sign conventions for first law of thermo

[dudewheresmymd]

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Can someone clarify the sign conventions for first law of thermo?

Example 11-1: For a perfectly insulated system, what are the values of delta E and q if w = + 100J?

Answer is delta E= +100J and q= 0

But I got -100 J

Delta E= Q-Wsys = Delta E=-Wsys = -100J

Princeton Revieww used Delta E = q +w = 0 + 100 J = +100 J so would this be the work done on the system?


2) For an isothermal process, how does one derive Q < 0 for compression and Q > 0 for expansion using Delta E= Q-Wby system?

3) TPR says: "expanding gases cool. compressed gases warm." but for an isobaric process doesn't compression lead to lower temperatures and expansion lead to higher temperatures due to charles law?

 

[labqi]

Hey, I'll try to answer these best I can

W = work done by the gas, when work is greater than 0 (based on the formula I list below) we have an expansion. But in your situation, I am not sure what the W refers to, work done by the system or done to the system.
Adiabatic q=0, E = -W. If you could clear up the W, and maybe post the question.
It seems like the +100, would in essence make this a compression (w<0).

2. E = q-w(by the gas)
Isothermal q=w


Compression: w<0 (work is done to the gas). If w<0 then q<0.

3. Isobaric : Charles law
Compression: volume decreases temperature decrease.

I think the scenario in which expansion cools, is more a generalized version, and is true to adiabatic systems.

If someone could please correct me that would be great.

 

[dudewheresmymd]

Hey, I'll try to answer these best I can

W = work done by the gas, when work is greater than 0 (based on the formula I list below) we have an expansion. But in your situation, I am not sure what the W refers to, work done by the system or done to the system.
Adiabatic q=0, E = -W. If you could clear up the W, and maybe post the question.
It seems like the +100, would in essence make this a compression (w<0).

2. E = q-w(by the gas)
Isothermal q=w


Compression: w<0 (work is done to the gas). If w<0 then q<0.

3. Isobaric : Charles law
Compression: volume decreases temperature decrease.

I think the scenario in which expansion cools, is more a generalized version, and is true to adiabatic systems.

If someone could please correct me that would be great.

yeah i'm not sure it seems Princeton review is defining w as the work done on the system? they don't specify if the w is work done by the system or on the system which is confusing.

 

[dudewheresmymd]

if someone else can clarify the sign conventions

Another example from TBR: how do we know if the PdeltaV here is work done by the system or work done on the system (Q+W) or (Q-W) ?

 

[TXKnight]

It's been a while, but don't a positive pdeltav is the work done by the system?

 

[Blueprint MCAT Tutor]

There's like a 99.9% chance on your real MCAT that they will use normal english words to describe what's going on. They won't say "Q = + 22.2 kJ" but will instead specify that "22.2kJ of heat flows into the system".

Or there will be some other description (e.g. they talk about the piston moving upwards, expanding the gas) to make it clear whether work is being done by the system or on the system.

Sign conventions here are more confusing than they are helpful. Just make sure you understand that in an expanding gas, joules are flowing out of the system. When the environment is compressing the gas, it's pushing joules into the system. And heat flow is just joules flowing into or out of the piston by flowing directly through the walls of the container.

That basic description in normal english words will let you answer (or reason your way to the answer) on nearly any real MCAT problem.

Questions that directly test the sign conventions just aren't test-like and you shouldn't worry about them (note that this isn't true for all areas of science content - the test can and will expect you to have memorized the sign conventions for lenses and WILL expect you to use them when solving questions).

Hope this helps!