# Ch 14.2 Help

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## Re: Ch 14.2 Help

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

Kathleen
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## question 2

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

sorry...

helpp
Guest

## Re: Ch 14.2 Help

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.

periwinkle

Posts : 22
Join date : 2008-09-17

## Question #2

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.

krolik
Guest

## Question 2

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.

Jones
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## Re: Ch 14.2 Help

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

Guest01

Posts : 133
Join date : 2008-09-19

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.

ryan240
Guest

it's correct, thanks ryan:)

salou
Guest

## Q#2

question 2 should be positive

guest_01
Guest

## Re: Ch 14.2 Help

Sorry but 2 still doesn't work

guest22
Guest

## Question 2

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

dl
Guest

## question 2 is right!

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

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

thanks to everyone else too ^^

luvely
Guest

## # 4

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

lshenry
Guest

## # 4

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.
How to solve: solution above

lshenry
Guest

## Question 2

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.

dl
Guest

The Show
Guest

## Re: Ch 14.2 Help

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

BTW, 14-3 will be coming soon

The Show
Guest

## different question2

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!!!!

boo
Guest

## How do you plug in answers

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?

S
Guest

## Re: Ch 14.2 Help

I'm glad the excel is helpinng people 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 This should alleviate some load off you guys.

The Show
Guest

## question 2

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.

student
Guest

## Re: Ch 14.2 Help

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.

S
Guest

## THANK YOU!

To: "The Show"

THANK YOU!

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

The Show
Guest

## Clarification

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 The Show is amazing and I just wanted to thank that person!!

thankful
Guest

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

nightnig
Guest