Work done BY vs. Work done ON  [ENDORSED]

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Emily Warda 2L
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Work done BY vs. Work done ON

Postby Emily Warda 2L » Fri Jan 12, 2018 1:28 am

As I read through the textbook, it is very clear that we must be able to differentiate between work done by a system and work done on a system. Typically, we are given examples of work done by a system such as gas in a cylinder expanding and pushing a piston but I am having a harder time visualizing the work done on a system. It is easy to think of energy leaving a system and entering its surroundings but can someone provide me with a way to visualize and comprehend work done on a system?
Furthermore, will these values (of work done by vs. work done on) always be opposite of one another as they are presented in figure 8.7 of the area below the curve of a graph?

Michelle Nguyen 2L
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Re: Work done BY vs. Work done ON

Postby Michelle Nguyen 2L » Fri Jan 12, 2018 7:56 am

I believe compressing a gas would be considered doing work on the gas. Since there are different ways we can do work on gases and different ways gases can do work, I don’t think work done by and work done on a gas are always equal; this is only the case for a net work done/on of 0.

Chem_Mod
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Re: Work done BY vs. Work done ON  [ENDORSED]

Postby Chem_Mod » Fri Jan 12, 2018 10:31 am

I will discuss this in detail in class Week 2.

Example of work done on a gas: compressing the air in a bicycle pump

Hammad Khan 2B
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Re: Work done BY vs. Work done ON

Postby Hammad Khan 2B » Fri Jan 12, 2018 11:08 am

From what I understand. when a system DOES work it loses energy, thus the work has to be negative as you lost it. While having work DONE to a system means that energy is being added to the system, thus the work is positive. Picture a car trying to move, it has a certain amount of gas (internal energy) and if it moves it has to expend energy or lose gas to move therefore it is doing work to move. But if another car were to push it then work is being done to the system and the car gains energy as it doesn't exert any work and it is positive.

AtreyiMitra2L
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Re: Work done BY vs. Work done ON

Postby AtreyiMitra2L » Fri Jan 12, 2018 11:10 am

This is how I think about it. Imagine work as the direction of the push. If the push is on the system (such as compression as the volume is becoming smaller), it would be work done on the system. Because work is done on the system, we know the work must be done by the surroundings. If the push is on the surroundings (such as expansion as the volume is now taking up the space of the surroundings), then it is work done on the surroundings. Because work is done on the surroundings, we know it must be done by the system.

Wenxin Fan 1J
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Re: Work done BY vs. Work done ON

Postby Wenxin Fan 1J » Fri Jan 12, 2018 11:48 am

Work done on the system results in a positive value for joules while work done by the system results in a loss of heat of the system (negative) and into the surroundings.


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