External Work done by a tank containing air

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Homework Help Overview

The problem involves a tank containing air that is stirred by a paddle wheel, with specific values for work input and heat transfer to the surroundings. The question seeks to determine the external work done by the system.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants discuss the implications of the work input and heat transfer, questioning whether the external work done is zero due to the fixed volume of the air in the tank. There is an exploration of the relationship between volume change and external work.

Discussion Status

The discussion is ongoing, with participants providing insights into the assumptions regarding the fixed volume of the air and its implications for external work. Some participants express clarity on the reasoning behind the answer being zero, while others continue to explore the underlying concepts.

Contextual Notes

There is no explicit mention of a change in volume of the air, leading to assumptions about the system's ability to do work. The problem context suggests constraints related to the fixed nature of the tank and the air contained within it.

Idris Sayyad
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Homework Statement



A tank containing air is stirred by a paddle wheel. The work input to the paddle wheel is 9000 kj and the heat transferred to the surroundings from the tank is 3000 kj. The external work done by the system is ?



Homework Equations



dU = Q-W



The Attempt at a Solution




Work input = -9000 kj
heat given to the surroundings = -3000 kj

The answer given is Zero, but no explanation has been given .
Is it zero because there is no movement of the boundary in this case?
 
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Idris Sayyad said:

Homework Statement



A tank containing air is stirred by a paddle wheel. The work input to the paddle wheel is 9000 kj and the heat transferred to the surroundings from the tank is 3000 kj. The external work done by the system is ?



Homework Equations



dU = Q-W



The Attempt at a Solution




Work input = -9000 kj
heat given to the surroundings = -3000 kj

The answer given is Zero, but no explanation has been given .
Is it zero because there is no movement of the boundary in this case?

Welcome to PF Idris Sayed!

The air is in a tank, which I assume is a fixed volume. Does the volume of air change? If not, how can the system do work?

AM
 
Thank you sir :)

There is no mention of the change of volume of air in the question. So if the answer is Zero, then volume is fixed has to be assumed I guess in this case.
 
Idris Sayyad said:
Thank you sir :)

There is no mention of the change of volume of air in the question. So if the answer is Zero, then volume is fixed has to be assumed I guess in this case.
I think it is the other way around. If the air is in a tank, its volume does not change. External work can only be done if there is a change in volume (i.e. a pressure applied over a change in volume = force x distance). Since there is no change in volume, no external work is done by the air.

AM
 
Andrew Mason said:
I think it is the other way around. If the air is in a tank, its volume does not change. External work can only be done if there is a change in volume (i.e. a pressure applied over a change in volume = force x distance). Since there is no change in volume, no external work is done by the air.

AM

Thank you sir , This cleared my doubt :)
 

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