Results are out! Find what you need...fast. Get quick advice or join the chat
Hey there Sign in to join this conversationNew here? Join for free

Thermodynamics 2nd Law Confusion

Announcements Posted on
    • Thread Starter
    • 4 followers
    Offline

    ReputationRep:
    Ok, I'm looking over my notes and my lecturer has given an example of proving a process is irreversible. This is how it goes:

    Assume a sudden compression (called process 1-2). Then assume the same compression, but slow and reversible (called process 1-2R). One of the tests is to check that W_12 < W_12R (which is to be proved wrong), and another is to show that W_12 is not equal to W_12R. This would then imply that W_12 had to be greater than W_12R, implying irreversibility.

    I am confused with the first test, showing that W_12 < W_12R.

    He says, from the 1st law, Q-W=\Delta U. Therefore -W_{12}=\Delta U_{12} as the process is adiabatic. Then, similarly -W_{12R}=\Delta U_{12R}. The next step then says that this implies T_{12} &lt; T_{12R}. I do not understand this. Surely as U = C_vT, C_vT_{12}=-U_{12} (and similarly for the 12R process). Therefore, -C_vT_{12}&lt;-C_vT_{12}. Then when you remove the minus signs, the inequality sign flips, and you get the opposite to what my lecture proposed.

    Can someone tell me what I am doing wrong here?

    EDIT: Also, can someone explain why, if W_12 > W_12R this implies irreversibility?
    • 19 followers
    Offline

    ReputationRep:
    It's been a while since you posted and nobody's replied yet...maybe you should check out MarkedbyTeachers.com, TSR's sister site. It has the largest library of essays in the UK.

    They've got over 181,000+ coursework, essays, homeworks etc.. all written by GCSE, A Level, University and IB students across all topics. You get access either by publishing some of your own work, or paying £4.99 for a month's access. Both ways give you unlimited access to all of the essays.

    All their documents are submitted to Turnitin anti-plagiarism software, so it can't be misused, and the site's used by hundreds of thousands of UK teachers and students.

    What's more, you can take a look around the site and preview the work absolutely free. Click here to find out more...

Reply

Submit reply

Register

Thanks for posting! You just need to create an account in order to submit the post
  1. this can't be left blank
    that username has been taken, please choose another Forgotten your password?
  2. this can't be left blank
    this email is already registered. Forgotten your password?
  3. this can't be left blank

    6 characters or longer with both numbers and letters is safer

  4. this can't be left empty
    your full birthday is required
  1. By joining you agree to our Ts and Cs, privacy policy and site rules

  2. Slide to join now Processing…

Updated: May 21, 2012
New on TSR

Have a UCAS application question?

Post it in our dedicated forum

Article updates
Reputation gems:
You get these gems as you gain rep from other members for making good contributions and giving helpful advice.