Need help with thermodynamics

In summary: To make sure the units are consistent, convert the given value to a value in cubic feet per pound.In summary, the conversation revolved around a student seeking help with two thermodynamics problems. The first problem involved determining the gage pressure given the atmospheric pressure, gas constant, and specific volume. The second problem required finding the volume of a rigid container after a gas expansion. The student was instructed to use the ideal gas law formula and convert units appropriately.
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Need help with thermodynamics!

im studying building systems engineering technology. yeh i know it's only a technology program at the cegep level but when I'm done with it, I'm going to do software engineering in university. anyways I'm having trouble to understand two problems in the book. if anyone can help me with this, it would be greatly appreciated.

1) Determine the gage pressure if the atmospheric pressure is 14.7 psia, the gas constant is 96 ft-lb/lb R, and the specific volume is 10 ft^3/lbm.
The answer is 31.9 psig

2) 70 lbs mass of gas are contained in a rigid container at 200 psia and 80 F. The gas is then expanded to fill a 2000 ft^3 volume at a pressure pf 20 psia and a temperature of 70 F. Determine the volume of the rigid container.
The answer is 203.7 ft^3.

Can someone show me the steps on how they got the answer because i already know the answer. i just need help on how to solve the problem.

for the first question, the formula is pv = mrt. we'll i know right away to find P = mRT. i would multiply 96ft-lb*700 * R (I don't know how to get that)/ Volume (I also don't know how to get that but i know it's something to do with the specific volume of 10 ft^3)

for the second question, i multiplied (70)(80)/200. and from there I'm lost and don't know what to do.
 
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  • #2
For #1, what's the definition of specific volume...?
For #2, you were taught another version of the ideal gas equation for dealing with that situation - find it in your notes/book. Hint: pv/mrt=constant.

Btw, we have a homework section...
 
  • #3
russ_watters said:
For #1, what's the definition of specific volume...?
For #2, you were taught another version of the ideal gas equation for dealing with that situation - find it in your notes/book. Hint: pv/mrt=constant.

Btw, we have a homework section...

specific volume is the volume per unit mass. (ft^3/slug)
for number 1ok i think when it says the specific volume is 10 ft^3/lbm, i did 1/(10ft^3/lbm) to get the density.
then i mutiplied (0.1)(96)(700) = 6720. but the answer is 31.9 psig so i know I'm really off even though i didn't even finished my answer.
 
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  • #4
As russ_watters pointed out, there is another variation of the ideal gas law. The one you posted PV = mRT is one variation, and you can rewrite the equation to solve this problem. It should also be in your textbook.

Look at the units that you have. You are given ft and the answer is in psig which is pounds per square inch gage. Also they are asking for the gage pressure, not absolute and you don't need to find density.

Also yes specific volume is the volume per unit mass. (ft^3/slug) in BG units. But the question is in EE units.
 

1. What is thermodynamics?

Thermodynamics is the branch of science that deals with the relationships between heat, energy, and work. It explains how energy is transferred between different forms and how it affects the properties of matter.

2. Why is thermodynamics important?

Thermodynamics is important because it helps us understand and predict the behavior of matter and energy in various systems, such as engines, power plants, and chemical reactions. It also has practical applications in fields like engineering, physics, and chemistry.

3. What are the laws of thermodynamics?

The first law states that energy cannot be created or destroyed, only transferred or converted from one form to another. The second law states that the total entropy of a system and its surroundings will always increase over time. The third law states that the entropy of a pure, perfect crystal at absolute zero temperature is zero.

4. How do you calculate thermodynamic quantities?

There are various equations and formulas used to calculate thermodynamic quantities, such as temperature, pressure, volume, and energy. Some common ones include the ideal gas law, the heat capacity formula, and the Clausius-Clapeyron equation.

5. What are some real-world applications of thermodynamics?

Thermodynamics has many real-world applications, including in the design and operation of engines, power plants, refrigeration systems, and chemical processes. It is also used in the study of weather and climate, and in the development of new materials and technologies.

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