LECTURE UNIT 002Basic Properties of Thermodynamic Substances      Two general classifications:             1. Intensive pr...
Mass Density or simply “Density”, ρ       mass per unit volume.                              Units:      English Units:   ...
Temperature           - is used to indicate the amount of energy within the molecules of the substance.           - measur...
11.     A vacuum gage indicates that the pressure of air in a closed chamber is 0.2 bar (vacuum). The pressure of the     ...
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002 basic properties_of_thermodynamics

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002 basic properties_of_thermodynamics

  1. 1. LECTURE UNIT 002Basic Properties of Thermodynamic Substances Two general classifications: 1. Intensive property - is one that is independent of the mass of the substance Pressure, Density, Specific Volume, Specific Weight, Specific Gravity, Temperature, and Specific Internal Energy. 2. Extensive property - is one that is dependent on the magnitude of the mass of the substance. Mass, Weight, Volume and Energy. Mass, m - is the quantity of matter in the body. Weight, W - is the force exerted by gravity on a given mass. Volume, V - is the amount of space occupied by the matter. Energy, E - is the capacity of a body to produce physical effects external to the body. Pressure is the normal force exerted by the fluid per unit area of the surface. F Units: English Units: lbf /in2 P= A SI Units: N/ m2 or Pa Where: F = normal force exerted by the fluid, lbf, N A = area normal to the force, in2, m2 Other units of pressure: 1 bar = 105 Pa = 100 kPa = 0.1 Mpa = 0.9869 atm 1 torr = 1 mm Hg = 133.322 N/m2 Graphical Representation of Pressure: Pabs Pgage Pressure Patmosphere Pgage (negative) or Pvacuum Pabs Bourdon Gage - is commonly used to measure gage pressure. Manometer - can measure either gage or vacuum (negative) pressure. Pabs = Patm + Pgage Pabs = Patm - Pvacuum Diagram relating pressure and density in a manometer. Patm go P1 - Patm = ρ L gc Using gage pressure; Patm = 0 gage L P1 = γ L P1 Standard atmospheric or Barometer (at sea level) Eng’g units SI units 14.7 psia 101.325 kPaa 29.92 in Hg abs 760 mm Hgabs 1 atm “The first step to wisdom is silence; the second is listening.”
  2. 2. Mass Density or simply “Density”, ρ mass per unit volume. Units: English Units: lbm /ft3 ρ= m V SI Units: kg/ m3 1. Mass density of water at sea level. Eng’g units SI units lbm kgm ρw = 62.4 3 = 1000 ft m3 2. Mass density of water at ant temperature. 1 ρw = vf @ tw Where: tw = temperature of water. vf = specific volume at saturated liquid condition.Specific Volume, v volume per unit mass. V Units: English Units: ft3/lbm v= SI Units: m3/ m kg Other units of volume: 1 ft3 = 7.481 gal 1 gal = 8 pt = 3.7854 L 1 m3 = 1000 L 1 drum = 55 gal (petroleum, unrefined) 1 barrel = 42 gal (refined petroleum products and other liquids)Specific density or Specific weight or Unit weight force exerted by gravity per unit volume. Units: English Units: lbf /ft3 γ= F SI Units: N/ V m3 1 slug = 32.2 lbm = 14.594 kgmSpecific Gravity (SG) or Relative Density (RD) is the ratio of the mass density of a substance to the mass density of an equal volume of water at 4oC (temperature at its maximum density). ILLUSTRATION: Unknown substance H2O @ 4oC (39.3oF) ms ms CG Vs CG Vs Fs = Ws Fs = Ws For liquids and solids: ρs SGs = ρ w For gases: MWg SGg = MWair Where: MW = molecular weight of gas or air. “A leader is the one who climbs the tallest tree, surveys the entire situation, and yells, ‘Wrong jungle!”
  3. 3. Temperature - is used to indicate the amount of energy within the molecules of the substance. - measures the hotness or coldness of the substance. Arbitrary Scale, t o C = 5/9 (oF - 32) Absolute Value, T o R = oF + 460 K = oC + 273PROBLEM SET :1. An object has a mass of 20 kg, Determine its weight, in N, at a location where the acceleration of gravity is 9.78 m/s2. [ ]2. When an object of mass 1.5 kg is suspended from a spring, the spring is observed to stretch by 3 cm. The deflection of the spring is related linearly to the weight of the suspended mass. What is the proportionality constant, innewton per cm, if g = 9.81 m/s2. [ ]3. A spring compresses in length by 0.04 in. for every 1 lbf of applied force. Determine the deflection, in inches, of the spring caused by the weight of an object whose mass is 50 lb. The local acceleration of gravity is g = 31.4 ft/s2.[ ]4. An object whose mass is 3 kg is subjected to an applied upward force. The only other force acting on the object is the force of gravity. The net acceleration of the object is upward with a magnitude of 7 m/s2. The acceleration ofgravity is 9.81 m/s2. Determine the magnitude of the upward force, in N. [ ]5. An object whose mass is 7 lb is subjected to an applied upward force of 20 lbf. The only other force acting on the object is the force of gravity. Determine the net acceleration of the object, in ft/s2, assuming the acceleration of gravityis constant, g = 32.2 ft/s2. is the net acceleration upward or downward?6. The weight of an object on an orbiting space vehicle is measured to be 42 N based on an artificial gravitational acceleration of 6 m/s2. What is the weight of the object, in N, on earth, where g = 9.81 m/s2? [ ]7. A spherical balloon has a diameter of 10 ft. The average specific volume of the air inside is 15.1 ft3/lb. Determine the weight of the air, in lbf, at a location where g = 31.0 ft/s2. [ ]8. A vacuum gage at the intake duct to a fan gives a reading of 6 in. of water. The surrounding atmospheric pressure is 14.5 lbf/in2. Determine the absolute pressure inside the duct, in lbf/in2. The density of water is 62.39 lb/ft3, and the acceleration of gravity is 32.0 ft/s2. [ ]9. The absolute pressure inside a tank is 0.2 bar, and the surrounding atmospheric pressure is 101 kPa. What reading would a Bourdon gage mounted in the tank wall give, in kPa? Is this a or reading? [ ]10. The figure below shows a tank within a tank, each containing air. Pressure gage A is located inside tank B and reads 1.4 bar. The U-tube manometer connected to the tank B contains mercury. Using the data on the diagram,determine the absolute pressures inside tank A and tank B, each in bar. The atmospheric pressure surrounding tank Bis 101 kPa. The acceleration of gravity is g = 9.81 m/s2. [ ]1 “Everything comes to those who hustles while they wait.”
  4. 4. 11. A vacuum gage indicates that the pressure of air in a closed chamber is 0.2 bar (vacuum). The pressure of the surrounding atmosphere is equivalent to 750-mm column of mercury. The density of mercury is 13.59 g/cm3, and the acceleration of gravity is 9.81 m/s2. Determine the absolute pressure within the chamber in bar. [ ] 12. Determine the difference in pressure, in lbf/in2, between the top and the bottom of a 6-ft sewer pipe filled with rainwater. Assume the density of water is 62.2 lb/ft3 and g = 32.2 ft/s2. [ ] 13. A new absolute temperature scale is proposed. On this scale the ice point of water is 150oS and the steam point is 300oS. Determine the temperatures in oC that corresponds to 100o and 400oS, respectively. [ ] 14. A pressure cooker operates by cooking food at a higher pressure and temperature than is possible at atmospheric conditions. Steam is contained in the sealed pot, with a small vent hole in the middle of the cover, allowing steam to escape. The pressure is regulated by covering the vent hole with a small weight, which is displaced slightly by the escaping of steam. Atmospheric pressure is 100 kPa, the vent hole is area 7 mm2, and the pressure inside the should be 250 kPa. What is the mass of the weight? [ ]15. The vertical frictionless piston-cylinder shown below contains a gas at an unknown pressure. The piston has a mass of 10 kg and a cross-sectional area of 75 cm2. The spring exerts a downward force of 100 N on the piston, and theatmospheric pressure is 100 kPa. Determine the pressure of the gas. [ ] Spring Piston Gas16. A barometer is used to measure an airplane’s altitude by comparing the barometric pressure at a given flying altitude to that on the ground. Determine an airplane’s altitude if the pilot measures the barometric pressure at 27.55 in Hg absolute,the ground reports it to be 29.92 in Hg absolute, and the average air density if 0.077 lbm/ft3. g = 32.174 ft/sec2. [ ]17. A new temperature scale is desired with freezing of water at 0oX and boiling at atmospheric pressure occurring at 1000oX. Derive a conversion between degrees Celcius and degrees X. What is absolute zero in degrees X? [ ]18. A piece of experimental apparatus is located in an airplane flying at 20 000 m, where g = 9.75 m/s2. The air flow in this apparatus is measured by determining the pressure drop across an orifice with a mercury manometer. Themanometer level difference reading is 250 mm, and the local temperature is -10oC (density of mercury at -10oC is 13 3620 kg/m ). What is the pressure drop in kPa? [ ]19. It is estimated that the earth has a mean radius of 6.37x106 m, and the air comprising the atmosphere above it is 8.05x104 m high. Atmospheric air is generally composed of Nitrogen, Oxygen, and Argon with volumetric proportions of 78, 21,and 1 percent, and densities of 1.165, 1.33 and 1.783 kg/m3, respectively. Determine (a) the density ofatmospheric air; (b) its total mass. [ ]20. 100 g of water is mixed with alcohol (density = 790 kg/m3). Calculate (a) the specific gravity of the mixture (b) the specific volume of the solution. [ ] “You will never find time for anything. If you want time you must make it!”

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