Saturday, April 01, 2006
HW #2 - P #5 - PVT Relationship Applied to an Automobile Tire
The pressure in an automobile tire depends on the temperature of the air in the tire. When the air temperature is 25°C, the pressure gage reads 210 kPa. If the volume of the tire is 0.025 m3, determine the pressure rise in the tire when the air temperature in the tire rises to 50°C. Also, determine the amount of air that must be bled off to restore pressure to its original value at this temperature. Assume atmospheric pressure is 100 kPa.
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7 comments:
Graham:
Your assumptions for part (a) are correct.
Your other statements are also correct EXCEPT that T does NOT double. Remember that you must use absolute temperature in the IG EOS. Kelvin, not Celsius !
For this problem do we need to convert the gage pressure to absolute pressure? That is add atmospheric pressure to it the gage pressure?
Anonymous @8:54 PM:
Yes, convert gage to absolute pressure before using ANY EOS.
Yes, Pabs = Pgage+Patm
Dr. B,
For the second part of the problem, I am having trouble figuring out how much air to bleed off. We correctly found part A and the new pressure. Originally, I thought about uisng the fact that it is a constant temperature process, and finding the change in volume and using the density of air to determine the mass, but this did not give us the correct result. Any help???
adam 11:33 PM:
Assume the T remains 50 degC as the air is bled off. Determine the moles left in the tire when P is restored to the original value, 210 kPa gage. Use this to find the mass of air that must be bled out of the tire.
I am getting an answer for part a on the magnitude of 300kPa, this is different from the answer on the homework that says 30kPa is this just a typo or did I do something wrong?
Katie @ 7:38 PM
I am guessing that you determined P2 ~ 300 kPa. But the problem asks for the CHANGE in P. DeltaP = Pfinal - Pinit or P2 - P1
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