The h Bar - 2017-01-11 (page 1 of 5)

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2:11 AM

0

Q: Any checks for possible nefarious use?

Not that I'm suggesting censorship, but are there any checks in the Physics Stack Exchange system that exclude questions seeking answers that might be used for evil or destructive purposes? I suppose people capable of answering such questions (and of sounder mind) would flag or else avoid answer...

3:07 AM

It's quiet tonight.

4 hours later…

7:24 AM

Storage of summer heat for winter using sodium hydroxide

@JohnRennie You there? Wanna answer my noob question =).

Hi :-)

In my book:
"Effect of pressure on boiling point"
Suck water with a syringe, close the the tip/mouth of the
syringe with your finger, and pull the piston.

So this should create a partial vacuum on the top
side of the syringe? So boiling is what happens when the
vapor pressure of the liquid is equal to the external
pressure.

So if there's lower pressure on the top side of the
syringe i could see how evaporation might be easier.

But i can't see how this would lower the pressure within
the water and make it boil.

You need to be a bit careful about the exact meaning of the word boiling.

Isn't this experiment supposed to show how the pressure of the water gets lowered and makes it boil?

7:29 AM

To me this means the phase transition between liquid and gas at a constant temperature e.g. for water it's the phase transition occuring at 100C at atmospheric pressure.

user228700

Ah, here we go again! :-) @JohnR: Morning, and good luck!

But water can turn to water vapor at any pressure, and I would refer to this as evaporation not boiling

What in the name of chromatophores is a-goin' on around here?

Yeah, but isn't this experiment supposed to show how the pressure of the water gets lowered and makes it boil?

@Kaumudi.H is laughing because we went through this at length a few weeks ago :-)

user228700

7:31 AM

@DanielSank Thermodynamics, as always.

I can't see how the pressure of the water would go down.

@LuBu Suppose you have a sphere of water floating in zero g in a vacuum. What would the pressure inside the water be?

@JohnRennie Asking such hard questions)=.

user228700

@JohnRennie I appreciate the vast amounts of patience that you possess. U say you don't particularly even enjoy thermodynamics and here you are, explaining to Lu Blu, the same sort of stuff we went through in horrible depth just a few weeks ago. U're awesome :-)

@LuBu well the pressure outside the water is zero because it's ina vaccum. Yes?

7:33 AM

@Kaumudi.H OOooooooh, I always wanted to understand that.

Seems interesting.

@JohnRennie Well yeah, the external pressure would be 0 right?

user228700

@DanielSank .__. Were u not around when JohnR and I dissected some bits of thermodynamics a few weeks ago?

@LuBu Yes. It wasn't a trick question. The pressure in a vacuum is zero.

@JohnRennie But you talked about withing the water.

@JohnRennie Isn't that different.

@Kaumudi.H I was not. Are you a whale again?

7:35 AM

@LuBu And if the pressure in the water is greater than the external pressure that pressure difference will make the water expand. Yes?

user228700

@DanielSank Why yes I am, Mr. Octopus.

@JohnRennie Yeah, it probably would.

user228700

Oh, @DanielS: How does 5 hours of sleep sound?

0

Q: Prisms and light outside the visible spectrum

Can a prism, of the proper material for a range of frequencies outside of the visible spectrum, but of interest, be constructed that will separate those frequencies, in that range, in a manner analogous to the visible light prism`s operation?

electromagnetic-radiation

@JohnRennie But if the water sphere is at a certain temperature, how would it exert a 0 pressure.

7:36 AM

@LuBu So the water will expand until the pressure inside it equals the pressure outside it. Yes?

What do you think diffraction gratings are for, then?

@JohnRennie Yeah, that's probably the case.

@JohnRennie Seems logical.

@LuBu So the pressure inside the water must be zero.

In fact, the pressure inside the water must be equal to the external pressure.

Because otherwise the water would expand or contract until the internal and external pressures were the same.

@JohnRennie If you state it like that, it seems it must be true. But how is it that the pressure would be 0 if the water is at a certain temperature.

@Kaumudi.H Doesn't sound like enough for most people.

Usually people in your age range need more like eight.

::shrugs::

user228700

7:38 AM

@DanielSank But it's genetic, isn't it?

I don't know.

user228700

I read that it is.

I do know that some people can get on with considerably less.

@LuBu I guess the question is why you think the pressure in the water would not be zero. What would be the mechanism for this?

@JohnRennie Well, what im thinking is that if the particles have a certain momentum they should apply some pressure, right.

user228700

7:40 AM

@DanielSank Those people are very lucky.

@LuBu In a liquid particle motion is relatively slow. Particles can mostly be treated as if they are vibrating. In the liquid the atoms/molecules are held together by an attractive force (that's why it's a liquid not a gas)

@JohnRennie But doesn't the motion in a liquid create pressure?

At zero pressure the attractive forces exactly balance out the vibratory motion. So on average the particles neither want to increase nor decrease their separation.

Note that this is completely different to a gas, because in a gas the attractive forces between atoms/molecules are negligable. In a liquid they are dominant.

You, sort of, think of the motion as creating a positive pressure and the intermolecular forces as creating a negative pressure. The two balance out to give zero pressure.

@JohnRennie Well i can see how it would be a definite statement that the internal pressure is 0 if the water doesn't expand.

@JohnRennie So where were we going with this.

So that's why in your syringe experiment the pressure inside the water is decreasing. It's because the pressure in the water is the same as the external pressure.

Though in the syringe expt the external pressure isn't decreased all the way don to zero of course. It's just decreased a bit.

7:48 AM

@JohnRennie Yeah, so what i was thinking is that there is a partial vacuum on TOP of the water.

@JohnRennie But isn't it in contact with the atmosphere trough the plastic of the syringe.

@LuBu I would guess you're suppose to regard the walls of the syringe as totally rigid.

@JohnRennie So the atmospheric pressure wouldn't get you then?

Correct

@JohnRennie So the atmospheric pressure doesn't travel equally well in different substances?

@JohnRennie Like if we would have a rigid wall.

Obviously not or you couldn't build vacuum chambers. Or look at it the other way. The reason we can have pressurised air in a skin diver's air tanks is because the metal walls of the tanks don't transmit the pressure.

user228700

7:52 AM

Say, what's a "skin diver"?

Scuba diving is a mode of underwater diving in which the scuba diver uses a self-contained underwater breathing apparatus (scuba) which is completely independent of surface supply, to breathe underwater. Scuba provides the diver with the advantages of mobility and horizontal range far beyond the reach of an umbilical hose attached to surface-supplied diving equipment (SSDE). Scuba divers engaged in armed forces covert operations may be referred to as frogmen, combat divers or attack swimmers. Unlike other modes of diving, which rely either on breath-hold or on breathing supplied under pressure...

user228700

Ah, same thing. Wokay, thanks.

Q. How do you circumcise a whale?
A.You need four skin divers

user228700

Hehehe, that's actually pretty clever!

Sadly not my invention. It's a very old joke :-)

user228700

7:55 AM

I see :-)

@JohnRennie So in a syringe then, the plastic walls insulate the air pressure somewhat.

user228700

Has anybody here tried polyphasic sleeping , BTW?

@LuBu To a good approximation you can take the walls of the syringe to be totally rigid.

@JohnRennie Anyway, ty for the explanation. On some level this probably would've seemed obvious, but i really needed the explanation.

You're welcome. Things always seem obvious once you know them :-)

8:01 AM

@JohnRennie That might be true. I'm probably just too lazy to think about these things myself, so i let you to explain them to me =).

user228700

@JohnR: Quick question (I am only taking a short break, I swear): Have u read THG/Divergent series?

I haven't. My niece has read both and says they're very good. Actually my mother (aged 84!) has also read the first book of the Hunger Games and says it's very good.

@JohnRennie I'm also pondering about something.

user228700

Why haven't u?!

So boiling is what happens when the
vapor pressure of the liquid is equal to the external
pressure.
Isn't this the general definition of boiling?
And it is defined this way, because if the pressure
of the vapor is equal to the external pressure, it is
able to disperse.

8:10 AM

@Kaumudi.H I suspect they'd be a bit too young adult/girlie for me ...

I mean, come on, a teenage girl main character?

user228700

@JohnRennie Fair enough.

That isn't a criticism of the books, just a statement that aging nerds aren't their main target audience :-)

user228700

I read both trilogies about three years ago and like them very much. I read THG first and enjoyed them. Then came Divergent and contrary to popular opinion, I liked them even better than THG trilogy.

user228700

However, I will be quick to add that the fact that a teenage girl is the protagonist of both stories doesn't change the experience/story that much.

If it wasn't for the weight of physics revesion currently bearing down on you there's loads of books I'd like to recommend, but that should probably wait until after May 21st.

I wonder if it would be worth trying reading for an hour before going to bed? I've found that helps me settle down to sleep.

user228700

8:14 AM

@JohnRennie Let's not forget Math and Chemistry now! Yes, I will ask for recommendations but I'm not too sure if I'll be able to find them all at my local library.

@Kaumudi.H I might allegedly know someone who allegedly has copies of the e-books they could loan you :-)

user228700

@JohnRennie An hour is a bit too much but I've tried sleeping after reading random crap for about 15 minutes but it hasn't helped me that much, to tell you the truth.

@JohnRennie My question is disappearing to the top )=.

Why is an hour too much. I doubt that 15 minutes is long enough to relax you much.

user228700

@JohnRennie :-) Sure, then. I will save up my allowance money for all the books.

user228700

8:16 AM

@LuBu Fret not! I shall begone in just a bit.

user228700

@JohnRennie An hour is too much because I am now contemplating sleeping for 5 hours between 9PM and 2AM because I am running short on time.

@Kaumudi.H The question is probably not hard enough for him to be able to have a conversation at the same time =).

user228700

Sure. Up to him :-)

user228700

@LuBu Ooh, I see that you have wandered into that area in which Gibbs Free energy rules all.

@LuBu I need to take a short break to attend to something at work ...

8:19 AM

@Kaumudi.H If that's the case i will not probably understand the answer. =)

@JohnRennie Sure.

user228700

I would advice you, actually, to search the transcript for the world "boiling" said by me because JohnR and I have had pretty much the same discussion before.

@JohnRennie Wait, you're working at the same time?

user228700

@LuBu Yeah, Ikr?

@Kaumudi.H But if i try to search for it, wouldn't i have to find the correct day?

user228700

This may be it:

user228700

8:24 AM

Nope.

user228700

This, perhaps:

user228700

Dec 17 '16 at 4:39, by Kaumudi. H

> "The temperature at which the vapour pressure of the solid becomes equal to the vapor pressure of the liquid"

@Kaumudi.H A plugin that would let you search the whole text, would probably be a good implementation.

user228700

@LuBu Oh, there is! How dyou think I'm finding these? Lookie here:

user228700

chat.stackexchange.com/…

8:26 AM

@Kaumudi.H Too bad it isn't implemented on the chat interface.

Not that it makes any difference.

user228700

@LuBu What dyou mean? Dyou see the "Search" bar at the top right-hand corner on this page? You can type the keyword over there and you will be redirected to a page on which you can search for "When said by X person".

@Kaumudi.H Oh yeah, didn't notice.

I'm back!

@LuBu remember that the Gibbs free energy is kind of a potential energy?

@JohnRennie All that i understand at this point is that it is somesort of potetential energy of a thermodynamic system and that the system wants to minimize it.

@JohnRennie (For whatever reason it wants to minimize it.)

@JohnRennie This might be a little too difficult for me at this point.

In your syringe you have two phases, liquid water and water vapour. Those two phases each have a free energy $G_l$ and $G_v$. OK so far?

8:36 AM

@JohnRennie I'm probably not going to understand this, but ok.

At equilibrium these free energies must be equal i.e. $G_l = G_v$

Because if they weren't equal the system would change to make them equal.

i.e. either some liquid would evaporate into vapour, or some vapour would condense into liquid, until the free energies of the two phases were equal.

Does that make sense so far?

@JohnRennie If that's what falls from the minimizing, then maybe they will be equal.

Well suppose you have two tanks of water, one on the ground and one raised by a metre. What happens if you connect the two tanks by a pipe?

@JohnRennie Probably gravity happens.

That's a little abstract. What I expected you to say was that water flows through the pipe from the upper tank to the lower tank ...

8:41 AM

It's just that i hope you don't expect me to grasp this free energy concept, i think it might be too difficult for me at this point =).

But yeah, it prob does do that=).

OK, maybe we'll let is go for now. To be fair students don't learn about free energy until the first year of a physics or chemistry science degree.

@JohnRennie You mean at university lvl?

Yes

user228700

@JohnR: I feel the time has home for me to reveal that I used to be obsessed with Twilight between ages 12 and 13. I bring this up because while having a teenage girl who is obsessed with sparkly non-farting 108 year-old old men in the disguise of a ravishing 17 year-old is something that literally shapes the story in the Twilight series, the gender of neither of the characters in THG/Divergent series matters very much.

@JohnRennie How much do universities generally expect you to grasp about the concepts in physics?

@JohnRennie To me it seems that here where i live the school before uni doesn't really prepare you for anything.

user228700

8:48 AM

For example, would u not have read the HP series if it were from the POV of Hermione?

user228700

I would have, because Hermione is a badass and so are Katniss and Tris.

@Kaumudi.H that's a good question.

user228700

I am not forcing you to read them or anything, I am simply arguing against your point that the fact that the story is narrated from the POV of teenage girls doesn't change anything in books like THG/Divergent (And not Twilight!!)

The first HP book I read was the Goblet of Fire and I read it by accident.

I needed a book to read on a train, and I bought GOF because it looked a nice thick book that would take me a while to read.

I really enjoyed it, an went on to read all the other books.

So if the book had been from the perspective of Hermione I probably would have read at least GOF. Whether I would have read the other books I'm not sure, but I probably would have.

user228700

@JohnRennie Oh, so you didn't read them in order?! Phew, now I feel more comfortable to admit that the first HP book that I read was Deathly Hallows. Well, no, I didn't read it, really; my mum bought it for me without knowing that it was the last book in a series. Of course I didn't understand anything so I went back to the first one and read them all.

user228700

8:51 AM

@JohnRennie Ah, GOF is an excellent book to have read first (of all the others) because the plot really thickens at the end.

@LuBu Universities are well used to dealing with fresh new students. They will teach you everything you need to know. You need not worry that you'll arrive at university unprepared. Especially if you hang around the Physics Stack Exchange :-)

user228700

^ True that.

@Kaumudi.H Yes, I think GOF is the best book of the series.

And I think Order of the Phoenix is, well I don't want to say worst, but certainly the least amazing :-)

user228700

Unfortunately, I don't particularly remember the experience of reading any of the books, so I can't comment on which book is my favorite :-(

@JohnRennie Well i was hoping if i spend time here, i might absorb at least some knowledge.

user228700

8:54 AM

The Order of the Phoenix is certainly the thickest of all of 'em. Man, that book is huge.

@LuBu if you hang around here you can't avoid absorbing physics knowledge. It just seeps in :-)

user228700

@JohnR: What dyou think about this:

user228700

40 mins ago, by Kaumudi. H

@JohnRennie An hour is too much because I am now contemplating sleeping for 5 hours between 9PM and 2AM because I am running short on time.

I don't think that's a good idea.

user228700

Me either, but I think I'm going to ahead and do it.

8:58 AM

Daniel seems to be the best chap to talk to about sleeping, but for what my opinion is worth I think you work a lot less efficiently when you're tired.

I think you will get less work done, not more, if you reduce your sleep to five hours.

user228700

Hmm, why is it that DanielS knows so much about sleeping?

Although, do you take a nap in afternoon?

user228700

@JohnRennie I'm not too sure if this is really the case. You see, last year, I only slept for about 6 hours (combined) on the 3 days before my Physics final.

user228700

@JohnRennie If it gets to the point where I cannot function, yes, I'm capable of taking so-called "power naps" in the afternoon...

Well, as I said, I don't consider myself an authority and my advice is worth roughly what you've paid for it.

But I think you learn best when you're relaxed and happy.

Actually I think chjatting about Harry Potter, Coldplay, etc is a good way to relax and increase your happiness levels, which is why the time you spend here isn't as much of a waste as you probably think.

user228700

9:02 AM

@JohnRennie That is true, but only works when you've got plenty of time on your hands.

Cor! My fence has just blown down!

user228700

@JohnRennie WTF, really?!

user228700

@JohnRennie Yes, probably :-) Maybe not so much in the afternoon tho...eek, it's about 2 PM. I gots to finish this chapter on AC!

Yes. It is very windy in the UK at the moment, and as I was typing the fence suddenly fell down!

Luckily it is actually the neighbour's fence i.e. he is responsible for keeping it in good condition.

@Kaumudi.H Back to work then! Now you're relaxed and happy :-)

user228700

:-P I see.

user228700

9:04 AM

@JohnRennie :-) Yes!

Met Office issues yellow warning for wind and snow in UK

user228700

-10 degrees? Uff.

Back to work!! :-)

user228700

Alright, I received this reply from "Anonymous" yesterday:

user228700

19 hours ago, by anonymous

Remember that $b-a=nw$ since the region between $a$ and $b$ is divided into $n$ parts of width $w$.

user228700

9:10 AM

And this, ladies and gentlemen, was the only part about the formula that I find difficult to understand. Only this.

user228700

Look, I even asked about it here:

user228700

20 hours ago, by Kaumudi. H

Wait, hang on, does $\int_a^b dx$ give me the total number of divisions I made b/w $a$ and $b$ ?

user228700

Hmm, I think it does make sense... almost. @JohnR: Thoughts?

There is an argument for just shrugging and moving on. You'll often find you get hung up ona subject but if you just shrug and move on then when you go back to it a few weeks later it suddenly seems clear.

Especially if you're under a bit of time pressure.

user228700

No no, this is the last bit and I feel that it is something I ought to understand now before delving into a chapter that is flooded with integrals.

9:14 AM

$\int_a^b dx$ is just $b-a$

user228700

OK, why is it that $b-a = nw$?

Let's make it concrete. Suppose the limits of integration are 0 and 1. and we divide it into 1000 chunks i.e. $n = 1000$.

Then the width of each chunk is $w = 0.001$. Yes?

user228700

Ooh, yes. I'm very stupid. Thanks very much, I think I understand it now.

user228700

Huzzah! Now I can move on!

user228700

When I said this:

user228700

9:17 AM

21 hours ago, by Kaumudi. H

@JohnRennie OK, no, hang on, does this: "Hang on, so we're finding the area by integrating...and then dividing by the base just leaves us with...the sum of all the heights, right? :-| " make any sense at all?

user228700

I was asking about this:

user228700

19 hours ago, by anonymous

Now, when you are dividing by total width what you are actually doing is $$\frac{\int_a^bf(x)dx}{b-a}=\frac{w(h_1+h_2+...+h_n)}{nw}=\frac{h_1+h_2+...+h_n‌}{n}$$=**AVERAGE HEIGHT OF ALL THE RECTANGULAR STRIPS** (i.e. average value of the function in the given interval).

user228700

What I was trying to do yesterday, was understand where the $n$ comes from so that we end up with the average height instead of just the sum of all heights.

user228700

Anyhoo, OK, moving on.

That's a nice way to describe the process :-)

user228700

9:19 AM

:-) And my brain was actually onto it! I was just confused about $n$.

user228700

Oh, @anonymous: Thanks very much! :-)

Pissed off layman

in Homotopy Theory, Dec 8 '16 at 19:48, by Charles Rezk

A "concrete" description is one that helps us cement our understanding ...

Groan :-)

Pissed off layman

:-)

Hey @JohnRennie, maybe put the final nail in the coffin on this one?

or did you already vote on it?

9:35 AM

@EmilioPisanty I've already voted to delete. Where are the moderators when you need them :-)

@JohnRennie they'll get to it when they get to it ;-)

it's not like it's really screaming for immediate deletion

it's merely strongly calling for deletion

user228700

Sigh, this chapter is almost depressing:

user228700

user image

user228700

9:50 AM

I have a quick-ish question. What is the difference between periodic DC and AC?

Can you define what you mean by periodic DC

user228700

This:

user228700

user image

user228700

Is it that it periodic DC doesn't change direction like AC does?

Periodic DC is just AC plus a constant offset.

If you integrate AC wrt time then over a cycle it averages to zero.

i.e. the positive and negative parts of the cycle sum to zero.

user228700

9:55 AM

Right, but this isn't the case with periodic DC because it never changes direction, right? There would be a non-zero average, yeah?

DC does change direction in the sense that it moves up and down

user228700

Sure, but only its magnitude changes, no?

Well it's value never goes negative.

It's a bit of an arbiitrary distinction.

e.g. in India the mains voltage is 220V (I think)

user228700

Yeah, that's what I mean; increases, then decreases, but never increases in the opposite direction, yeah?

I want help in vectors

in Mathematics, 6 mins ago, by Ramanujan

If we have a matrix which gives a vector then transpose of that matrix gives same vector?

9:57 AM

So the mains AC signal is a sine wave with a peak ampitude $\pm 220\sqrt{2}$

user228700

@JohnRennie Right.

user228700

Wokay, it's fine. Probably reading too much into it, as always. Thanks!

If you add a constant $220\sqrt{2}$ you now have a sine wave displaced up the $y$ axis so it oscillates between zero and $+440\sqrt{2}$ with an average value of $+220\sqrt{2}$

That would be periodic DC, but it's just AC with a constant added.

user228700

Right, yeah, OK. Thank you :-)

12:05 PM

@Kaumudi.H @JohnRennie I looked trough the old conversations and i think i
understood how osmosis works (lol). I also read how John
explained the raise in the boiling point due to a solute
in terms of the equation for GFE. I think it made sense
in terms of the equation that i don't understand.

@JohnRennie What i might wanna ask at this point is that does the
definition of the boiling point as the vapor pressure
equaling the external pressure come out of GFE or is
there a simpler explanation for why the boiling point
is defined that way.

I think the Gibbs free energy is the simplest way to understand things like vapour pressure.

It is a hard concept to get started with, but once you understand it all sorts of things suddenly become easy to explain.

@JohnRennie Yeah, i suppose you gotta start builing the mental picture and in the end it will probably come together.

One reopen vote still missing; physics.stackexchange.com/questions/240979/… This is related with this question; physics.stackexchange.com/questions/304243/… The question is heretic, but it is completely mainstram-physics to ask the reason for the measured/observed data.

user228700

12:26 PM

@JohnR: Is there no webcam application installed in this thing?

Morning

@Kaumudi.H Generally speaking apps that use the webcam have the webcam support built in. For example Skype will use your webcam without needing any extra software.

Are you looking for an app to take pictures through the webcam, like the camera app on your phone?

How do i paste a image onto the chat.

Save the image somewhere on your PC then click the Upload button

use the upload button, which is to the right of the send button (the text box must be empty)

@JohnRennie, beat me to it =P

12:40 PM

I can't see an upload button.

Are you using a mobile phone?

Nope.

user image

That's what the web site looks like on my PC

Maybe i don't have the privilege of sending pictures yet.

user228700

@JohnRennie Precisely.

12:42 PM

Don't things need some sort of reputation here.

user228700

@LuBu Privilege? Can u upload a... dammit. Is there no "Upload" button on ur screen?

There is no upload buttons in sight.

I think you needed 20 rep to join a chat room anyway.

@Kaumudi.H you have mail

user228700

@JohnRennie Do I need to click there every time I need to use it? Plus, it doesn't look like I can record anything...

No. Download the app and save it somewhere on your disk.

user228700

12:46 PM

See, I did a fair bit of googling.

@JohnRennie Anyway i prob don't need a picture to illustrate this but:

@JohnRennie This might sound stupid, but if we have that beaker and
the vapour exerts a vapour pressure, isn't the vapour
pressure always equal to the atmospheric pressure,
because the vapour is in contact with the atmosphere
and shouldn't the pressure be transferred to the vapour.

@Kaumudi.H I can't remember if that app takes video. I think it only takes stills. Do you want to record video?

user228700

@JohnRennie Oh, I see. Um, yeah, I was hoping I could record video diary entries :-P

@LuBu The atmosphere above the beaker is a mixture of air and water vapour, and the total pressure is equal to the partial pressure of the air plus the partial pressure of the water vapour.

Partial pressure

In a mixture of gases, each gas has a partial pressure which is the hypothetical pressure of that gas if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal gas mixture is the sum of the partial pressures of each individual gas in the mixture. It relies on the following isotherm relation: V x V t o t ...

At equilibrium the partial pressure of the water vapour is equal to the vapour pressure.

@Kaumudi.H I can have a look around. Right now I need to take a break to go shopping.

@Kaumudi.H not that you're procrastinating :-)

user228700

@JohnRennie :-P Hey! I just, yeah, it's not very pressing. It's OK, u can point me to some source (I'm telling u, I couldn't find anything) later on, then.

user228700

12:50 PM

And thanks! :-P (For the link and for reminding me not to procrastinate :-P)

@Kaumudi.H Will do. I don't use that type of oftware myself so I don't have anything to hand.

user228700

Have fun shopping!

user228700

@JohnRennie Thanks :-) It's OK tho, it's not very pressing. Take ur own sweet time...even months :-P

Hello.

@SanjuktaGhosh Hi.

12:59 PM

To anybody who can help.

I'm actually struggling with a question.

@SanjuktaGhosh Aren't we all.

This is it:

0

Q: 2nd law of thermodynamics for an open system

Ecosystems are open systems, they receive solar energy and other materials from outside the ecosystem and migration of animals is also witnessed. I read Physics uptil class XII. I found this book 'Environment By Peter H. Raven, Linda R. Berg, David M. Hassenzahl' that says that when energy is tr...

thermodynamics biophysics

Hi

@JohnRennie If you find some time can you take a look?

@SanjuktaGhosh He probably will, but he went to shopping.

1:10 PM

Can anyone help me with my question?

@Siddharth You don't need to ask for permission to ask. What's your question?

@Siddharth Some1 will prob answer it if they understand it. So i think you can just throw the question into the chat. But our professional physicist @JohnRennie left to shopping so you might want to wait for him.

This is the question physics.stackexchange.com/questions/304393/…

@LuBu I see.

@Siddharth They are identical inductors?

1:20 PM

No they aren't.

@Siddharth What are their specifications, then?

Well you consider their inductance as L1 and L2 .

@Siddharth Do you know anything about their internal resistances? If you're interested in the steady state after many time constants $\tau = RL$ then the inductance won't matter any more.

No there's no internal resistance mentioned.

@Siddharth What is the source for this problem?

1:25 PM

This is a problem from FIITJEE review package.

I can post the original question if you want.

@Siddharth Is the original the image here?

Yup thats it.

Hmmm, that's interesting. I'll edit & comment on your question in a minute.

Alright.

reddit.com/r/Showerthoughts/comments/5n8sq0/…

I like the way these people think

1:32 PM

@SirCumference It kind of does:

The geoid is the shape that the surface of the oceans would take under the influence of Earth's gravity and rotation alone, in the absence of other influences such as winds and tides. This surface is extended through the continents (such as with very narrow hypothetical canals). All points on the geoid have the same gravity potential energy (the sum of gravitational potential energy and centrifugal potential energy). The force of gravity acts everywhere perpendicular to the geoid, meaning that plumb lines point perpendicular and water levels parallel to the geoid. Specifically, the geoid is the...

1:55 PM

@rob You have a favourite geoid definition? Mine is NAVD88, but WGS84 is a very close second

I should get back to writing that CTC book

user image

So many nice topics

2:14 PM

@Jim I've never actually had to use a geoid for anything, so I haven't given any thought to the standards. Why do you prefer one to another?

2:26 PM

@rob each one is more appropriate in different parts of the world. NAVD88 makes a better approximation to the geoid where I am. WGS84 is better on average for all parts, which is why it's a good runner up

1 hour later…

3:54 PM

@JohnRennie So you're back. Let me continue:

@JohnRennie But doesn't the water vapor just end up
assuming the atmospheric pressure and end up pushing
the atmosphere with the atmospheric pressure.

@LuBu Was it you I answered a question for talking about the microscopic origin of pressure?

@JohnRennie I'm not really sure. You did answer to this: physics.stackexchange.com/questions/302250/…

aha, yes, that's the question I was thinking of.

In my answer I explained that pressure is just molecules bouncing off a surface. Yes?

@JohnRennie Yeah, i think so.

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