Why can't this process run forever?

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Discussion Overview

The discussion revolves around a cyclic process occurring in a system within an isolated box, questioning why this process cannot run indefinitely. Participants explore the implications of heat flow, system descriptions, and the nature of cyclic processes, with a focus on theoretical and conceptual aspects.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant describes a cyclic process where net heat flows into the system from the surroundings, questioning the feasibility of perpetual operation.
  • Multiple participants express confusion regarding the clarity of the problem statement, highlighting the need for precise definitions of the cycle and the system's boundaries.
  • Concerns are raised about whether the box is truly isolated or if heat can flow from outside, suggesting that the nature of the box's walls is critical to understanding the system.
  • Questions are posed about the meaning of heat being "consumed" by the cycle, indicating a need for clarification on thermodynamic principles involved.
  • Another participant suggests that the original question should be posted verbatim to facilitate better understanding and discussion.
  • There is a mention of reviewing course notes on cyclic systems and considering the role of entropy in the context of the discussion.

Areas of Agreement / Disagreement

Participants generally agree that the problem statement is vague and incomplete, leading to confusion. However, there is no consensus on the specifics of the cyclic process or the implications of the system's description.

Contextual Notes

The discussion highlights limitations in the problem description, including assumptions about the system's isolation and the nature of heat flow. Participants note the importance of precise terminology and definitions in thermodynamic contexts.

get_physical
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A cyclic cycle occurring in a system immersed in surrounding, all inside an isolated box. Each cycle, net heat flow into the system from isolated surroundings in the box and the box has an inlet which just enough heat flows to balance that consumed by the cycle. why can't this process run forever?

attempt:

I know that Q=-W in this case. But I don't know how to explain.
 
Last edited:
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The situation is incompletely described:

get_physical said:
A cyclic cycle
... is there any other kind?
What sort of cycle? What causes the cycle?

... occurring in a system immersed in surrounding,
... "in surrounding" what?

... all inside an isolated box.
The box is all by itself? Or do you mean an isolating box? i.e. a box with adiabatic walls?

Each cycle, net heat flow into the system from isolated surroundings in the box and the box has an inlet which just enough heat flows to balance that consumed by the cycle. why can't this process run forever?
Heat can flow from outside the box - so not adiabatic walls - so the box is not isolating??
How can heat energy get "consumed" by the cycle? (i.e. what does that mean?)

The place to start with this is to be more careful about describing the system under study.
Be pedantic about it.
 
Last edited:
Simon Bridge said:
The situation is incompletely described:

... is there any other kind?
What sort of cycle? What causes the cycle?

... "in surrounding" what?

The box is all by itself? Or do you mean an isolating box? i.e. a box with adiabatic walls?

Heat can flow from outside the box - so not adiabatic walls - so the box is not isolating??
How can heat energy get "consumed" by the cycle? (i.e. what does that mean?)

The place to start with this is to be more careful about describing the system under study.
Be pedantic about it.

I'm just as confused as you are. That's how the question was given to me by my teacher...
 
Simon Bridge said:
The situation is incompletely described:

... is there any other kind?
What sort of cycle? What causes the cycle?

... "in surrounding" what?

The box is all by itself? Or do you mean an isolating box? i.e. a box with adiabatic walls?

Heat can flow from outside the box - so not adiabatic walls - so the box is not isolating??
How can heat energy get "consumed" by the cycle? (i.e. what does that mean?)

The place to start with this is to be more careful about describing the system under study.
Be pedantic about it.

The question just says "consider a cyclic change occurring in a system that is immersed in surroundings, all inside an isolated box.
 
I think you'd better post the question verbatim - i.e. copy it out word for word.
Mind you - you won't always be given everything you need to know as part of the problem statement: some of it will be in the course notes and/or delivered in class.

But if it is really that vaguely worded, I don't think we can help you much - have you tried discussing it with your classmates? If everyone has the same problem then you can confront the teacher as a group.

Anyway there are still some things you can figure out, like:
What do you know about cyclic systems? (review course notes: have you covered heat engines?)
Consider how entropy factors into this?
 

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