# Zeroth law of thermodynamics

1. Dec 16, 2015

### Jansen

This is not a homework problem. I want to relay a statement made by my professor. From his online script:

"0th law of thermodynamics: If two subsystems I,II are separately in thermal contact
with a third system, III, then they are in thermal equilibrium with each other."

He stated this in class and I protested because I have always heard that system I and II must seperately be in thermal equilibrium with system III. He basically told me it didn't matter it means the same. I don't see how though, if I have three systems, eg bricks. One is in a freezer, one is in an oven and brick III is at room temperature. I take all bricks out and put I and II in contact (here I am considering thermal contact as physical contact) with brick III but not each other. Brick I and brick II are most certainly not in thermal equilibrium.

Can you guys help me? What am I missing?

2. Dec 16, 2015

### Bystander

"Thermal contact" is/means "thermal contact."

3. Dec 16, 2015

### JeremyG

They will be, in time. As long as heat can flow from brick I to brick II (or vice versa) using brick III as a medium, they are considered to be in thermal contact. It does not mean physical contact, as u/Bystander pointed out.

4. Dec 16, 2015

### BvU

I think prof means equilibrium, not just contact. Perhaps if you bring it in a bit tactically (e.g. as a question) he can overcome his hesitation.
Your "Brick I and brick II are most certainly not in thermal equilibrium" I can only agree with -- until both equilibria ( I and III and III and II ) are established.

'All' our thermodynamics is equilibrium thermodynamics....

5. Dec 16, 2015

### Jansen

I will reply to every one with one message instead of individually.

First u/bystander, I do not like to say that thermal contact means thermal contact. This means thermal contact can mean anything which is not true, or that meaning of the word thermal contact is irrelevant which is also false. Two systems are said to be in thermal contact if they can exchange energy with each other through the process of heat. Now this does not necessarily mean that the two systems must have physical contact. For example, a cake is not in physical contact with the heating element of an oven but it is in thermal contact because the energy of the heating element is transfered into the cake by a process called heat via the medium of the air contained in the oven. I stated I am taking thermal contact here to mean physical contact because it was the method I chose to establish thermal contact in my example. Physical contact is, indeed, a meathod by which one system can exchange energy with another by the process of heat. If you don't believe me I propose an experiment. Turn on your stove and leave it for 5 minutes after 5 minutes establish physical contact with the stove with your hand. Then see if any energy is exchanged. :)

u/JeremyG Yes, I know that in time themal equilibrium will be reached. Probably as governed by Fourier's law. That is not what the law as my professor says. The quotation I posted is precicely what is in his script and precicely what he said in lecture. As I said before I chose physical contact as a method in which thermal contact could be established.

u/BvU I did ask him that. I said, "when you say system I and system II are in thermal contact with system III should they not also be in thermal equilibrium with system III?" His reply was to laugh and throw his hands up in the air and said, " this is not a precise definition, we haven't defined system, thermal contact or thermal equilibrium." My belief is that you can use words and words mean things and depending on how you string those words together it is a true statement, a false statement or a (sorta) true statement. The implication of saying that when two systems are in thermal contact (seperately) with another system then they are in thermal equilibrium means that thermal contact and thermal equilibrium are two distinct ideas. I guess, what he means is similiar to what you have said which is that they are in some sort of contact and some time has passed and all systems have equal energy so energy transfer by heat no longer occurs. However, when I asked weather that was true he did not answer but instead scoffed at me. What I assume that means is I am missing something or it is stupid to believe that the clarification of equilibrium being reached is necessary for the statement to be true. I am wondering if my interpretation of the two statements:

"0th law of thermodynamics: If two subsystems I,II are separately in thermal contact with a third system, III, then they are in thermal equilibrium with each other."

and

0th law of thermodynamics: If two subsystems I, II are separately in thermal equilibrium with a third system, III, then they are in thermal equilibrium with each other."

are, in fact, equivilent statments or not. If they are equivilant does that then mean thermal contact implies that there is some time associated with it? IE the time that it takes to acheive thermal equilibrium?

Edit: In the interest of completeness this is the script (p 83): http://home.uni-leipzig.de/tet/wp-content/uploads/2015/02/datei.pdf [Broken] From what I can tell of the mathematical description of what follows it appears that thermal contact and thermal equilibrium are used interchangeably.

Last edited by a moderator: May 7, 2017
6. Dec 16, 2015

### BvU

We distinguish equilibrium thermodynamics from something one could call non-equilibrium thermodynamics. You are learning equilibrium thermodynamics. There thermal contact means equilibrium.

Teacher should be able to explain this to you without having to resort to laughing and scoffing. It seems to me you two are in an all out fight. No one will be better off because of it; perhaps you can assume part of the responsiblilty for improvement of the relationship. To me you appear to be smart enough to do so.

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