Results 1 to 1 of 1

Thread: Thermodynamics: heat transfer?

  1. #1 Thermodynamics: heat transfer? 
    New Member
    Join Date
    Aug 2014
    Can you help me figuring this exercise on thermodynamics?

    "A cylinder with a movable piston contains a certain quantity of helium. With a very slow transformation - represented in a V-p graph as a straight line - it goes from state to state . It is then subjected to a costant-volume transformation followed by a costant-pressure one, returning to the initial state".

    I had a graph but couldn't post it. Anyway, from there it was pretty simple to find the work done (area of the triangle of the three states):
    It was a counterclock-wise transformation, hence the negative work.

    It then asks for , which I calculated from the ideal gas law. I didn't know the amount of substance, but I took that from state A.
    then .

    Finally, it wants the heat transfer with the outside during .
    At first, I wanted to use , but the specific heat gave me some doubts: which one should I use when the transformation is not particular (I'm referring to the fact that there are specifc specific heats when the transformation is with costant pressure/costant volume)?
    Then I thought I could use . I could calculate the difference in internal energy:

    Since it is work done on the gas, it is more internal energy, not less: . Also, we are only talking about the work done from A to B, which includes the area underlying the triangle: . But I can't figure out the sign of the Q: the work causes a compression, but I don't know how much. I mean: it could be so much that the temperature went much higher than 375K, arriving then at that level giving off heat; but it could also be that the work brought the temperature under 375K and that it arrived at that level absorbing heat.
    And anyway, whatever sign I use the answer , the book says) isn't right. And I'm stuck here.

    Where is my mistake?
    Thanks in advance!
    (If something isn't clear, please tell me)

    Reply With Quote  


Similar Threads

  1. Transfer of heat
    By kraman.157 in forum Physics
    Replies: 29
    Last Post: January 12th, 2014, 10:11 PM
  2. Heat transfer for air
    By ChaosD.Ace in forum Physics
    Replies: 4
    Last Post: September 26th, 2013, 06:27 AM
  3. Heat transfer gel??
    By rusty in forum Chemistry
    Replies: 5
    Last Post: November 3rd, 2009, 02:35 PM
    By fbrad2 in forum Chemistry
    Replies: 1
    Last Post: May 13th, 2009, 08:24 AM
  5. molecular heat transfer system
    By einsteintr in forum Chemistry
    Replies: 1
    Last Post: January 6th, 2009, 07:47 PM
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts