Thermo Problems Flashcards
An Elevator with a mass of 2500 Kg rests at a level 10 m above the base of an elevator shaft. It is raised to 100 m above the base of shaft, where the cable holding it breaks. The elevator falls freely to the base of the shaft and strikes a strong spring. The spring is designed to bring the elevator to rest and to hold the elevator at the position of maximum spring compression. Assuming the entire process to be frictionless, calculate:
a) The potential energy of the elevator in its position relative to the base of the shaft.
b) The potential energy of the elevator in its highest position relative to the base of the shaft.
The work done in raising the elevator.
c) The work done in raising the elevator.
d) The velocity and kinetic energy of the elevator just before it strikes the spring?
e) The potential energy of the compressed spring?
a. 245 000 J
b. 2 205 000 J
c. 2 450 000 J
d. 44.27 m/s
e. 0
How Many Degrees of Freedom of the ff. System?
a. Liquid water with its vapor
b. Liquid water mixture with vapor and nitrogen
c. Liquid solution of alcohol with its vapor.
a. 1
b. 2
c. 2
A horizontal piston/cylinder arrangement is placed in a constant-temperature bath. The piston slides in the cylinder with negligible friction, and an external force holds it in place against an initial gas pressure of 14bar. The initial gas volume is 0.03 m3. The external force on the piston is reduced gradually, and the gas expands isothermally as its volume doubles. (a) If the volume of the gas is related to its pressure so that the product PV is constant, what is the work done by the gas in moving the external force? (b) How much work would be done if the external force were suddenly reduced to half its initial value instead of being gradually reduced?
a. -29 112
b. -21 000
A gas is confined in a cylinder by a piston. The initial pressure of the gas is 7 bar(700kPa), and the volume is 0.10m^3. The piston is held in place by latches in the cylinder wall.The whole apparatus is placed in a total vacuum. What is the energy change of the apparatus if the restraining latches are removed so that the gas suddenly expands to double its initial volume, the piston striking other latches at the end of the process?
No Work And Q is done in the process.
W=0
Q=0
If the system described in the previous problem is repeated, not in a vaccuum but in atmospheric pressure of 101.3 Kpa, what is Q and W?
W=10.13kj
Q=-10.13kj
Calculate ∆U and ∆H for 1Kg water vaporized at constant temperature and
pressure ( 100○ C , 101.33 KPa) the specific volumes of liquid and vapor water at
these condition are 0.00104 and 1.673 m 3
kg -1
,for this change , heat in the
amount of 2,256.9 kJ is added to water.
Delta H= 2256.9 Kj
Delta U= 2087.5 Kj
Show that equation 2.29 reduces to equatiom 2.3?
d(mu) = Qdt + Wdt
Anwers: Delta U = Q + W
Acetone at 293.15 K and 1 bar has =1.487 x 10-3 K-1
, = 62 x 10-6 bar-1
, and V
= 1.287 x 10-3 m3 kg-1
a) Find the value of dP/dT
b) The pressure generated when acetone is heated at constant volume from
293.15 K and 1 bar to 303.15 K
c) The volume change when acetone is changed from 293.15 K and 1 bar to
273.15 K and 10 bar
a. 24 bar/k
b. 241 bar
c. -0.0038x10^-3 m3/kg
What pressure is generated when 1 kmol of methane is stored in a volume of 0.125 m3at 50°C (323.15 K)? Base calculation on each of the following:
(a) The ideal gas equation
(b) The Redlich/Kwong equation
(c) A generalized correlation
a. 214.9 bar
b. 189.65 bar
c. 191.3 bar
what is the initial temperature when heat in the amount of 422 MJ is added to 11.3 kmol of ammonia initially at 533.15 K in a steady flow process at aporoximately 1 atm?
T= 1250 K
methane gas at 550 K and 5 bar undergoes a reversible adiabatic expansion to 1 bar. Assuming methane to be an ideal gas at these conditions, find its final temperature.
T= 411.34K
A 40kg piece of steel (Cp = .5kJ kg^-1 K^-1) at temperature 450C is quenched in 150kg of oil (Cp = 2.5kJ kg^-1 K^-1) at temperature 25C. If there are no heat loses what iss the change of entropy of (a) the steel, (b) the oil. and (c) both together.
a. 16.33 KJ/Kg
b. 26.13 KJ/Kg
c. 9.80 KJ/Kg
In a steady-state flow process, 1 mol/s of air at 600 K and 1 atm is continuously mixed with 2 mol/s of air at 450K and 1 atm. The product stream is at 400K and 1 atm. Determine the rate of heat transfer and the rate of entropy generation for the process. Assume that air is an ideal gas with CP= (7/2)R, that the surroundings are at 300K, and that kinetic- and potential-energy changes are negligible.
Q= -8729.7 J/s or watts
Delta S = 10.446 J/K s
an inventor claims to have devised a process which takes in only saturated steam at 373.15 K (100 C)…. Mahabang problem
The Value of is smaller in magnitude the -2000 kj/kg claimed. One also notes that the inventor’s claim implies a negative rate of entropy generation.
Impossible
Propane gas at 20 bar and 400 K is throttled in a steady-state flow process to 1 bar. Estimate the final temperature of the propane and its entropy change.
T= 385 K
Delta s = 23.80 J/mol K
