# Is the 3rd law of thermodynamics necessary?

1. Mar 4, 2006

### Feynmanfan

Here's a question my thermodynamics teacher was unable to answer. I present it to you and see where the flaw of my argument is.

Is it possible to derive the 3rd law of thermodynamics from the 2nd law?

Carnot's theorem (no other machine than Carnot's has maximum efficiency) tells us that when the process is reversible (Carnot engine)
Q2/Q1=T2/T1 and therefore it is possible to define an absolute temperature. Now if there were a thermal reservoir at T2=0, it would be possible to have a more efficient engine than Carnot's. T2=0 -> Q2=0 and efficiency=1
This argument holds the other way around, it is impossible to have Q2=0 (2nd law) therefore T2=0 cannot be zero.

Our teacher argues that Einstein tackled this problem and demostrated that it has no physical sense to talk about ADIABATIC and ISOTHERMAL processes at the same time (I mean simultaneously). This, I do not understand.

What do you think? Why do we need a 3rd law? Thanks for your help.

2. Mar 5, 2006

### sampath_phy

Of course it is necessary
the 3rd law states that
" As the temperature tends to absolute zero the entropy also tends to zero. "

It must remembered that the entropy gives the direction of a process.
therefore, if entropy = o there is no thermodynamic process and the third law gives the relation temperature and entropy.
Sampath

3. Mar 6, 2006

### dextercioby

No, it isn't possible. Else it would be stated as a theorem in theoretical thermodynamics. It can be however proven as being valid for every thermodynamical system for which one derives the thermodynamics starting from (equilibirum) statistical mechanics...

Daniel.

4. Mar 7, 2006

I get it.
Sampath

5. Mar 8, 2006

I don't.

~Kitty