Learning undergraduate engineering thermodynamics might be less painful with a blog. I hope that students, faculty and interested observers will share their thoughts on the laws of thermodynamics, phase and chemical equilibrium and many related topics.
Wednesday, March 29, 2006
HW #1 - P#5 - Altitude and Barometric Pressure
Please post any questions about this problem as comments on this posting.
6 comments:
Anonymous
said...
Im having trouble with HW problem number 5. Ive converted everything to AE because that makes g/gsubc one. Then I know that P2=P1+density air(density merc dosent matter because all ther mercury does is tell you what the pressure is...I think)times height. Then I just try to solve for height and then convert the answer to meters. Any suggestions as to if my method is wrong or if im just making a unit mistake??
Let me see if I understand what you did. Did you convert the two pressures in mmHg to kPa ? You can consider this a unit conversion OR you can use the barometer equation and the density of mercury to do this: P2=P1+density*(g/gc)*h. When converting say h=755 mmHg to kPa, use P2 = zero in the barometer eqn to calculate P2 in kPa.
Anyway, once you have converted the two P's to kPa, you can apply the barometer eqn to the building to determine h using the density of air.
I am going to guess that your problem has to do with switching to AE units, but I could be wrong. Let me know how this works out for you.
In converting the pressures from units of mmHg to Pa, does 900mmHg equal 1.2atm (I went from mmHg to atm to Pa)? Also, I am getting a height of 291.87m...is this okay or should I have gotten an answer even closer to 300m?
900 mmHg = 1.184 atm = 120.0 kPa But I am not sure what this has to do with this problem.
My answer of 300m is the correct answer, but only to TWO significant figures. So, your answer may be just fine. Then again, it may not be ok because I am not sure how you got it !
I need a little more information about how you solved the problem in order to say for sure whether you did it right.
i just happen to like SI units also. i converted the pressures to kPa and used the barometer equation. with SI units you ignore gc and have to divide by gravity yes? i did this and got an answer of 288.22 meters. is this close enough?
6 comments:
Im having trouble with HW problem number 5. Ive converted everything to AE because that makes g/gsubc one. Then I know that P2=P1+density air(density merc dosent matter because all ther mercury does is tell you what the pressure is...I think)times height.
Then I just try to solve for height and then convert the answer to meters. Any suggestions as to if my method is wrong or if im just making a unit mistake??
Let me see if I understand what you did. Did you convert the two pressures in mmHg to kPa ? You can consider this a unit conversion OR you can use the barometer equation and the density of mercury to do this: P2=P1+density*(g/gc)*h. When converting say h=755 mmHg to kPa, use P2 = zero in the barometer eqn to calculate P2 in kPa.
Anyway, once you have converted the two P's to kPa, you can apply the barometer eqn to the building to determine h using the density of air.
I am going to guess that your problem has to do with switching to AE units, but I could be wrong. Let me know how this works out for you.
In converting the pressures from units of mmHg to Pa, does 900mmHg equal 1.2atm (I went from mmHg to atm to Pa)? Also, I am getting a height of 291.87m...is this okay or should I have gotten an answer even closer to 300m?
900 mmHg = 1.184 atm = 120.0 kPa
But I am not sure what this has to do with this problem.
My answer of 300m is the correct answer, but only to TWO significant figures. So, your answer may be just fine. Then again, it may not be ok because I am not sure how you got it !
I need a little more information about how you solved the problem in order to say for sure whether you did it right.
I hope this helps.
i just happen to like SI units also. i converted the pressures to kPa and used the barometer equation. with SI units you ignore gc and have to divide by gravity yes? i did this and got an answer of 288.22 meters. is this close enough?
Yes, 288.22 m is good enough. You don't want to have 5 significant figures in the answer you put a box around though, right ?
Post a Comment