Why can't thermal energy be converted completely to motion

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Discussion Overview

The discussion revolves around the limitations of converting thermal energy into motion, particularly in the context of thermodynamics and the second law. Participants explore the reasons behind the assertion that thermal energy cannot be completely converted into usable energy, examining both theoretical and practical implications.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant notes that thermal energy is described as the kinetic energy of particles at the microscopic level, questioning the apparent contradiction in its conversion to motion.
  • Another participant suggests that the inability to convert thermal energy entirely into usable energy is due to the random motion of particles, referencing the second law of thermodynamics and the inefficiency of engines.
  • A participant introduces the example of a hydroelectric dam, prompting a discussion on whether all stored energy can effectively drive a turbine.
  • A later reply emphasizes that if all thermal motion of a gas were converted to kinetic energy of a piston, it would result in the gas reaching absolute zero, which is deemed impossible due to quantum fluctuations.

Areas of Agreement / Disagreement

Participants express varying viewpoints on the conversion of thermal energy to motion, with some agreeing on the limitations imposed by thermodynamic principles while others explore specific examples and implications. The discussion remains unresolved regarding the nuances of these concepts.

Contextual Notes

Participants reference the second law of thermodynamics and the concept of absolute zero, indicating limitations in understanding the full implications of thermal energy conversion. There are also assumptions about the efficiency of engines and the nature of particle motion that are not fully explored.

osnarf
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In my thermo class I'm currently taking, the professor constantly says thermal energy cannot be completely converted into energy of motion. Just to be clear, I'm not arguing this point. I'm just wondering why we say that, when earlier in the course it said that thermal energy at the macroscopic level is the kinetic energy of the particles at the microscopic level. It seems to me that heat transfer is just transfer of kinetic energy... so what am I missing?
 
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I think what he means is that it can't be converted entirely into "usable" energy (e.g. to drive an engine). You can't get all the randomly moving particles to all of a sudden move in tandem to drive a machine because of the random nature of the particles. This is the second law of thermodynamics. There are not 100% efficient engines (where all the heat is converted to mechanical energy), there will always be excess heat left over moving into your cold reservoir from your hot reservoir.
 
Consider a hydroelectric dam. Can all of the stored energy drive a turbine? Why/why not?
 
I see what you're saying russ, thanks for the response, but matterwave hit it on the head. I was just making sure I wasn't missing something important. Thanks again.

SOLVED
 
If 100% of the thermal motion of a gas were converted to the kinetic energy of a piston, the gas would be left at absolute zero temperature, which is impossible. We can get very close to absolute zero, but never finally there because of quantum fluctuations.
 

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