### Negative Work

Posted:

**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?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=75&t=58520

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Posted: **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?

Posted: **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.

Posted: **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

Expansion is work done by the system

Compression is work done on the system

Posted: **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.

Posted: **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! :)

Posted: **Fri Feb 07, 2020 6:55 pm**

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

Posted: **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).

Posted: **Fri Feb 07, 2020 8:34 pm**

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

Posted: **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

Posted: **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

Posted: **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.

Posted: **Sun Feb 16, 2020 11:14 pm**

Work is being done by the system

Posted: **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).

Posted: **Mon Feb 17, 2020 2:19 pm**

What does it mean when we have negative work?

Posted: **Tue Mar 10, 2020 8:54 pm**

work is typically negative when work is done ON the system

Posted: **Sat Mar 14, 2020 3:59 pm**

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

Posted: **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.

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.

Posted: **Sat Mar 14, 2020 5:04 pm**

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

Posted: **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.