Heat and work not state functions

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ryan_macdougall_3B_2C
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Heat and work not state functions

Postby ryan_macdougall_3B_2C » Fri Jan 12, 2018 12:43 pm

Hi,
In the textbook it states that internal energy of a system is a state function because it is constant in its current state. Why then are work and heat not considered to be state functions?

(Ryan MacDougall 14B 2C)

Lourick Bustamante 1B
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Re: Heat and work not state functions

Postby Lourick Bustamante 1B » Fri Jan 12, 2018 12:48 pm

Heat and work are not state functions. Work can't be a state function because it is proportional to the distance an object is moved, which depends on the path used to go from the initial to the final state. Same with heat. I hope this is helpful!

Mika Sonnleitner 1A
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Re: Heat and work not state functions

Postby Mika Sonnleitner 1A » Fri Jan 12, 2018 12:59 pm

Heat and work both depend on the transfer of energy, regardless of the substance. This means that heat and work are not specific to a particular substance, i.e. they are not intrinsic properties so they cannot be state functions. Only properties like volume and pressure that are specific to the substance can be state functions. Work is proportional to the distance that the substance is moved, and heat depends on the transfer of energy between substances, so neither of them depend on their final and initial values.

Yixin Angela Wang 2H
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Joined: Thu Jul 27, 2017 3:00 am

Re: Heat and work not state functions

Postby Yixin Angela Wang 2H » Fri Jan 12, 2018 6:53 pm

Another way of thinking about it is by using the mountain climbing metaphor Dr. Lavelle went over in class. The change in altitude would be a state function but the distance traveled by foot of the hiker would not be, because the hiker's distance could vary based on the nature of the path he took. If he took the scenic route from point A to point B, he would travel a greater distance than he would have if he had traveled directly between points A and B. However, because A and B are unchanging start and end points, the change in altitude would be B minus A, irrespective of which path the hiker took.

Sirajbir Sodhi 2K
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Re: Heat and work not state functions

Postby Sirajbir Sodhi 2K » Fri Jan 12, 2018 9:30 pm

One more way to think about it:

There are lots of ways to change the energy of system, and those ways of adding energy to a system can be thought of as heat or work. Now, there are lots of combinations of ways you can add energy to a system to achieve a certain state. And depending on how the energy is transferred, the amount of work done or heat added will vary -- they are path dependent. Thus, they can't be state functions.

Naomi Jennings 2H
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Re: Heat and work not state functions

Postby Naomi Jennings 2H » Fri Jan 12, 2018 11:57 pm

Yet another way to think about it:

The difference between a state function and a path discussion is the difference between the distance between point a and b and the distance walked. If you start at Pauley and walk to Hedrick and then to CS50, the distance between your start and endpoint will only be the difference between Pauley and CS50. However, the distance you walked depends on the route you took- it's path dependent.
Since work and heat are asking about how things moved from one stage to another- the path they took- it matters how far they "walked" in between point a and b. Therefore, they aren't state functions, which are functions where all that matters is the distance between point a and point b.

Jennie Fox 1D
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Re: Heat and work not state functions

Postby Jennie Fox 1D » Sat Jan 13, 2018 12:21 pm

I found this online and found it to be very helpful.

"Heat and work, unlike temperature, pressure, and volume, are not intrinsic properties of a system. They have meaning only as they describe the transfer of energy into or out of a system.
"Heat & work are the forms of energy in transit. They appear only when there occurs any change in the state of system and the surroundings. They don't exist before or after the change of the state."

Hope this helps!

Hellen Truong 2J
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Re: Heat and work not state functions

Postby Hellen Truong 2J » Sun Jan 14, 2018 12:29 pm

Are there any other path functions we should know?


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