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kalsh
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#1
Report Thread starter 8 years ago
#1
Im currently studying for my physics exam but im stuck on this question just wondering if anyone can help me. ive done part (i) need help on part (ii). i think its the 1/30 thats throwing me off

7
(a) A Diesel engine is a heat engine.
(i) Explain, briefly, what a heat engine is.
(ii) In a particular Diesel engine air is compressed to 1/30 of its initial volume in the compression stroke. If the initial temperature and pressure of the air are 30°C and 110kPa and the final temperature is 500°C what it the pressure at the end of the compressions stroke?

cheers for your help people!
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roar558
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P1V1/T1=P2V2/T2, from gas laws substitute to find a P

Edit-Don't forget T is in kelvins!!!
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roar558
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To check i got 8.42MPa
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kalsh
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Report Thread starter 8 years ago
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yep! see that what i thought it was but im not getting the answe of 8420kPa, so im still not sure what im doing wrong, also ive been converting the temperature into kelvins.
this is what im doing to get my answer:

P1V1/T1=P2V2/T2

(110 x 1/30)/303=(P2 x 1/30)/737

which then gives: ((110 x 1/30)/303) x 737 = P2 x 1/30

then i do: ((110 x 1/30)/303) x 737 divided by 1/30 to leave you with P2.

hmm i hope thats correct but yeh im getting a answer of 267.55 which i no where near 8420kPa
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kalsh
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Report Thread starter 8 years ago
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i just saw your message on your answer how did you get that. ok im like well annoyed now as im probably doing somthing stupidly wrong
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roar558
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#6
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(Original post by kalsh)
yep! see that what i thought it was but im not getting the answe of 8420kPa, so im still not sure what im doing wrong, also ive been converting the temperature into kelvins.
this is what im doing to get my answer:

P1V1/T1=P2V2/T2

(110 x 1/30)/303=(P2 x 1/30)/737

which then gives: ((110 x 1/30)/303) x 737 = P2 x 1/30

then i do: ((110 x 1/30)/303) x 737 divided by 1/30 to leave you with P2.

hmm i hope thats correct but yeh im getting a answer of 267.55 which i no where near 8420kPa
You went wrong here, 3 reasons
1.The original pressure was in kPa so the actual pressure is 110x10^3
2.500degreesC=773K
2.Say the initial volume=V then the final volume would be V/30
in other words it should be
(110x10^3x1)/303=(P2x1/30)/773
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kalsh
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Report Thread starter 8 years ago
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hey thanks alot i can clearly see how i messed that one up, think biggest mistake was not converting into pascal forst totally forgot that, but i now get it thanks again
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roar558
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you're welcome
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FranticMind
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#9
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Wait is this what applied physics looks like?

We were forced to do the Astrophysics option


Looks a bit too applied to be thermal part in unit 5
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