Is Work Being Done on or by a Closed System? - Understanding Thermodynamics

In summary, the conversation discusses how to determine if work is being done on or by a closed thermodynamic system based on a diagram. It is explained that work done by a force is positive if the force is in the same direction as the displacement, and that the greater the pressure, the greater the magnitude of the work. The conversation also touches on how to explain the difference in magnitude between positive and negative work, and it is mentioned that the area underneath a section of the pV diagram represents the magnitude of the work.
  • #1
theintarnets
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Homework Statement


I am supposed to determine whether work is being done on or by a closed thermodynamic system from a diagram that's sort of like this, and provide justification for my answer:
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My problem is I have no idea how to tell if that system is doing work or if work is being done on it based on the graph. I heard that if it's going in the clockwise direction, that means the work is being done by the system, but that isn't a proper justification. Can someone help me understand this better?
 
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  • #2
Work done by a force is positive if the force is in the same direction as the displacement. So, if you have a gas, say, in a cylinder with a piston and the piston moves outward (increase volume) then you can see that the force of the gas on the piston is in the same direction that the piston moves. So, you see that this implies that if the volume increases, the system does positive work on the environment (which is equivalent to saying that the environment does negative work on the system). The greater the pressure, the greater the magnitude of the work.

So, if you look at your diagram, which leg of the "triangle" corresponds to positive work done by the system?

Which part corresponds to negative work done by the system? Which part to zero work?

Which is greater in absolute value, the positive work or the negative work?
 
  • #3
Thank you for the explanation. If I'm understanding this correctly, the segment from 2 to 3 represents positive work being done by the system, and the segment from 3 to 1 represents work being done on the system, which I guess is negative. I'm not sure about 1 to 2 though...
 
  • #4
theintarnets said:
Thank you for the explanation. If I'm understanding this correctly, the segment from 2 to 3 represents positive work being done by the system, and the segment from 3 to 1 represents work being done on the system, which I guess is negative. I'm not sure about 1 to 2 though...

Yes, that's right. Now, from 1 to 2 there is no change in volume. Since forces do work only when there is a displacement, there is no work done from 1 to 2.

Hopefully, you can see that if you have a closed loop ("cycle") on the diagram, then the net work done by the system will be positive for a clockwise cycle and negative for counterclockwise.
 
  • #5
But how do I explain why the positive work done here is greater than the negative work? I mean, aside from the fact that the hypotenuse is always the longest side of a triangle...
 
  • #6
theintarnets said:
But how do I explain why the positive work done here is greater than the negative work? I mean, aside from the fact that the hypotenuse is always the longest side of a triangle...

The pressure during the positive work is higher than the pressure for the negative work and the magnitude of the volume change is the same. Greater pressure (force) for the same displacement means greater work.

It is possible to show that the magnitude of the work for a section of the pV diagram is just the area underneath the section. So, the area underneath the line from 2 to 3 is greater than the area underneath the line from 3 to 1. "Area underneath" means area down to the V axis, as shown below.
 

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  • #7
Wow, thank you so much! I understand it a lot better now.
 

FAQ: Is Work Being Done on or by a Closed System? - Understanding Thermodynamics

What is the definition of work done on or by a system?

The work done on or by a system is a measure of the energy transfer that occurs during a process where the system undergoes a change in its state or configuration. It is a scalar quantity that is calculated by multiplying the magnitude of the force applied to the system by the distance over which the force is applied.

What is the difference between work done on a system and work done by a system?

Work done on a system refers to the energy transfer that occurs when an external force acts on the system and causes it to undergo a change in its state. On the other hand, work done by a system refers to the energy transfer that occurs when the system itself performs work on its surroundings, causing a change in the surroundings.

Can work be both done on and by a system at the same time?

No, work can only be done on a system or by a system at any given time. If an external force is acting on a system, then work is being done on the system. If the system is performing work on its surroundings, then work is being done by the system.

How is the sign of work determined for a system?

The sign of work for a system is determined by the direction of the force and the displacement of the system. If the force and displacement are in the same direction, then the work done is positive. If the force and displacement are in opposite directions, then the work done is negative.

What is the relationship between work done and the energy of a system?

Work done on a system increases the energy of the system, while work done by a system decreases its energy. This is due to the fact that work is a form of energy transfer, and according to the first law of thermodynamics, energy cannot be created or destroyed, only transferred between systems.

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