Thermodynamics Assignment Solutions: Pressure Concepts, Lecture notes of Thermodynamics

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ANSWER KEY IN

THERMODYNAMICS

1 (ASSIGNMENT 2)

Prepared by: Adam C. Macapili

Problem 1-34C. What is the difference between

gage pressure and absolute pressure?

Answer:

The pressure relative to the atmospheric pressure

is called the gage pressure, and the pressure

relative to an absolute vacuum is called absolute

pressure.

Problem 1-36C. Someone claims that the

absolute pressure in a liquid of constant density

doubles when the depth is doubled. Do you agree?

Explain.

Answer:

No, the absolute pressure in a liquid of constant

density does not double when the depth is

doubled. It is the gage pressure that doubles

when the depth is doubled.

Problem 1-40. A vacuum gage connected to a chamber reads 35 kPa at a location where the atmospheric pressure is 92 kPa. Determine the absolute pressure in the chamber. Analysis The absolute pressure in the chamber is determined from

The pressure difference corresponding to a differential height of 28 in. between the two arms of the manometer is

Then the absolute pressures in the tank for the two cases become: (a) The fluid level in the arm attached to the tank is higher (vacuum): (b) The fluid level in the arm attached to the tank is lower: Note that we can determine whether the pressure in a tank is above or below atmospheric pressure by simply observing the side of the manometer arm with the higher fluid level.

Note that jumping horizontally from one tube to the next and realizing that pressure remains the same in the same fluid simplifies the analysis greatly.

Problem 1-43. Determine the atmospheric pressure at a location where the barometric reading is 750 mm Hg. Take the density of mercury to be. Properties The density of mercury is given to be. Analysis The atmospheric pressure is determined directly from

Note that the gage pressure in a given fluid is proportional to depth.

Problem 1-45. The absolute pressure in water at a depth of 5 m is read to be 145 kPa. Determine (a) the local atmospheric pressure, and (b) the absolute pressure at a depth of 5 m in a liquid whose specific gravity is 0.85 at the same location. Assumptions The liquid and water are incompressible. Properties The specific gravity of the fluid is given to be

. We take the density of water to be. Then density of the liquid is obtained by multiplying its specific gravity by the density of water, Analysis (a) Knowing the absolute pressure, the atmospheric pressure can be determined from

Problem 1-46E. Show that. Analysis Noting that , , and , we have and