## Mashy 2 years ago Second law and carnot engine Can someone explain how second law prevents carnot engine from having 100 percent efficiency?.. why the carnot engine HAS to reject some about heat to the sink??

1. JamesJ

Thermodynamics is a subtle subject. Can I recommend this little book: http://www.amazon.com/The-Laws-Thermodynamics-Short-Introduction/dp/0199572194 It is explained there. I could write out an answer myself, although to do it justice, it requires a few paragraphs I would just steal from that book anyway.

2. Mashy

oki thanks james :)

3. Mashy

Oh well is it because.. if the engine doesn't reject the heat, then the engine can never lower its temperature.. and so then the heat can never flow from the source .. and hence it wouldn't work?

4. christbot

The second law has to do with entropy, and establishing that time only travels one way. The third law though says you can never reach the ideal operating temperature of 0K. So, I think the previous answer is addressing the 3th law and the Carnot engine.

5. christbot

Oh sorry Mashy, that was you, not another answer, lol!

6. christbot

Also, because of the 2nd law, The max efficiency of an engine is $1-\frac{T _{L} }{T _{H} }$ , So that's specifically why the Carnot Engine is impossible. This is more along the lines of the answer you'd want.

7. Mashy

yea.. i see that.. but THEORETICALLY its possible to get TL = zero right??.. i mean thats how we define.. 0K.. THEORETICAL minimum attainable temperature!!!!.. So theoretically the max efficiency SHOULD be 1- 0 = 1.. !! thts what i don't get