Understanding the Laws of Thermodynamics as described by Feynman

In summary, Mr. Feynman argues that if we have an ideal engine with no friction, and we put a gas cylinder on two heat pads at different temperatures, then the temperature of the cylinder does not change, but heat Q2 flows from the cylinder into the reservoir at the temperature T2. The reservoir has a very high thermal inertia, so the temperature increase is negligible, and the work done by my hand when I press the cylinder is included in the calculation.
  • #1
bryanso
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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.
 
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  • #2
bryanso said:
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?
 
  • #3
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?
 
  • #4
bryanso said:
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.
 
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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.
 

1. What are the three laws of thermodynamics?

The three laws of thermodynamics are:

  1. The first law, also known as the law of conservation of energy, states that energy cannot be created or destroyed, only transferred or converted from one form to another.
  2. The second law, also known as the law of entropy, states that the total entropy (or disorder) of a closed system will always increase over time.
  3. The third law, also known as the law of absolute zero, states that the entropy of a pure crystalline substance at absolute zero temperature is zero.

2. How did Feynman contribute to our understanding of thermodynamics?

Richard Feynman, a Nobel Prize-winning physicist, made significant contributions to our understanding of thermodynamics through his work on quantum mechanics and statistical mechanics. He helped to develop a deeper understanding of the laws of thermodynamics and their application to various physical systems.

3. What is the relationship between thermodynamics and energy?

Thermodynamics is the study of energy and its transformations. The laws of thermodynamics describe how energy behaves in various systems, including heat, work, and other forms of energy. Thermodynamics also helps us understand how energy can be converted from one form to another.

4. How do the laws of thermodynamics apply to everyday life?

The laws of thermodynamics have many practical applications in our daily lives. For example, they help us understand how engines and refrigerators work, how heat is transferred in our homes, and how our bodies use energy. The laws of thermodynamics also play a crucial role in fields such as chemistry, biology, and environmental science.

5. What are some common misconceptions about thermodynamics?

One common misconception about thermodynamics is that the laws only apply to large-scale systems. In reality, they also apply to small-scale systems, such as individual molecules. Another misconception is that the laws of thermodynamics only apply to heat and energy, when in fact they also apply to other forms of energy, such as chemical and nuclear energy. Additionally, the third law of thermodynamics is often misunderstood to mean that absolute zero is unattainable, when in fact it is a theoretical limit that cannot be reached in practice.

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