Struggling With Internal Energy..

[Tom Booth]

It's been a long time since I worked on that PV diagram, trying to work out the temperatures and thus the isotherms, using the ideal gas law.

But as I recall, for every calculation, when I just increased the volume, the calculator would update the other values. The moles would go up automatically.

So even with that, I had to keep deleting the moles and paste in the fixed value. Over and over for every incremental change. It was very aggravating and time consuming and I still do not trust the results.

If a calculator does not even have a field for moles, I'm not sure how you would know what the calculator might be automatically updating to make things "easier".

[VincentG]

This is my go to calculator.

https://www.calculatorsoup.com/calculat ... as-law.php

[Tom Booth]

The reason I say I don't trust the results I spent so much time plotting is that the ideal gas law does not account for things like forced expansion due to momentum and cooling from work during expansion.

Take for example my previous calculation for Jack.

The calculator said the volume was 1 liter, though quite obviously that would not be the case, as the volume started at 1 liter and 600°K then expanded to 300°K.

At that temperature, with a fixed number of moles expanded until the pressure equalized with 1atm at BDC the volume should have been greater than the starting volume.

Likewise for every incremental change in the course of an adiabatic expansion with work output.

PV=nRT "assumes" the system is at rest not at the end of a forced expansion due to momentum, with additional cooling due to conversion of internal energy to work.

In other words, the plotted isotherms are not realistic. The temperature drop at (around/ & after) BDC logically should be colder.

I did take a brief look at the calculator Matt linked to.

I noticed immediately there was a note about how it was unnecessary to input moles as the calculator would compute moles for you.

Needless to say, I'm a bit skeptical, though admittedly I haven't tried it out or done any kind of thorough error checking.

I don't suppose it has a special button for Stirlings, or some way to specify moles must be held constant. ?

[Tom Booth]

This is my go to calculator.

https://www.calculatorsoup.com/calculat ... as-law.php

Funny,

That's the same one I used and recommended to Jack, and caught a lot of flack for it from "everybody" especially you VincentG.

viewtopic.php?p=23703#p23703

viewtopic.php?p=23729#p23729

So now your recommending the exact same calculator.

That's funny. Not Ha Ha funny. But peculiar funny.

[VincentG]

Tom you are a trip.

That's because he needed this combined gas law calculator, not PV=nRT.


https://www.thecalculator.co/others/Com ... r-716.html

[Tom Booth]

Tom you are a trip.

That's because he needed this combined gas law calculator, not PV=nRT.


https://www.thecalculator.co/others/Com ... r-716.html

Really?

How so?

From that page:

The combined gas law is almost identical to the ideal gas law: we only ignore the effect of the number of moles,

For a Stirling engine, you need to hold moles constant.

So why would you want to ignore the effect of the number of moles?

You really know how to put your foot in your mouth.

[VincentG]

Good lord where's the face palm emoji??

The ideal gas calculator is a state function.

The combined gas law calculator is for a path, which is exactly what Jack needed.

[matt brown]

The CalcTool "Combined Gas Law Calculator" is one of the best online calculators for several reasons:

(1) includes process field

(2) includes internal energy

(3) includes common gas table to nix guess/check whether mono or diatomic

(4) includes manual mole option via advanced mode button

The only issue I currently have with this calculator is that I don't think J value is working correct and I'll be testing this over the next few days.

Most of these online calculators are worthless since they're either extremely limited or bogus code akin clickbait where a bunch of clowns have all poached the same bad code. A simple omni calculator with moles took forever to 'wake up' and output n values. Until this sucker woke up, the n value sat frozen while I entered various other values.

If I ever have a Bill Gates bank account, I'll hire some guys to make an integrated app with calculator and PV...

[Tom Booth]

....
(4) includes manual mole option via advanced mode button

...

Ah, thanks.

As it turns out though, Jacks question didn't actually require any calculator.

BTW Matt, referring to "your" calculator as "more elaborate" was in no way intended as a criticism. Nothing wrong with more detail, more options, including an advanced mode.

I'm just curious what might be behind VincentG's scathing criticism over a simple recommendation that using an online calculator might be of some assistance to someone.

The fact that you put forth a basic ideal gas law calculator makes me question your actions here, or at least your understanding of the subject.

Sometimes the acceptable right answer is "I don't know".

A somewhat over the top, over reaction, don't you think ?

My "actions" were simply to point out the fact that a Stirling engine is a closed system with a fixed number of moles, so when using ANY online calculator, ideal gas law OR combined gas law, it is necessary to make sure the damn thing doesn't automatically change the moles, which is a problem I've run into with many many many of these calculators.

If you've never run into that particular issue, VincentG, I'd have to say you haven't had much experience. We're not aware of the problem and have probably been getting wrong answers.

Personally, after making baseless insinuations in regard to someone's actions or motives, If I had been guilty of such a thing, I'd feel I owed that person an apology.

[matt brown]

Tom - I saw Jack's question as relating to adiabatic expansion where few calculators would help, then Fool dropped in the 2 point buzz which had eluded me. The thing that lost me about Jack's question is that I read it it as free expansion and thought huh, only to find out later that's what he meant (and yeah, no calculator req'd).

[matt brown]

I have a screenshot of another excellent online calculator, but it's buried amongst 1000s of screenshots. Hopefully, I can find it over this long weekend (been procrastinating HD cleanup for many months).

The biggest problem with everything online is trust. What good is a calculator if you can't trust the answer ? Whenever I use one of these buggers, I always test it first by scaling from U=15J per 100cc and 300k 1 bar air, but this always makes me feel like a Chinese merchant using his abacus to check his computer.

[VincentG]

Tom, I have manually entered moles each and every time I do a gas calc to make predictions on outcomes. I have checked and rechecked over and over at times. I have had experimental results that reach over 75% of the ideal gas law.

You are a professional gaslighter, and after having just accused me of staging a fake and second screen name, you are asking me for an apology over questioning your wrong answer to a simple question that you did not read and understand.

[Tom Booth]

Tom, I have manually entered moles each and every time I do a gas calc to make predictions on outcomes. I have checked and rechecked over and over at times. I have had experimental results that reach over 75% of the ideal gas law.

You are a professional gaslighter, and after having just accused me of staging a fake and second screen name, you are asking me for an apology over questioning your wrong answer to a simple question that you did not read and understand.

Actually it was your response to Jack that threw me off when you said:

Jack if you heat 1 liter of 300k air to 600k and expand it adiabatically...

Having been busy with "fool" I assumed your answer was actually based on his question.

So you want to "correct" your own mistakes by rewriting the definition of thermodynamic terminology?

It would help to have some index of proper terms.

To me, for the context of this forum, a "free expansion" is air contained within a piston and cylinder, pushing outwards against a buffer pressure of 1ATM...

"Proper" terms?

Under A I suppose in VincentG's new index of terms we should include adiabatic:

Adiabatic; verb; when a gas is heated and allowed to expand against atmospheric pressure.

What other thermodynamic terms do you propose we rewrite for special use on the forum so you don't look stupid?

Gaslighting: when someone who actually knows what their talking about has a different opinion from, or disagrees with VincentG

[VincentG]

That was all in response to you asking Jack "how do you expand gas adiabatically?".

I am not intending to rewrite anything.

From previous conversation with Jack you should identify that he has a hands on approach, similar to me, you, Ian SC, Bumkin, etc.

Maybe the problem is that I understood what he was asking early on and saw your responses as nothing short of demeaning.

I see now that "free expansion" is by definition into a vacuum.

From conversation with Stroller it seemed to me that the baseline was expansion against a 1 bar buffer pressure.

[VincentG]

All I was trying to do is bring in some real world viewpoints.

When is "free expansion" ever occurring inside a Stirling engine?

How does worrying about free expansion help keep to isothermal or adiabatic ideals?