Understanding the Laws of Thermodynamics as described by Feynman

[bryanso]

Hi, there is no other topics in my adventure in Feynman Lectures that makes me so loss in thoughts (https://www.feynmanlectures.caltech.edu/I_44.html). I seem to understand every sentence. But the whole thing is completely unintelligible. Let me start by asking one fundamental question. I am sure after that I would have a lot more. Thanks for support.

This question is about "magical assumptions". So Mr. Feynman assumes we have an ideal engine with no friction, I'm fine with that. Now he assumes there are two magical "heat pads" at temperature T1 and T2.

https://www.feynmanlectures.caltech.edu/img/FLP_I/f44-06/f44-06_tc_big.svgz

At point (c) he wrote,

The gas cylinder has now reached the temperature T2, so that if we put it on the heat pad at temperature T2 there will be no irreversible changes. Now we slowly compress the gas while it is in contact with the reservoir at T2, following the curve marked (3) (Fig. 44–5, Step 3). Because the cylinder is in contact with the reservoir, the temperature does not rise, but heat Q2 flows from the cylinder into the reservoir at the temperature T2.

• Everything else is assumed to be ideal... I would think if heat Q2 flows from the cylinder into the reservoir at temperature T2 then obviously the reservoir's temperature would start to increase too? Why can he make such an assumption that it doesn't?
• It seems like he's making assumptions here and there that just happen to work with his argument, quite arbitrarily.
• He said "we slowly compress the gas while it is in contact with the reservoir at T2". But later on, all the calculation seems to ignore there are "external work"... done by "we". Why is that left out?

Sorry I'm really very confused. May be another introductory text is necessary... Please advise.

 

[Chestermiller]

Summary:: Completely loss -- Laws of Thermodynamics in Feynman Lectures

Hi, there is no other topics in my adventure in Feynman Lectures that makes me so loss in thoughts (https://www.feynmanlectures.caltech.edu/I_44.html). I seem to understand every sentence. But the whole thing is completely unintelligible. Let me start by asking one fundamental question. I am sure after that I would have a lot more. Thanks for support.

This question is about "magical assumptions". So Mr. Feynman assumes we have an ideal engine with no friction, I'm fine with that. Now he assumes there are two magical "heat pads" at temperature T1 and T2.

https://www.feynmanlectures.caltech.edu/img/FLP_I/f44-06/f44-06_tc_big.svgz

At point (c) he wrote,

The gas cylinder has now reached the temperature T2, so that if we put it on the heat pad at temperature T2 there will be no irreversible changes. Now we slowly compress the gas while it is in contact with the reservoir at T2, following the curve marked (3) (Fig. 44–5, Step 3). Because the cylinder is in contact with the reservoir, the temperature does not rise, but heat Q2 flows from the cylinder into the reservoir at the temperature T2.

• Everything else is assumed to be ideal... I would think if heat Q2 flows from the cylinder into the reservoir at temperature T2 then obviously the reservoir's temperature would start to increase too? Why can he make such an assumption that it doesn't?

The reservoir is assumed to have a very high thermal inertia (mass times heat capacity) so that its temperature increase is negligible.

• He said "we slowly compress the gas while it is in contact with the reservoir at T2". But later on, all the calculation seems to ignore there are "external work"... done by "we". Why is that left out?

I don't understand this question. Can you please re-word?

 

[bryanso]

Thanks.

The second question is about the definition of Work done by this engine.

When I read that "The gas cylinder has now reached the temperature T2, so that if we put it on the heat pad at temperature T2 there will be no irreversible changes. Now we slowly compress the gas while it is in contact with the reservoir at T2, following the curve marked (3) (Fig. 44–5, Step 3). Because the cylinder is in contact with the reservoir, the temperature does not rise, but heat Q2 flows from the cylinder into the reservoir at the temperature T2."

To me it means:

Someone (a person's hand pressing it) presses the cylinder for some time, very slowly so nothing vigorous occurs. Q2 transfers from the cylinder to that reservoir.

Later Feynman states "Incidentally, it is easy to find out what the total amount of work is, because the work during any expansion is the pressure times the change in volume"

$W = \int p\, dV$

Is that all? Shouldn't we add to the above the work done by my hand when I press the cylinder?

 

[Chestermiller]

Thanks.

The second question is about the definition of Work done by this engine.

When I read that "The gas cylinder has now reached the temperature T2, so that if we put it on the heat pad at temperature T2 there will be no irreversible changes. Now we slowly compress the gas while it is in contact with the reservoir at T2, following the curve marked (3) (Fig. 44–5, Step 3). Because the cylinder is in contact with the reservoir, the temperature does not rise, but heat Q2 flows from the cylinder into the reservoir at the temperature T2."

To me it means:

Someone (a person's hand pressing it) presses the cylinder for some time, very slowly so nothing vigorous occurs. Q2 transfers from the cylinder to that reservoir.

This is correct.

Later Feynman states "Incidentally, it is easy to find out what the total amount of work is, because the work during any expansion is the pressure times the change in volume"

$W = \int p\, dV$

Is that all? Shouldn't we add to the above the work done by my hand when I press the cylinder?

First of all, the equation should have a minus sign (if this is the work done on the gas).

Secondly, your hand is doing work on the (massless, frictionless) piston, and the piston is doing an equal amount of work on the gas. So no net work is done on the piston, but work is done on the gas. This equation is just the force integrated over the displacement: Fds=PdV

If you really want to understand thermo, I do not recommend studying it from Feynman. I recommend Fundamentals of Engineering Thermodynamics by Moran et al. This is a wonderful book that is worth adding to your collection, but it is also available for download online.

 

[bryanso]

Thanks for the recommendation. I was just trying to complete all his lectures due to "completionism". I do sense it's too terse as a formal overview of this topic. Thanks again.