Ch 14.2 Help

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Re: Ch 14.2 Help

Post  Kathleen on Mon Dec 08, 2008 1:09 am

Well can someone with the alternative question 2 post their question again with the answer given so that we can work on it?

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question 2

Post  helpp on Mon Dec 08, 2008 1:59 am

the question that i posted is from maple ta not the cd so I dont have what the answer should be....

sorry... Sad

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Re: Ch 14.2 Help

Post  periwinkle on Mon Dec 08, 2008 2:46 am

I'm sorry, I got the questions and answers from the website that engelmann posted up on blackboard in the beginning of the semester to allow us to do 'practice' questions. So far, the questions have been the same so I didn't realize that #2 was a different on this quiz this time. Although, I still don't know how to do question 2, I'll repost the correct question.

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Question #2

Post  krolik on Mon Dec 08, 2008 10:00 am


An ideal gas changes its pressure and volume as shown in the PV-diagram starting at point A and returning to it. The pressure PAB is 2.503 atm and volume VB is 299.1 cm3. The pressure at C is 1 atmosphere. How many calories are exchanged with the environment? (Hint: What is the relationship between the work done and the area of the triangle shown?) Indicate with a positive (negative) sign whether heat is put into (taken out of) the gas.

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Question 2

Post  Jones on Mon Dec 08, 2008 11:20 am

If someone knows how to do this question please post an answer because many of us have this question and don't know how to do it. Thanks.

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Re: Ch 14.2 Help

Post  Guest01 on Mon Dec 08, 2008 12:50 pm

Are you able to post the correct answer? What formulas have you used so far that have proven to be incorrect/ineffective?

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other question 2 answer

Post  ryan240 on Mon Dec 08, 2008 4:21 pm

So I didn't have this question so I don't know if my logic is 100% accurate, but this is what I would do if I had this problem:

The question gives a hint: (Hint: What is the relationship between the work done and the area of the triangle shown?)
Since it is a closed cycle, the total change of the internal energy is zero. So by the first law of thermodynamics, Q=W.
The work done by the gas is equal to the area enclosed by the cycle from A-->B-->C-->A.
So Q=W=Area of triangle.

Area of a triangle=(1/2)*(base)*(height)
the base can be represented by the change in pressure from C-->A (press. of C=1atm, press. of A=2.503atm)
base=2.503atm-1atm=1.503atm*101300= 152253.9Pa

the height can be represented by the change in volume from B-->A (vol. of A=.000100m^3, vol. of B=.0002991m^3)
height=.0002991-.000100= 1.991E-4 m^3

W=A=(1/2)(152253.9)(1.991E-4)= 15.15687575J
convert to calories: 15.15687575J/4.187= 3.619984655calories
and it should be negative

Again, I'm not sure if this is the correct way to solve it but it's an idea.

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question #2 answer

Post  salou on Mon Dec 08, 2008 8:47 pm

it's correct, thanks ryan:)

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Q#2

Post  guest_01 on Mon Dec 08, 2008 10:24 pm

question 2 should be positive Smile

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Re: Ch 14.2 Help

Post  guest22 on Tue Dec 09, 2008 4:56 am

Sorry but 2 still doesn't work

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Question 2

Post  dl on Tue Dec 09, 2008 8:11 am

Yeah I have the same problem it still doesn't work positive or negative.

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question 2 is right!

Post  luvely on Tue Dec 09, 2008 6:16 pm

ryan240 posted the EXACT right answer but it's POSTIVE.

it worked for my on mapla ta. thank you!!

thanks to everyone else too ^^

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# 4

Post  lshenry on Tue Dec 09, 2008 9:13 pm

Q_H = 19.37 Kj * 1000= 19370 J
e= 58.81%
Q_L= 19370 J / .5881 = 32936.57541
W= Q_H - Q_L
W= 19370 - 32936.57541 = - 113566.57541

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# 4

Post  lshenry on Tue Dec 09, 2008 9:15 pm

4) A heat engine converts 58.81 % of the input heat into mechanical work. How much heat is released into the environment when the engine does 19.37 kJ of work (see sheet 16,17) ? Indicate with a negative (positive) sign whether the the efficiency of the engine is always (not always) less than 1.
Answer: -13566.575412344839
How to solve: solution above

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Question 2

Post  dl on Tue Dec 09, 2008 9:37 pm

Question 2 now finally worked, so the posted above solution is correct. I wonder why it didn't work the other ten times I tried though. God I HATE Maple TA.

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Re: Ch 14.2 Help

Post  The Show on Tue Dec 09, 2008 11:59 pm


The Show
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Re: Ch 14.2 Help

Post  The Show on Wed Dec 10, 2008 12:02 am

Okay, so I figured out this site doesn't allow links. So this is the new exccel sheet for 14-2 for people that just want answers. DON'T CONVERT ANYTHING!!!!!!!!!!!!!!!!!!!!!!! The questions are on different sheets on the bottom.

http://www. sendspace. com/ file/ i7vsk4

There are 4 spaces (1 after each period and 1 after each backslash) in the link provided so cope and paste and delete the spaces and go download. Good luck on the final Smile

BTW, 14-3 will be coming soon Smile

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different question2

Post  boo on Wed Dec 10, 2008 12:04 am

RYAN u were right!!!! but it is positive and it took me four times to finally get it right. THANK U FOR ALL UR HELP!!!!

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How do you plug in answers

Post  S on Wed Dec 10, 2008 1:55 am

First off, thank you for the excel file. i have a huge final this week, and could really use the break from maple ta. that said, how do you plug in the variables?

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Re: Ch 14.2 Help

Post  The Show on Wed Dec 10, 2008 1:59 am

I'm glad the excel is helpinng people Smile First off, all you do is plug in the numbers that it asks for in the "Number" section. Stuff like "first number" means just type in the first number that was given in your problem. It could sometimes say "kilojoules" which means to plug in the number that has kilojoules as its units (I'll make sure there's only one number with that unit in the problem).

There are NO UNIT CONVERSIONS. Just plug in the numbers that the problem gives and the answer will be given in the "answer" section.

Good luck on your final Smile This should alleviate some load off you guys.

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question 2

Post  student on Wed Dec 10, 2008 12:19 pm

my question 2 does not give the Wbc. it just says pressure = 1am so shouldnt that be:
1*101300 = Pressure
Vb-Vc = value-(1x10^-1) = deltaV
and then DeltaV*101300?
For that i would get a positive value then add it to the negative value found for Wab.
Wca = 0
then convert to J by diving by 4.187

I don't know why i keep getting this wrong.

Answer's + right?

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Re: Ch 14.2 Help

Post  S on Wed Dec 10, 2008 12:22 pm

It worked. Thank you, again for all the help-- sometime the Physics teachers don't understand that we have other classes to juggle-- other, hard, science classes.

Thanks again.

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THANK YOU!

Post  The Show on Wed Dec 10, 2008 2:19 pm

To: "The Show"

THANK YOU!

From: An appreciative student with a little less work to do Smile

The Show
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Clarification

Post  thankful on Wed Dec 10, 2008 2:24 pm

Apparently "The Show's #1 Fan" doesn't fit as a user name...so instead of a thank you post from "The Show's #1 Fan", maple ta has a way of being a pain sometimes and it looks like The Show is a loser and is writing himself or herself thank you notes haha Well, just to clarify, that is not the case Smile The Show is amazing and I just wanted to thank that person!!

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question 2 reply

Post  nightnig on Wed Dec 10, 2008 3:22 pm

ryan 240 is 100% correct if you dont trust him u'll be screwed. Thank you and 2 is positive.

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Re: Ch 14.2 Help

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