Compression of a gas

[pbure9]

So I've been having some problems with this whole concept mainly bc I see that different sources have different formulas for this (relating heat and work)...

Anyway, my question is related to destroyer 47 in the chem. If a gas is compressed from 40 to 16 liters at 4 atm what is the work?

If a gas is compressed, that means that the system is doing the work on the gas (as opposed to expansion where the gas is doing the work), so shouldn't work be negative not positive?

Thanks guys

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[RetiredEngineer]

In thermodynamics negative work means that the system loses energy. Compression of gas increases the internal energy and therefore the work is positive.

 

[tsitneD]

Formula for work is W= -P (delta V)

so that mean when it is compressed, our final volume is less than initial so the change is negative giving us positive work.

 

[americanpierg]

In thermodynamics negative work means that the system loses energy. Compression of gas increases the internal energy and therefore the work is positive.

yes, thats why you dont mix chemistry and physics, negative means positive and positive means negative

 

[pbure9]

This is why I hate this topic.

 

[americanpierg]

In thermodynamics negative work means that the system loses energy. Compression of gas increases the internal energy and therefore the work is positive.

this is incorrect

 

[RetiredEngineer]

this is incorrect

So what is correct then?

 

[americanpierg]

So I've been having some problems with this whole concept mainly bc I see that different sources have different formulas for this (relating heat and work)...

Anyway, my question is related to destroyer 47 in the chem. If a gas is compressed from 40 to 16 liters at 4 atm what is the work?

If a gas is compressed, that means that the system is doing the work on the gas (as opposed to expansion where the gas is doing the work), so shouldn't work be negative not positive?

Thanks guys

i think where you're getting off track on this is that you're not realizing that the gas IS part of the system. when the gas is compressed, the surrounding is doing work on the system.

-work is if the system is doing work. +work is if something is doing work on the system

 

[americanpierg]

So what is correct then?

well its on the right track but using "internal energy" just totally changes things. the internal energy of the system can increase even if doing work if the heat transfer into the system is larger than the work done being done by the system.

energy lost by work + energy gain by heat = total internal energy

w=-pdeltav is more fitting

 

[pbure9]

Alright, I think this sorta summarizes where I was confused.
http://videolectures.net/mit560s08_bawendi_lec02/

from the 30 min mark he's talking about work. I think I was going by the "old" convention where expansion was +w, and compression was - w.

 

[RetiredEngineer]

My statement is correct..and yes the gas is part of the system and is doing work on the surrounding. The reason i used the word energy is because the question states that heat is released.

It's all about convention.

 

[americanpierg]

kk

 

[RetiredEngineer]

heat given off doesnt necessarily mean work is positive (cant just pick positive answer if you see "heat is given off"), unless the problem specifically says temperature stayed constant as well. constant temperature = constant kenetic energy = constant internal energy. internal energy of gas = kenetic energy of gas. delta U = q + w.

delta U = 0 = q = -w, THEN heat given off will always = positive work. otherwise, in a non isothermal expansion/compression, negative heat doesnt always equal positive work.

I edited that part out when i realized non isothermal conditions.