Conceptual Question on Elementary Thermodynamics

In summary, the conversation discusses the problem of external forces acting on a thermodynamical system and the resulting change in internal energy. It is highlighted that in the case of a container filled with gas and a movable piston, the gas particles colliding with the piston causes a change in internal energy, but there are no external forces acting on the system. This leads to a flaw in the thinking process, which is clarified through further questioning. It is concluded that the internal energy of the system does not change in this scenario.
  • #1
Ricky2357
23
0
1. The problem statement

Suppose we have a thermodynamical system whose state is modified by external forces. This results in a change of the internal energy of the system. If we designate by [tex]W_{ext}[/tex] the total work done during the process by the external forces acting on the particles of the system, then the conservation of energy requires that
[tex]\Delta U=W_{ext}[/tex]

In the classic example of the container filled with gas whose volume can change by means
of a movable piston, the particles of the gas collide with the surface of the piston causing it to move by a small distance [tex]\Delta x[/tex] (see figure attached).
Since no external forces act on this particular system we must have
[tex]W_{ext}=0[/tex] and thus [tex]\Delta U=0[/tex]. But this can not be since the internal energy of the system clearly changes.

2. Relevant Questions

Where is the flaw in my thinking?
 

Attachments

  • fig11.jpg
    fig11.jpg
    3.3 KB · Views: 465
Physics news on Phys.org
  • #2
Most of your reasoning is fine. Your attachment isn't visible yet, but if you've defined your system as the gas only, then the piston's mass and friction result in external forces. If you've defined your system to include the piston, then the internal energy of the system doesn't change; the gas cools slightly and the piston heats up slightly from friction. Does this help?
 
  • #3
If we define the system as the container, the gas and the piston and assume that there are not any forces of friction present, then no external forces act on this system, yet the internal energy of the system changes.Is this right?
 
  • #4
No, the internal energy doesn't change. How could it?

EDIT: To be more clear, the gas would be cooler and the piston would be moving.
 
Last edited:

1. What is thermodynamics?

Thermodynamics is the study of the relationship between heat, energy, and work. It involves understanding how energy is transferred and transformed within a system.

2. What are the laws of thermodynamics?

The laws of thermodynamics are fundamental principles that govern the behavior of energy in a system. They include the first law, which states that energy cannot be created or destroyed, only transferred or converted, and the second law, which states that the total entropy of a closed system will always increase over time.

3. What is the difference between heat and temperature?

Heat is a form of energy that is transferred from a hotter object to a cooler object, while temperature is a measure of the average kinetic energy of the particles in a substance. In other words, heat is the transfer of energy, while temperature is a measure of the amount of energy present.

4. How does thermodynamics relate to everyday life?

Thermodynamics has many practical applications in everyday life, such as the operation of engines, refrigerators, and air conditioners. It also plays a role in chemical reactions, weather patterns, and the behavior of materials.

5. What is the difference between an open and closed system in thermodynamics?

An open system can exchange both matter and energy with its surroundings, while a closed system can only exchange energy. This means that in an open system, matter can enter or leave, while in a closed system, the amount of matter remains constant.

Similar threads

  • Introductory Physics Homework Help
Replies
13
Views
716
Replies
8
Views
218
  • Introductory Physics Homework Help
Replies
1
Views
265
  • Introductory Physics Homework Help
Replies
28
Views
1K
  • Introductory Physics Homework Help
Replies
1
Views
841
Replies
7
Views
236
  • Introductory Physics Homework Help
Replies
10
Views
1K
  • Introductory Physics Homework Help
Replies
12
Views
52
  • Introductory Physics Homework Help
Replies
15
Views
300
  • Introductory Physics Homework Help
Replies
1
Views
1K
Back
Top