Help with when work is done on/by

[m25]

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I always have problem figuring out when work is done by or on system/gas.
Here's how I understand it, pleasure correct me if they are wrong:

Work done by system/gas indicates expansion(volume increases,) and work is positive.
Work done on system/gas indicates compression(volume decrease) and work is negative.

Also, I am guessing that work done by "system" and work done by "gas" is the same thing, but is this assumption even correct? I was looking through past forums on this topic only to be confused even more. Please help!

 

[Cawolf]

It's confusing because engineers and chemists use different viewpoints.

There are really only two situations with PV work that require an understanding.

Gas Expansion: + Work done by gas (system) or - Work done on the gas (system)

Gas Compression: - Work done by the gas (system) or + Work done on the gas (system)

When a gas compresses it gains energy and when it expands it loses energy (in isobaric expansions).

The gas IS the system in PV work problems (generally).

 

[m25]

It's confusing because engineers and chemists use different viewpoints.

There are really only two situations with PV work that require an understanding.

Gas Expansion: + Work done by gas (system) or - Work done on the gas (system)

Gas Compression: - Work done by the gas (system) or + Work done on the gas (system)

When a gas compresses it gains energy and when it expands it loses energy (in isobaric expansions).

The gas IS the system in PV work problems (generally).

Oooh, I think I get it. There's also something about the equation U=Q-W or U=Q+W depending on how the W is defined, right? How is W defined in each case?

 

[Cawolf]

Right.

It is pretty straightforward, you should work through it to make sure you understand it.

U = energy of the system. This will increase when the gas does negative work or has work done on it (compression).

 

[m25]

Right.

It is pretty straightforward, you should work through it to make sure you understand it.

U = energy of the system. This will increase when the gas does negative work or has work done on it (compression).

Ok thanks, I will practice this!

 

[m25]

Right.

It is pretty straightforward, you should work through it to make sure you understand it.

U = energy of the system. This will increase when the gas does negative work or has work done on it (compression).

Actually, I just went through the work chapter on my kaplan, and this one question does not match with what you have said, and I was wondering if this is just a mistake on kaplan's part or I somehow did not understand correctly. Going by your rule, answer should be B but the answer stated in the book is A for some reason:

I am sorry for so many posts, I'm just really excited to be able to finally understand this whole work on/by naming convention that bugged me for years.

 

[Cawolf]

Everything I have ever learned was that negative work on a gas involved expansion.

Here is a simple link I found from BU that agrees with me (and disputes Kaplan).

http://physics.bu.edu/~redner/211-sp06/class-thermodynamics/thermo_work.html

I am also confused by the answer key you posted. It says, "When a gas is compressed, positive work is being done on the gas, so the value for the work done by the gas will be negative".

The answer choices clearly ask for the work done on the gas, which according to answer key is A, but the explanation lends itself to choosing B - the answer I would have picked.

I could be wrong of course, but I don't believe I am.

 

[m25]

Everything I have ever learned was that negative work on a gas involved expansion.

Here is a simple link I found from BU that agrees with me (and disputes Kaplan).

http://physics.bu.edu/~redner/211-sp06/class-thermodynamics/thermo_work.html

I am also confused by the answer key you posted. It says, "When a gas is compressed, positive work is being done on the gas, so the value for the work done by the gas will be negative".

The answer choices clearly ask for the work done on the gas, which according to answer key is A, but the explanation lends itself to choosing B - the answer I would have picked.

I could be wrong of course, but I don't believe I am.

Hmm, I guess it's one of those errata then, thank you!

 

[Cawolf]

@m25 I don't know if taking my word for it being an error is correct.

I can simply provide my take on the problem based on what I have learned and sources I can review. Maybe they have a place where errors are listed?

 

[m25]

@m25 I don't know if taking my word for it being an error is correct.

I can simply provide my take on the problem based on what I have learned and sources I can review. Maybe they have a place where errors are listed?

As far as I know kaplan doesn't have errata and I've seen mistakes in other parts of the book, so I am hoping this is is just another one of their errors.