External Work done by a tank containing air

In summary, a tank containing air is stirred by a paddle wheel, with a work input of 9000 kj and heat transferred to the surroundings of 3000 kj. The external work done by the system is zero, as there is no change in volume of the air and therefore no external work can be done.
  • #1
Idris Sayyad
4
0

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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  • #2
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
 
  • #3
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.
 
  • #4
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
 
  • #5
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 :)
 

Related to External Work done by a tank containing air

1. What is external work done by a tank containing air?

The external work done by a tank containing air refers to the amount of energy required to compress the air inside the tank by an external force. This work is typically measured in joules (J) or foot-pounds (ft-lb) and is dependent on the pressure and volume of the air in the tank.

2. How is external work done by a tank containing air calculated?

The external work done by a tank containing air can be calculated using the formula W = P(V2-V1), where W is the work done, P is the pressure, V2 is the final volume of the air, and V1 is the initial volume of the air. This formula is based on the ideal gas law, which states that the work done is equal to the change in pressure multiplied by the change in volume.

3. What factors affect the external work done by a tank containing air?

The external work done by a tank containing air is affected by several factors including the initial and final pressure and volume of the air, the type and efficiency of the compressor, and the temperature and humidity of the air. Additionally, the external work done can also be affected by external factors such as friction and heat loss during compression.

4. What is the significance of external work done by a tank containing air?

The external work done by a tank containing air is an important concept in various industries such as manufacturing, transportation, and energy production. It helps determine the amount of energy required to compress air for use in various applications, such as powering engines or pneumatic tools. Understanding this concept is crucial in optimizing the efficiency and performance of air compression systems.

5. How can the external work done by a tank containing air be reduced?

There are several ways to reduce the external work done by a tank containing air. One method is to use a more efficient compressor, which can compress the air with less energy. Another approach is to reduce the initial pressure and volume of the air before compression, which can also decrease the amount of work required. Additionally, minimizing external factors such as friction and heat loss can also help reduce the external work done by the tank.

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