to 85 km/h in 10 s. Would the energy transferred to the car be different if it were accelerated to the same speed in 5 s?
The work done is the same, but the power is different.
1-3) 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. Is there any truth to his claim? Does this process violate any thermodynamic laws?
There is no truth to his claim. It violates the second law of thermodynamics.
7-2) Describe an imaginary process that satisfies the first law but violates the second law of thermodynamics.
Transferring 5 kWh of heat to an electric resistance wire and producing 5 kWh of electricity.
7-1) A mechanic claims to have developed a car engine that runs on water instead of gasoline. What is your response to this claim?
Water is not a fuel; thus the claim is false.
5-2) Is the boundary work associated with constant-volume systems always zero?
Yes.
7-7) Consider the process of baking potatoes in a conventional oven. Can the hot air in the oven be treated as a thermal energy reservoir? Explain.
Yes. Because the temperature of the oven remains constant no matter how much heat is transferred to the potatoes.
T < Tsat or P > Psat or v < vf
compressed liquid Water (Table A-7)
Mechanical Energy of a flowing fluid per unit mass
emech = P/ρ + V^2/2 + gz
Density (ρ)
mass per unit volume ρ = m/V (kg/m^3)
T > Tsat or P < Psat or v > vg
super-heated vapor Water (Table A-6) R-134a (Table A-13)
specific volume (v)
volume per unit mass v = V/m = 1/ρ (m^3/kg)
Stationary Systems
z1 = z2 → ΔPE = 0 V1 = V2 → ΔKE = 0 ΔE = ΔU
Rate of mechanical energy of a flowing fluid (Power)
Ėmech = ṁemech = ṁ(P/ρ + V^2/2 + gz)
Kinetic Energy (ΔKE)
ΔKE = 1/2 ⋅ m(V2^2 - V1^2) (V is velocity)
Potential Energy (ΔPE)
ΔPE = m ⋅ g(z2 - z1)
Internal Energy (ΔU)
ΔU = m(u2 - u1)
Mass Balance for Steady-flow Processess
Σṁin = Σṁout (kg/s)
