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12:27 AM
@DanielSank, it's my birthday today and I got some pretty awesome stuff - Quantum Computation and Quantum Information by Nielsen and Chuang among them =D
 
What unit is " for the semimajor axis of a star
oh it's probably arcseconds
so then I guess conversion to meters is $2d\tan^{-1}(\theta /2)$
 
12:51 AM
@heather The Naive Set Theory is a very good book, but can be very hard at the latter chapters
 
@heather nice!
Wow
 
1:17 AM
@DanielSank, yeah, pretty awesome! =D
 
1:32 AM
I already started reading Quantum Computation and Quantum Information =)
 
@heather keep in mind that book is kinda old.
The basic stuff is good but the error correction stuff etc. is out of date.
 
Oh, okay. I didn't realize that. I mean, I knew it was older, I just didn't realize that some of it was significantly out of date. It'll still be good though!
 
1:53 AM
@heather For sure.
The basic algorithms, e.g. Shor's algorithm, are good to read about.
Also, Happy Birthday, @heather!
Να ζείσεις Χεθερ, και χρωνια πολά
Μεγαλι να γείνεις με άσπρα μαλια
Πανδου να σκορπεισεις τιεσ γνώσεις το φος
και όλλη να λενε "Να μία σοφος!"
That's the happy birthday song in Greek, although I probably spelled almost every single word wrong.
 
thanks!
happy birthday in greek - that's cool. How do you even type characters like that in the chat box without MathJax?
 
Just set my keyboard map to Greek.
Stack Exchange chat supports unicode.
The Greek birthday song is the best. The translation is:
May you live many long years.
May you grow old with white hair.
May you spread the light of knowledge everywhere,
and everyone shall say "there goes a wise woman!"
2
Quite a step up from our version ;)
 
Indeed, that is much better than the English version. =)
 
2:14 AM
Teaching thermal physics again in the spring, and I've been reading texts to try work out a presentation better suited to my students than just trying to plow through Zemansky as fast as possible.
I had to write myself a crib sheet.
Admittedly those aren't all on my reading list, but once I got started I went ahead with all the books in reach.
Let's try a slightly higher resolution:
user image
2
 
rob
2:59 AM
@dmckee Have you read/used Schroeder (2000)? It's very well-done.
 
@rob No. Nor had anyone suggested it yet. The 2014 Styer in the table is a set of incomplete notes toward a book I found on-line. The author has given me permission to use them and they will form part of my strategy as they are at just the right level.
 
rob
@dmckee Yes, I just found Styer's notes and they look promising. Schroeder was new when I used it as an undergraduate and it was one of my favorite texts as a student; our undergraduates are using it here this year.
 
3:15 AM
Reading an excerpt of Schroeder online. The definition of enthalpy as "the energy of the system plus the work need to make room for it at constant pressure" is very nice. That alone make the book worth looking at.
 
@heather ok, back
@heather There's a nerdy boy who likes you then
 
3:36 AM
@heather anyway, best not be picky here
my plan will ruin his life
@dmckee Did you write that? Typo in the last line (should be 'its').
 
user218912
3:52 AM
hello
 
4:18 AM
@dmckee what level thermal physics?
personally i like the idea behind kittel but the book somewhat lacking if this is for an undergraduate class, I REALLY think stat mech first is a better way to do things
 
4:51 AM
@heather: Happy birthday! :-) Also, dyou reckon there are any girls here apart from both of us?
 
5:26 AM
physics.stackexchange.com/questions/290956/… Can someone help me with this question of mine?
 
6:05 AM
@user507974 Undergrad. And yeah, I want to go stat mech first.
 
Anonymous
6:27 AM
@Kaumudi heather is a girl ? :O :O .....first time I saw a girl whose profile picture is a rocket And @heather happy birthday :)
 
@S007 Yes, she is :-)
 
Anonymous
@Kaumudi i can't believe still. A middle school girl learning quantum computing by herself :O .... hats off girl :-P @heather
 
I'd ammend that ^ to "A middle schooler learning Quantum Computing by herself" :-) Anyhoo, I have a quick question. May I ask u? @S007
No, never mind :-P
 
Anonymous
yeah you may ask :-D .... i am feeling so useless now..... seeing so many smart kids on stack exchange :-P
 
Anonymous
@Kaumudi
 
6:41 AM
Never mind, I figured it out with some help, thanks :-)
Yeah, so many of u guys :-P
 
Anonymous
@Kaumudi Ah, okay :-D never mind
 
Anonymous
"so many of u guys"----what ?
 
@All physicists here: Is 100 research papers a good number of them?
 
user116211
7:06 AM
@user507974 Did you like Kittel? Hmm.
 
user116211
Well, like yesterday, the transcript is too long ;/
 
user116211
@heather WTH is that?
 
user116211
This is boring.
 
user116211
And tiring ;/
 
user116211
Ohh.
 
user116211
7:12 AM
May The Force be with @heather.
 
user116211
Okay, I'm nervous T__T
 
im back with some questions!!
ok
first
so
when you life an object in a gravitational field, you do work on it
after you life it to a certain height , that work that you have done on it gets converted into evergy and the energy is stored in that object...the energy = mgh
m= mass of object, g = acceleration due to gravity and h = height that you lifted it to
and then when you let go the object, it falls and while falling, the P.E gets converted in Kinetic evergy
 
user116211
No energy is stored in the object.
 
yeah
thats what i said
am i correct till now?
let me know if im correct, im gonna be right back
 
7:31 AM
@MartianCactus there's a subtle point about PE that can cause confusion so it's as well to be clear about. When you lift an object you don't do work on the object, you do work on the whole system. That is you do work on the Earth-object system.
When you lift an object you increase the distance between the object and the Earth, and that increases the potential energy of the Earth-object system.
The energy isn't stored in the object, or in the Earth. It's stored in the whole system.
 
8:18 AM
@JohnRennie and what do you mean by the system?
i dont really understand the concept of the earth object system..
 
@MartianCactus Everything that is involved in the interaction.
In this case the Earth is exerting a gravitational force on the object so the system is those two bodies.
 
i thought only us and the object were involved in the interaction..
oh
gravity
ok
so energy is just a stored for of work?
an object can ONLY do work if there is energy stored in it..
and if we do work on an object, we increase the amount of energy in the object?
 
@MartianCactus yes, that's not an unreasonable statement. Energy is always conserved so when you do work on a system you increase its energy. The work could go into increasing the potential energy or in some cases it might just end up as heat.
 
user116211
@MartianCactus You are working with conservative force here.
 
oh
so, we do work on an object..it decreases the energy is US but increases the energy in the other object?
 
8:22 AM
The vast majority of interesting forces are conservative, so I wouldn't worry too much about that just yet.
@MartianCactus doing work moves energy around, but when you start asking exactly where the energy is things can get complicated.
 
For example you mentioned lifting an object
 
well..then work and energy are essentially the same thing..just in diff. forms
 
There are three objects involved here (1) the Earth (2) the object and (3) you i.e. your muscles that did the lifting
Your muscles converted chemical energy (from the food you ate) to mechanical energy, and that energy went into increasing the energy of the Earth-object system.
@MartianCactus Yes, work and energy are both forms of energy, but the word energy doesn't have a precise meaning in physics and people tend to use it in a rather vague way. So some caution is needed when using the word.
 
oh
lets come back to the falling object example
as the object is falling, its PE gets converted to kinetic energy
 
8:27 AM
Yes
 
user116211
How dare you, @JohnR, to use \mathrm by yourself? There is one MAN here who does this job ;(
 
@MAFIA36790 :-)
It's your influence rubbing off on me :-)
 
and the moment it reaches the ground, all the PE has converted to KE?
 
Yes
 
user116211
Mafia spares John this time...
 
8:30 AM
well....i dont think ALL its PE has converted to KE as we are not at the center of gavity of earth
,just the PE which was added in during the lifting gets converted in PE
i think
am i right?
 
Generally speaking we only ever talk about changes in potential energy. The reason for this is a bit subtle - PE has what we call a gauge symmetry that means its absolute value isn't precisely defined, but changes in PE are always precisely defined.
So when you lifted the object you changed (increased) its PE by some amount (mgh)
When the object falls back the change in the PE converts to KE i.e. the KE is equal to mgh
 
but KE can only exist while te object is in motion right?
 
Yes
 
so what happens to the KE when the object hits the ground?
and stops..
 
What you're describing is called an inelastic collision.
The energy goes into deforming the earth and/or the object and ends up as heat.
That is the object gets a bit hotter as the KE turns into heat
 
8:38 AM
well,if the energy is deforming the earth, then what form is it converted afterwards?
 
Suppose the object is a rubber ball and you're dropping it on a hard surface
As the object hits the ground it deforms and the energy goes into the deformation. Then the ball sprinfgs back into shap and bounces back up.
So the energy gets converted back into KE.
 
But suppose now you are dropping a lump of soft clay. The clay will just go splat and won't bounce.
 
but if the energy deforms something..and then suddenly disappears.......then wont that mean that energy is being destroyed?
 
In that case the KE goes into deforming the clay but because the clay isn't elastic the energy is lost as heat and can't turn back into KE.
No energy is lost, it just turns into heat
 
8:44 AM
well won't the energy that goes into deforming the clay will be stored into clay as PE?
maybe..
 
In a perfectly elastic object all the energy used in deforming the object is stored as PE. This is what happens ina rubber ball and it's why the ball can biunce up again.
 
is it stored as PE in the deformed ground or the ball?
 
In a perfectly inelastic all the energy is lost as heat and none of it is stored as PE. A perfectly inelastic object can't bounce back at all.
Real objects are somewhere in between, but soft clay is pretty close to perfectly inelastic.
So in soft clay pretty much all the energy is lost as heat.
@MartianCactus I've talked about the deformation of the ball, but suppose you dropped a steel ball onto ground that was made from soft clay. In that case the majority of the energy would go into deforming the ground.
So exactly where the energy goes depends on what the ball and the ground are like.
 
oh
but the energy used in deforming the ground, some of it will be stored in ground right?
 
It depends. If the ground is soft clay all the energy will be lost as heat. If the ground is rubber then the energy will be stored as PE and the ground can bounce back into shape.
 
8:51 AM
oh
 
Just ask yourself does the collison go splat or boing?
 
its kinda hard to wrap your head around the concept that energy can move/ deform anything but still it only changes form and doesnt become less
 
If it goes splat chances are that all the KE is lost as heat. If it goes boing the KE is probably changed to PE then back to KE as the objects bounce apart again.
I suspect you're hung up ona definition of energy that doesn't exist.
There isn't a single thing called energy. There are lots of different things that can change into each other, and we use energy as a general term for them all.
 
oh
ok, lets come back to lifting the object and then falling scenario
 
8:55 AM
the only reason the ball gets PE(and then KE while falling) is becase we LIFTED it agains the force of gravitation
well what if its coming frm space?
like a meteorite
in this case it didnt have PE
but still when its falling, it has KE
whats up with that?
 
The universe started out a basically even distribution of gas.
Then that gas started clumping together to form starts, planets and eventually meteorites.
The potential energy of the meteorite relative to the Earth came ultimately from the PE of the gas that everything formed from.
i.e. that energy was built in when the universe was created
 
well.....its kinda hard
to get a correct notion
 
Yes, it's a complicated business and you're not the first student to find all this stuff confusing.
 
well then..why does the ball that we lift on earth has the PE frm those gases too?
 
I'm not sure what you're asking.
 
user116211
9:05 AM
No new posts in 30 min ;|
 
The matter that the ball was made from came from the Earth e.g. if it's a rubber ball then it was made from oil pumped out of the Earth.
So the ball and the Earth came from the same part of the universe.
But a meteorite was formed far from the earth.
 
It's the distance between the places where the objects were formed that determines their original PE
 
well....how does the meteor get the PE tho?
how do those gases get the PE in hte first place
 
Good question, and all we can say is that's the way the universe was created.
You're basically asking where did the energy to create the universe come from?
And we have no theory that describes this so we don't know.
 
9:09 AM
oh
well.. earth was also created frm those gases...then does earth have that PE too?
 
The Earth was created when a ball of gas collasped under its own gravity. That ball started out much bigger than the Earth and collapsed inwards getting denser and hotter as it fell.
So that ball of gas started with a large PE and as it collapsed the PE turned into KE. Then the KE turned into heat, which is why the Earth started out a molten ball of rock.
 
o wait u got the wrong idea
that was the son
a cloud of gas collapsed making sun
and then the remaining gas swirling the sun made the earth and other planets, including the astroid belt
 
@MartianCactus Yes, but in the large mass of gas that made the Sun there were smaller masses of gas, one of which made the earth.
What I've described is the collapse of one of those smaller masses of gas.
Are you asking about the PE of the Earth relative to the Sun?
 
different masses of gases made small rocks, thses small rocks smashed makin astroids ,those astroids smashed too and then slowly became the earth
i dont get how PE can be relative to something
 
Well, if you go back to your problem of dropping the ball, we talk about the PE of the ball relative to the Earth. Yes?
 
9:17 AM
yeah
 
But what about the Sun? The earth and ball are orbiting the Sun. Shouldn't we talk about the much larger PE relative to the Sun?
And the sun orbits the Milky Way galaxy. Shouldn't we talk about the very very large PE relative to the centre of the Milky Way?
 
now its getting confusing
how does themeteorite get a PE relative to earth?
 
@MartianCactus Because it was formed out in space a long way away from the earth. It had the PE when it was created.
 
and we dont know how it got PE?
 
Because the universe started out with a lot of PE built in.
But we don't know where the PE needed to create the universe came from.
 
9:21 AM
oh
ok
well...my doubts are almost clear than
i will ponder bout that
 
I have to work for about an hour now, but I'll be around later.
 
10:18 AM
dU = nCvdT is true only of reversible processes right?
does it work for irreversible processes?
 
11:01 AM
Is Sakurai's 2nd edition so bad? I see quite some ppl still only use the revised 1st edition
 
Wtf, this place is so empty o.O
 
11:16 AM
"Work from our lab shows that when high school girls see Star Trek posters and video games in a computer science classroom, they are less interested than boys in taking the course. When the classroom is devoid of décor, girls still opt out. It is only when an alternate image of computer science is presented by replacing geeky objects with art and nature posters that girls become as interested as boys."
My sides
 
user116211
11:28 AM
-1
A: Why is the ratio dark matter / normal matter bigger today than in the past? Is it the dark matter that increased or the normal matter that decreased?

michael HarmerI the very early universe there was very little atoms. Atoms are mainly made from exploding stars. I the first pie chart there are less atoms and more photons. This is because that was the initial state and the universe. It was highly fence with photons and neutrinos going in all directions. They...

 
user116211
Is there any sense in it?
 
user116211
@Slereah WTH would one be less interested in Star Trek ;(
 
11:50 AM
2
Q: Gravity assist, Coordinate system transformation

residuenceI am currently learning about the physics and the mathmatics behind gravity assist. I have a question regarding the coordinate transformation between the planet frame and the sun frame. I want to simulate the gravity assist maneuver of a probe mass A in the gravity field of Planet B. So i took a...

Too broad? Homework-like? Revoke bounty?
 
user116211
Too many questions?
 
user116211
In a single post?
 
user116211
I noticed that the very first time it was posted; but didn't do anything. It seems the community didn't do anything either.
 
12:05 PM
@Slereah haha girls seem to be less geeky than guys. it sounds good to me.
 
@DavidZ thanks - I once knew about the bookmark feature, but I'd forgotten it
 
user116211
@Sanya You will get it in the bookmark list.
 
user116211
All bookmarks are listed here unless it is deleted, just for reference.
 
@MAFIA36790 thanks for the link :)
that's also useful
 
12:33 PM
Hey @JohnRennie: :-) Can u help me with s'thing?
(Wait, did I greet u in the morning, as usual? I don't remember :/)
 
@Kaumudi I have to dash out in about 20 minutes, but if it will take less than 20 minutes please ask your question.
Incidentally I fixed the laptop and it works fine :-)
@Kaumudi Hello?
 
Oh, crap.
 
That sound ominous :-)
 
I'm sorry! >.<
Became distracted by food.
2
@JohnRennie Nice :-D
I was gonna ask u about a definition in my textbook.
 
That's a good reason to be distracted :-)
What about the definition?
 
12:42 PM
:-) My textbook defines the term saturation like so:
"A gas or gaseous mixture is said to be saturated with the vapours of a liquid if the partial pressure of liquid vapours is equal to its (saturated) vapour pressure"
 
Dyou get that? O.o
 
Are you asking what it means?
 
I sure don't. Yeah, what does it mean?
 
Do you know what the vapour pressure of a liquid is?
 
12:47 PM
Affirmative.
 
OK suppose you have a liquid in contact with air at some pressure $P$. We can split the pressure into the contribution to the pressure from the air $P_a$ and the pressure of the vapour from the liquid $P_l$.
 
Alright...
 
The total pressure is just the sum of these two: $P = P_a + P_l$.
 
Yes, OK...
 
Imagine we flow fresh air contai ing no vapour into the vessel with the liquid. Then initially $P_a = P$ and $P_l = 0$.
And as the liquid evaporates $P_l$ increases.
 
12:50 PM
Yeah, OK...
 
So the amount of vapour in the air increases with time, but the partial pressure $P_l$ of that vapour cannot get higher than the vapour pressure.
 
Yes...
(Unless we do something to the temperature)
 
Because if $P_l$ was greater than the vapour pressure the vapour would condense back into liquid.
@Kaumudi yes, we're assuming constant temperature.
 
Yes...
Okay, I get it.
 
OK when the partial pressure of the vapour has risen to be equal to the vapour pressure we say the air is saturated with the vapour because it isn't possible to increase the amount of vapour any further.
 
12:53 PM
Ah, OK...
The air!
 
That's all your textbook is saying.
 
My textbook's wording is a little...incomprehensible for beginners.
Okay, thank you :-)
I assume it's lunch time over there? Have a good lunch!
 
It is lunch time and I'm about to have lunch. Pizza with Polish sausage. More meat I'm afraid ;-)
 
Polish Sausage, hmm :-P Anyway, have a good one!
 
Right I'm off. See you later.
 
12:56 PM
OK :-)
 
@Kaumudi, yeah, there's Sanya, for instance.
@MAFIA36790, what the heck is what?
 
Pls.. I would like to disturb u guys.. pls help me... what's the difference between cortex and gray matter? (I know it's not a sight of biology) .. but biology chat rooms are frozen...
 
cortex and gray matter, okay, one moment
The cerebral cortex is the cerebrum's (brain) outer layer of neural tissue in humans and other mammals. It is divided into two cortices, along the sagittal plane: the left and right cerebral hemispheres divided by the medial longitudinal fissure. The cerebral cortex plays a key role in memory, attention, perception, awareness, thought, language, and consciousness. The human cerebral cortex is 2 to 4 millimetres (0.079 to 0.157 in) thick. In large mammals, the cerebral cortex is folded, giving a much greater surface area in the confined volume of the skull. A fold or ridge in the cortex is termed...
Grey matter (or gray matter) is a major component of the central nervous system, consisting of neuronal cell bodies, neuropil (dendrites and myelinated as well as unmyelinated axons), glial cells (astroglia and oligodendrocytes), synapses, and capillaries. Grey matter is distinguished from white matter, in that it contains numerous cell bodies and relatively few myelinated axons, while white matter contains relatively very few cell bodies and is composed chiefly of long-range myelinated axon tracts. The colour difference arises mainly from the whiteness of myelin. In living tissue, grey matter...
@ffahim, the above might be useful to you.
 
Mew
1:11 PM
The cortex is mainly composed of grey matter
 
Composed of?
@Mew
 
Mew
Well the cortex is the outer layer brain tissue
and this outer layer mainly consists of neuronal cell bodies
which is grey matter
this contrasts with white matter which is the neuronal axons
 
U mean both are same? @Mew
 
Mew
no
The Cerebral cortex has both white and grey matter
But it is mostly composed of grey matter
 
That doesn't make sense ... @Mew the white matter is the inside part of brain ,isn't that?
 
Mew
1:25 PM
@ffahim, yes
all semantics
best to say the cortex has grey mkatter in it
rather than say cortex = grey matter
cortex = ourter layer of brain
grey matter = neural cell bodies that look grey on a specimen
clearly cortex is grey matter
but not all grey matter is cortex
hope that makes sense
laterz
 

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