You are Here: Home

# Thermodynamics 2nd Law Confusion

Announcements Posted on
Talking about ISA/EMPA specifics is against our guidelines - read more here 28-04-2016
1. 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, . Therefore as the process is adiabatic. Then, similarly . The next step then says that this implies . I do not understand this. Surely as , (and similarly for the 12R process). Therefore, . 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?

## Register

Thanks for posting! You just need to create an account in order to submit the post
1. this can't be left blank
2. this can't be left blank
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
1. Oops, you need to agree to our Ts&Cs to register

Updated: May 21, 2012
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Today on TSR

### How to predict exam questions

No crystal ball required