The h Bar

user228700

8:09 AM

@JohnRennie: I need to ask about boiling again, one last time. When u finish ur work, dyou want to..?

John Rennie

You can ask now. I'm between tasks so I have a few minutes.

user228700

Oh, okay.

user228700

Well, I was just discussing with @Doraemonドラえもん about what the phrase "pressure due to the surrounding air" really means in the definition of the term boiling.

John Rennie

OK?

user228700

It turns out that even after having discussed all this many many times, my brain doesn't understand what really happens. If u could tell me why the vapour pressure has to be equal to the pressure due to the surrounding air for the liquid to boil...

John Rennie

8:13 AM

It involves free energy .. you might want to run away screaming at this point :-)

user228700

:-) No, no, I actually like free energy now, thanks to you!

John Rennie

OK. At equilibrium the free energy of the liquid is the same as the free energy of the vapour.

If we heat the liquid its free energy goes up because it's enthalpy increases.

user228700

Right.

John Rennie

That means molecules can reduce their free energy by turning from liquid into vapour, so some of the liquid evaporates.

user228700

Right.

John Rennie

8:16 AM

But as the liquid evaporates this increases the partial pressure of the vapour, and that increases the free enery of the vapour (mainly due to entropy decrease though the details don't matter).

Eventually we get back to an equilibrium where the liquid is a bit hotter and the vapour has a higher partial pressure.

So far so good?

user228700

Almost. Gimme a minute, please...

user228700

@JohnRennie Yep.

John Rennie

All you really know is that the free energy of the liquid increases with temperature and the free energy of the vapour increases with pressure. And at equilibrium the two free energies have to stay equal.

So if we heat the liquid the vapour pressure must go up to keep the free energies equal.

Is that OK so far, because the next step explains the boiling?

user228700

Again, gimme like, 45 seconds.

user228700

@JohnRennie What dyou mean must? You mean for the system to attain equilibrium again?

John Rennie

8:22 AM

> You mean for the system to attain equilibrium again?

Yes.

user228700

Okay. Do go on...

John Rennie

The system can only be in equilibrium if the free energies of the liquid and vapour are equal.

@Kaumudi.H the next step is easy ...

user228700

Yes, OK...

John Rennie

The vapour pressure can't get any bigger than the external pressure. Yes?

user228700

Aha, I'm going to stop u right there.

user228700

8:24 AM

Will u please tell me what this "external pressure" is?

John Rennie

If you're boiling the liquid in an open pan then the external pressure is atmospheric pressure.

user228700

And what if I'm boiling it in a closed vessel?

John Rennie

If you're boiling your lentils in a pressure cooker then it's whatever the pressure release valve on the pressure cooker sets it to.

user228700

(There are only two case, phew. Right..?)

user228700

@JohnRennie Huh? I don't understand...

John Rennie

8:27 AM

@Kaumudi.H A pressure cooker has a valve on it. If you watch a pressure cooker cooking you can normally see steam escaping through ther valve and hear the hissing.

user228700

Yes, right...

John Rennie

The valve opens at a set pressure e.g. 2 atmospheres, so it controls the pressure inside by stopping it get higher than (in this case) 2 atmospheres.

user228700

Right, but this pressure...it's caused by the vapours of the liquid, right?

John Rennie

@Kaumudi.H yes.

user228700

So what did mean when u said:

user228700

8:29 AM

> "The vapour pressure can't get any bigger than the external pressure."

user228700

The whole problem is that this pressure is caused by the vapours of the liquid itself. This fact is sort of causing a block in my brain.

John Rennie

In the case of the pressure cooker the external pressure is due to the force exerted by the valve. The valve has a spring (or sometimes just a weight) that exerts a force on the steam and holds it in the pressure cooker. That's how the pressure inside can be greater than atmospheric pressure.

user228700

Riight...

John Rennie

The external pressure means something in the environment is exerting a force/pressure on the vapour that is boiling off the liquid.

user228700

Right, but won't the vapour pressure always be equal to this external pressure? (As always, I'm very sorry)

John Rennie

8:33 AM

That might just be the surrounding atmosphere (in an open pan) or could be the valve and walls of the containing vessel if the system is inside some closed vessel.

@Kaumudi.H no. Remember that below boiling the gas above the liquid is a mixture of vapour and air.

user228700

Always? In every case there's some air too?

John Rennie

The sum of the partial pressures of the vapour and air is equal to the external pressure.

@Kaumudi.H well, you could have your liquid in a vacuum chamber where you pump out all the air. Is that what you are thinking of?

user228700

Yeah. Okay, there are only two cases and then two sub-cases or whatever. Either the vessel is open. Or it's inside a pressure cooker and there's some air too and then there's the vacuum chamber.

user228700

In every single case, I keep thinking "How can the external pressure be anything other than the liquid's vapour pressure at that temp.?" And then I realize I probably dunno what external pressure even means and...yeah, that's the dilemma.

John Rennie

Ok, yes.

Consider the open pan.

user228700

8:39 AM

Okay...

John Rennie

Hmm, I've realised I'm not entirely sure what you're asking.Let me have a think about this.

user228700

Dyou want me to rephrase my question?

John Rennie

OK, if you have a pan then the external pressure on the top of the liquid in the pan is 1 atm.Yes?

user228700

Yeah.

John Rennie

And the gas above the liquid is a mixture of air and vapour. Yes?

user228700

8:42 AM

Yep.

John Rennie

And that 1 atm external pressure is the sum of the partial pressure of the air and the partial pressure of the vapour. Yes?

user228700

Yeah.

John Rennie

So the external pressure isn't just the partial pressure of the vapour.

user228700

Not when there's air, no. That's clear now.

John Rennie

But the only way to stop air being present is to put the liquid ina sealed vacuum chamber. Yes?

user228700

8:45 AM

Yeah.

John Rennie

And in that case the steel walls of the vacuum chamber exert a force on the air ouitside to stop it crushing the vacuum chamber.

user228700

Okay. (How? God only knows :-| Not important now, tho)

John Rennie

So the pressure inside is the 1 atm pressure of the air outside minus the force the steel walls are exerting on the air to keep it outside the chamber.

Or put another way ...

user228700

Right...but to prevent the chamber from being crushed even slightly, those two forces must be equal, no?

John Rennie

The vapour pressure inside, plus the force the steel walls are exerting on the atmosphere, must add up to the 1 atm external pressure.

user228700

8:48 AM

Ohhhhhhhh, damn. Okay...

John Rennie

Yes. You have one inwards pressure, the atmosphere, and two outwards pressures, the vapour + the steel walls.

user228700

Okay. So, wait, what classifies as "external pressure" here? $$P_{atm} - F_{ExertedBySteelWalls}$$ ?

John Rennie

I guess it depends on exactly what you mean by external pressure. I suppose it would be the pressure exerted on the surface of the liquid, in which case yes it would be 1 atm minus the steel walls.

That is, if you had a pressure gauge floating on the liquid it would show a pressure reading of 1atm minus the steel walls.

user228700

I mean that pressure used in the definition for the term boiling point. That's all I really mean.

John Rennie

Remember that the term boiling point means the temperature at which when you pump heat into the liquid its temperature doesn't rise. Yes?

user228700

8:54 AM

Yeah.

John Rennie

Boiling points only exist in an open system.

If you have a liquid in a completely sealed vessel then there is no temperature at which when you pump more heat in the temperature doesn't rise.

user228700

::Gasps; eyes wide open::

user228700

@JohnRennie Why not?

John Rennie

Suppose you have water at 100C in a sealed vessel.

user228700

Okay.

John Rennie

8:57 AM

Now you pump some heat in, and some of the water boils. But because the steam is held in the sealed vessel the pressure inside goes up.

And as the pressure goes up the boiling point of the water rises.

So what happens is that as you pump more heat in the temperature of the water keeps rising.

The key feature of an open system is that the steam we boil off escapes into the surround atmosphere so the pressure stays at 1 atm no matter how fast we boil the water.

user228700

Yes, yes, yes, yes, thanks thanks thanks. God, this had been the confusion from the very beginning.

John Rennie

And because the pressure in an open system can't rise above 1 atm the boiling point is locked at 100C.

user228700

So, okay.

user228700

I finally understand. I think.

John Rennie

@Kaumudi.H this is why boiling points exist. As we pump heat in the free energy of the water increases

But the free enrgy of the vapour can't increase because its pressure can't increase.

So we can't have an equilibrium where the temperature of the water is above 100C.

user228700

9:02 AM

So when u said:

user228700

> "The vapour pressure can't get any bigger than the external pressure."

user228700

You meant how, in an open system, some of the vapour escapes and that's why, yes?

John Rennie

Yes, exactly.

user228700

@JohnRennie OhmGod, this makes so much sense.

John Rennie

:-)

Free energy is great :-)

3

user228700

9:05 AM

I starred that 'cause YAS, it's great!

user228700

Okay, one second. (Trying to figure it out for closed vessels)

user228700

There's no such max. pressure for a closed vessel? I mean, it'll explode or s'thing at one point, no?

John Rennie

@Kaumudi.H correct.

user228700

:-) Right.

user228700

Wait wait.

John Rennie

9:12 AM

Once it explodes it is of course no longer a sealed vessel.

user228700

So u increase the temp. but the pressure does increase and then equilibrium and then this keeps happening until it explodes?

John Rennie

Yes

user228700

Riight.

John Rennie

So there is no boiling point in the sense that it exists for an open vessel.

user228700

Oh my God, u have just helped me understand something that had been bugging me forever.

user228700

9:14 AM

_/\ _ _/\ _ _/\ _

John Rennie

It's quite a subtle point. I remember that it bugged me for ages as well.

user228700

Seriously, how dyou know this much? When was it that u understood this? Like, I need tips so that I can do well in college and all, and also so I don't have to ping u 24x7x365 :-P

John Rennie

These things can take a long time, sometimes many years, to work their way through ones skull. The main advantage I have is many years experience.

I'm afraid i don't know any short cuts, apart of course from asking someone old and wise ;-)

user228700

Hmm.

user228700

@JohnRennie :-D

John Rennie

9:17 AM

If I knew any shortcuts I would have taken them when I was a student and not waited to get grey hair.

Aakash Tomar

John, I guess you're a physicist?

John Rennie

@AakashTomar colloid scientist, though I'm retired now.

user228700

Okay, this may have been the last of my questions about pressure and crap, yaay! :-P I'm almost done with this friggin' chapter.

Aakash Tomar

Is a colloid scientist someone who studies colloidals?

G. Bergeron

@Kaumudi.H You'll be amazed at where you get with a physics degree.

Aakash Tomar

9:18 AM

Like, milk?

John Rennie

Like milk yes, though my work was more to do with synthetic polymers.

G. Bergeron

@JohnRennie Quite the patient educator. You seem to be the half official resident professional

user228700

Yeah, @JohnRennie: Do explain what "colloid scientist " means when u have the time. Even I don't understand what it is that u used to do :-)

John Rennie

@G.Bergeron I'm retired - I have the time to kill ...

user228700

@G.Bergeron Where I get?

G. Bergeron

9:20 AM

@JohnRennie Fair enough. Last week I was way too much on this website...

@Kaumudi.H university

Aakash Tomar

Oh, @G.Bergeron that's one of the, inner conflicts, I'm facing. I'm wondering if I should go for engineering or a physics degree.

G. Bergeron

@AakashTomar What do you want to do?

Aakash Tomar

Well, it wouldn't be conflict if I knew.

G. Bergeron

I mean, what is the basis of the dilemma?

Aakash Tomar

I like making stuff, but with my own hands. Not sitting in the cubicle and waiting for 5:00 P.M so I can get home. I don't like corporate jobs.

user228700

9:22 AM

@JohnRennie You know, I have a chap. on colloids that has always been very confusing and I think I might start that after this. Fancy a walk down memory-lane? :-P (Not now. In like, a week)

G. Bergeron

@AakashTomar I do that as a hobby because I like that too.

Aakash Tomar

Damn, I so wish I was Tony Stark or somethin.

John Rennie

@Kaumudi.H Wow, I certainly didn't study colloids at school, and I think they were only briefly menationed in my undergrad courses. It wasn't until I started work at Unilever that I really got into their study.

G. Bergeron

@AakashTomar Why?

Skyler

@G.Bergeron i kinda do wish engineering physics was a bit more a degree though, we really would kick ass with a little more rounding out

G. Bergeron

9:23 AM

@Skyler It is

Skyler

@G.Bergeron its rare though

i think 2 schools in CA (or at least prominent schools)

John Rennie

@AakashTomar well Tony Stark was an engineer ...

Aakash Tomar

Well, for one, he's a billionaire. He's got no bills to pay, so he can make anything he likes. Two, he's so, so smart. I mean, he's proficient at many subjects.

G. Bergeron

@Skyler You can round yourself out. To me, physics was the hardest to learn on my own

... so the choice was clear

@AakashTomar He's also fictious

user228700

@JohnRennie Ah, I see. Well, the stuff I've to learn is relatively easy :-)

Skyler

9:24 AM

@G.Bergeron this i agree with, and thats why I've been doing the learning engineering on my own with a physics degree

Aakash Tomar

Common. @G.Bergeron I bet you that there'll be people like him in less than 30 years from now.

Skyler

basically I think having the physics degree in the College of Engineering at schools would clear up most issues

cut down on the GEs somewhat, allow a bit more practical lower div years

then hit you with the upper div

G. Bergeron

@AakashTomar Billionaires, yes. Smart, yes. Proficient at many subject, yes. Make anything he likes? Absolutely not.

Aakash Tomar

I don't know. I'm a kid, so I got no clue.

John Rennie

James Dyson is the closest I know.

Maybe Elon Musk.

G. Bergeron

9:27 AM

Design and build a modern advanced piece of equipment from scratch and you'll know what I mean

Skyler

transferring into physics from a JC was silly compared to every other L&S degree. I had to take about 12 more classes than an english major

@G.Bergeron first build a modern basic piece

Aakash Tomar

John, elon musk is very humble.

G. Bergeron

@JohnRennie But he is the driver behind the project, not really the technical brain...

Aakash Tomar

He isn't a nick like stark.

Skyler

thats hard enough sometimes

@G.Bergeron he does ALOT of technical reading and work

Aakash Tomar

9:28 AM

But he doesn't actually make stuff Skyler.

He 'supervises'.

G. Bergeron

@Skyler Yes, but I don't think he's gonna be proposing the next warpdrive engine either

John Rennie

@AakashTomar There are no rich, successful engineers who aren't extremely hard working and committed!!

2

Aakash Tomar

I know John, of course musk is really hardworking. But the thing is, he didn't do electric cars.

Skyler

@AakashTomar you'll generally always then either work as a prototyper, creating something and handing it off to a business who hired you to do it, or making simple smaller systems

you cant lead a MASSIVE project and do all the details

G. Bergeron

That's the problem with character like Stark, like I would believe one guy has the deepest understanding of physics at all level, to be able to build doom machines, while also being on the cutting edge of AI, mechanical engineering, leading a company and on to of all a superhero

Aakash Tomar

9:30 AM

Marc and Martin are the electric car guys.

G. Bergeron

@Skyler My point exactly

Skyler

@G.Bergeron well thats why in most of his origin stories they have him being born with his entire body kind all being a distributed brain/nervous thinking system thingy

G. Bergeron

You can dabble in a lot of things but yeah

Aakash Tomar

Yeah, that's the thing @G.Bergeron. That is too much work. I mean, if an average guy like me sets out to become Stark, it'll take me decades. By then, I'll be 40ish and broke because I didn't work a job and all I did was study.

G. Bergeron

@Skyler Well, fine but then...

@AakashTomar But then you won't be him, no?

Aakash Tomar

9:32 AM

No, because I wouldn't have the friggin cash to make a suit, only knowledge.

G. Bergeron

Anyway, I think learning physics is one of the best bet to be optimally versatile with regard to sciency stuff

Aakash Tomar

And no investor funds iron man suits. Seriously.

I've tried calling one up.

Cold call.

G. Bergeron

@AakashTomar But they funded his company

Skyler

@AakashTomar kek

Aakash Tomar

Kek?

Skyler

9:33 AM

@AakashTomar like lol but a tad snarkier

G. Bergeron

I have friends in the startup business and trust me, it's no small amount of work if you want to have a remote chance of not going bankrupt

Aakash Tomar

Of course. Nobody wants to put money on a new horse, right?

Skyler

@G.Bergeron im in an early stage one atm

were taking it a bit slow but once we take in money things are going to get serious

Aakash Tomar

Wow, Skyler. So, so cool.

Skyler

I dont think ALL startups have to take a ton of work, but its really hard to find ones that dont take up a bunch of effort

G. Bergeron

9:35 AM

@Skyler Yeah either explode up or just bust in your face. It's kind of make it or break.

@Skyler Well the guy at the top usually does yeah.

Aakash Tomar

No, thing is. Either you have the next big thing, or you're ready to put in the hours. Only these two thing bring dollars.

G. Bergeron

Sucking up to investor is not work I would enjoy, but it is work

Aakash Tomar

Things*

Skyler

@AakashTomar not really, if you meet a need, a can market validate that, investors will pitch in

G. Bergeron

@AakashTomar But the next big thing without marketing and/or luck is nothing. There's plenty of good ideas floating around

I don't actually think people get rich because there really hard working and good at what they do. There is also an incredible amount of sheer luck.

Aakash Tomar

9:37 AM

Yeah, I get ideas every 4 hours. But thing is, I'm too lazy for execution. I mean every person has lots of ideas.

G. Bergeron

E.g. Facebook brought nothing new technologically speaking and not much in terms of idea

Skyler

@G.Bergeron its all something between confidence and initiative, luck is about seizing the right oppurtunities, people miss out on "luck" all the time

G. Bergeron

Just at the right time and with good marketing starting with Ivy Leagues and all.

Skyler

were literally bathing in good and bad luck, you just have to learn to control it

Aakash Tomar

For instance. I just thought about starting a business revolving around bio luminescence, but that already exists. So my lazy mind reasons that it's already done, and I'm back to playing Assassin's Creed.

When actually, there isn't one in my country. But it takes effort. The price of being rich is putting your ego and sleep in a dumpster.

And the Internet + Video game consoles.

John Rennie

9:41 AM

@AakashTomar Being a successful entrepreneur takes enormous effort and a certain arrogance. Not everyone can do it - I can't.

G. Bergeron

@AakashTomar ...and luck. I stand my point

On top of it all

Aakash Tomar

Of course, John. So correct. But most of all, it takes disobeying the society and sometimes your family. Right?

John Rennie

@AakashTomar it means not caring about anything except your current project.

Aakash Tomar

For example. My folks want me to go to the top college, have a nice job, have kids and a safe life.

G. Bergeron

But it is actually a plan B if academia doesn't work out... Data analysis, there is so much to be done

@AakashTomar They get less and less influence. It's your own life at some point

John Rennie

9:43 AM

@AakashTomar I have to say that I've met many successful entrepreneurs, and you don't strike me as that kind of character.

G. Bergeron

@Skyler But what's the difference between having not seized an oppurtunity and being unlucky?

Aakash Tomar

Exactly, but sometimes I feel I'm doing the wrong thing disobeying them. Maybe I should sit down and score marks. But then, self control is so, so difficult. So I either pick up my games, or read the physical principles behind something I want to make ( like a catapult ) and try to make it, with a failure rate of 99.99%.

Skyler

@AakashTomar trebuchet are better

John Rennie

@AakashTomar Sounds like the perfect qualifications to be a physicist :-)

Aakash Tomar

John, I know. I was the biggest jerk of my school.

G. Bergeron

9:46 AM

A the beginning of bitcoins (when it was a fringe geeky thing of the internet worth less than a cent each, if even), I liked the concepts and downloaded the software...

Trying to get it to work, I ran into several issues and left it as is thinking: "oh well would have been a nice thingy"

Aakash Tomar

Physicist? No. I mean I'm not the Harvard material. Or the MIT material.

G. Bergeron

Consider that back in those days you could mine several hundreds bitcoins easily

Had the software worked, I would have been probably halfway to being a millionaire

How is that not just luck

Skyler

@G.Bergeron on the outside maybe nothing very clearly, but i think as you do something you build mental machinery to reinforce the behavior. Being "oppurtunistic" reinforces being able to find opportunity and execute on it

Aakash Tomar

Like, Stephen Hawking wouldn't be talking Tony Stark on stackexchange. Those guys aren't so, idle.

G. Bergeron

You could say I could have seen the potential, but so many occasions like that presents themselves and looking back you can always think "If only..."

Like "If only I MADE THAT DAMN SOFTWARE WORK BACK THEN!!!!

Skyler

9:49 AM

I also tend to believe people are far more capable than they think they are at changing their behaviors, and besides a few terrible things I'm really working on stamping out of myself I think I'm pretty adaptable to the world I put myself in

G. Bergeron

But I must not think to much about that...

Well, I, for one would like to not be awake at 4:50 AM...

2

Aakash Tomar

@G.Bergeron I don't believe this, but Jobs said when you're old, you'll look back and see that the dots connected well.

G. Bergeron

@AakashTomar He was really more laidback before he had symptoms

Aakash Tomar

Symptoms?

G. Bergeron

@AakashTomar Oh yes, I like my life up to now but a lot of serious work gets done in weird conditions for me. Like the last two days, since monday morning, I passed out for 5 hours and work pretty much non-stop the rest of the time. This is clearly not the most advantageous conditons...

@AakashTomar His chair

Aakash Tomar

9:53 AM

Okay, I still don't get that statement, but never mind. At least you got a job, pal.

G. Bergeron

@AakashTomar PhD not really a job

Better 8D

Aakash Tomar

Why isn't it a job.

Skyler

@G.Bergeron was having a discussion with someone that went like that last night

G. Bergeron

@AakashTomar And I put myself there

Aakash Tomar

You're one the smartest people on Earth right now.

Be content, seriously.

G. Bergeron

9:54 AM

@AakashTomar Because it really isn't and I don't get salary. I live on scholarships

@AakashTomar I'm not discontent at all!

Skyler

@G.Bergeron i thought professorship is salaried

Aakash Tomar

Dude. There's guys sitting on the streets who don't even have scholarships.

Skyler

@AakashTomar but he doesnt have a mortgage either does he

G. Bergeron

@Skyler I'm not a professor

Aakash Tomar

Mortgage. Okay, that's something to be worried about.

G. Bergeron

9:56 AM

@AakashTomar I know, but what's the point. We were talking about bevavioural traits we have that we don't like

Skyler

yea

G. Bergeron

@Skyler Which I don't

I was not complaining about my condition

Skyler

for me I think I've been slowly realizing I have the behavioral traits that would make grad school unbearable

G. Bergeron

I could have just not worked in the last two days with no real consequences

But then I'm throwing away my chance to do research.

Aakash Tomar

I'll put my bet on this trait - I'm extremely, extremely prone to oscillating between a self harming recluse to an inspired innovator, but it's mostly the recluse.

G. Bergeron

9:57 AM

@Skyler Really depends on the advisor

Skyler

@G.Bergeron more so the structure, 2 classes, TAship, long omnibus project

and really deep focus on a minutia (most of the time)

G. Bergeron

You don't have to take TAships

Skyler

@G.Bergeron you also dont have to live in an apartment

G. Bergeron

the classes, yeah, but it's different then undergrad