Negative Work

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rachelle1K
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Joined: Sat Sep 07, 2019 12:16 am

Negative Work

Postby rachelle1K » Fri Feb 07, 2020 3:11 pm

In the homework problem 4B.7, the answer for work is -1626. However, I got 1626. When should I know to make work negative?

KBELTRAMI_1E
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Joined: Sat Jul 20, 2019 12:17 am

Re: Negative Work

Postby KBELTRAMI_1E » Fri Feb 07, 2020 3:55 pm

rachelle1K wrote:In the homework problem 4B.7, the answer for work is -1626. However, I got 1626. When should I know to make work negative?


I think you just need to know how much work is being done on the system, but I think that means that it was going from the Surroundings to the System vs the System to the Surroundings. In my notes it says that the Enthalpy can be negative if the energy is flowing from the system to the surroundings (exothermic), so I'm assuming the answer in the book says that because they wanted the Enthalpy value to be positive, so they just decided by negative 1.

Bella Townsend
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Joined: Wed Feb 20, 2019 12:18 am

Re: Negative Work

Postby Bella Townsend » Fri Feb 07, 2020 4:19 pm

a rule of thumb is:
Expansion is work done by the system
Compression is work done on the system

CynthiaLy4F
Posts: 103
Joined: Sat Jul 20, 2019 12:16 am

Re: Negative Work

Postby CynthiaLy4F » Fri Feb 07, 2020 4:31 pm

Work will be negative when the system is doing the work, such as through expansion. Work is positive when is being done on the system, such as through compression.

charleejohnson1L
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Re: Negative Work

Postby charleejohnson1L » Fri Feb 07, 2020 5:43 pm

I had the same problem :/ but my TA explained that the problem says that the work is done BY the system so that means it will be negative. It's just really important to make sure you pay attention to everything the problem is telling you before you start it, because that's how you forget the important details. I hope this helps! :)

Rebekah Alfred 1J
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Re: Negative Work

Postby Rebekah Alfred 1J » Fri Feb 07, 2020 6:55 pm

Here's a visual that shows an example of when work is negative:
Image

Leonardo Le Merle 1D
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Re: Negative Work

Postby Leonardo Le Merle 1D » Fri Feb 07, 2020 8:33 pm

The equations U=Q-W and U=Q+W are used interchangeably; instead of memorizing one or the other it helps to think about whether work is being done BY the system (in many contexts, expansion of a gas) or TO the system (compression).

205405339
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Joined: Thu Jul 11, 2019 12:16 am

Re: Negative Work

Postby 205405339 » Fri Feb 07, 2020 8:34 pm

just remember a system doing work isn't gaining work therefore its doing negative work

nicole-2B
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Re: Negative Work

Postby nicole-2B » Tue Feb 11, 2020 3:51 pm

CynthiaLy4F wrote:Work will be negative when the system is doing the work, such as through expansion. Work is positive when is being done on the system, such as through compression.

this helps a lot thank you! I have been confused about this

205389184
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Joined: Sat Sep 07, 2019 12:18 am

Re: Negative Work

Postby 205389184 » Sun Feb 16, 2020 10:10 pm

Since the system is doing work, your value should be negative whereas it would be positive if work was being done on the system

Keya Jonnalagadda 1A
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Re: Negative Work

Postby Keya Jonnalagadda 1A » Sun Feb 16, 2020 10:22 pm

You should use the context clues of the question to figure out if work is meant to be negative or not. If the system is doing work on the surroundings, such as expansion against a constant pressure, then it is right for your value to be negative.

Anushka Chauhan2B
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Re: Negative Work

Postby Anushka Chauhan2B » Sun Feb 16, 2020 11:14 pm

Work is being done by the system

Leyna Dang 2H
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Joined: Thu Jul 25, 2019 12:17 am

Re: Negative Work

Postby Leyna Dang 2H » Sun Feb 16, 2020 11:20 pm

When a system is doing the work, w is negative (ex. expansion), but when work is being done on a system, w is positive (ex. compression).

Natalie Benitez 1E
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Joined: Fri Aug 02, 2019 12:15 am

Re: Negative Work

Postby Natalie Benitez 1E » Mon Feb 17, 2020 2:19 pm

What does it mean when we have negative work?

WGaines_2E
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Joined: Mon Nov 18, 2019 12:18 am

Re: Negative Work

Postby WGaines_2E » Tue Mar 10, 2020 8:54 pm

work is typically negative when work is done ON the system

Ayushi2011
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Re: Negative Work

Postby Ayushi2011 » Sat Mar 14, 2020 3:59 pm

When a system does work, the value of work is negative.

Frank He 4G
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Re: Negative Work

Postby Frank He 4G » Sat Mar 14, 2020 4:59 pm

I think trying to intuitively understand what work is is also important. From what I understand, it's like the energy used or transferred to do something. So if you do work, then you're expending energy in order to make something happen. Thus if a system does (positive sign) work on the surroundings, then the system is using energy and thus it is lost, while the surroundings are gaining it.

Imagine if you have some marbles. You give them to someone else. So you would say that you gave some marbles. If you took them, then from a mathematical standpoint, it'd be the same as saying that you gave a negative amount of marbles. The same goes for work, where you say that work is done on or by a system. If the work done by the system is negative, then it's as if the system transferring energy to the surroundings was going in the opposite direction, so we'd interpret that as the system losing energy.

With that said, in order to make calculations ending with the right sign, you need to define whether the equation you're using is work done by or on the system. In this class, since we're using w = -PV, and P is the external pressure, then the we are saying that work is being done by the system. That is because the external pressure is pushing, or trying to do work, on the system, but since it has a negative sign added on, then we're saying that we are considering the opposite direction, where it's not work done on the system, but work done on the surroundings (by the system). Thus, the work done by the system is equal to the opposite (negative) of work done by the surroundings, or the external pressure times the change in volume.
Last edited by Frank He 4G on Sat Mar 14, 2020 5:16 pm, edited 1 time in total.

Amy Kumar 1I
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Re: Negative Work

Postby Amy Kumar 1I » Sat Mar 14, 2020 5:04 pm

The value for work would be negative if the system was doing work on the surroundings.

Alan Cornejo 1a
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Re: Negative Work

Postby Alan Cornejo 1a » Sun Mar 15, 2020 7:45 am

When work is done b the system against the surrounding the value is negative. When work is being done ON the system by the surrounding such as an increasing external pressure, then work is positive.


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