Why do solids maintain their individual identity when placed together?

[Graeme M]

Ahhh... I see, my problem is not understanding the meaning of 'weight' in context. I am hung up on the idea of weight being a measurable quantity as in a thing on a scale, which is your 'apparent weight'. But you are talking of weight as the force over the whole fluid arising from gravity. I'm not sure I explained that correctly but its right there in your equation
W=A(P(b)−P(a)).

I think I can adapt my earlier mental model to that sort of idea.

Now I get it, as much as I can get it.

Thanks to all, sorry I took so long to understand this one. Never having learned this stuff at school means I didn't have the hooks to hang this off...

 

[Dale]

I am glad I could help.

 

[Graeme M]

Aaaargh! I must be as dumb as a fence post. DaleSpam, thanks to your last post I grasped the difference in concepts of weight and how that applies in this context. But on rethinking it...

In words, this says that the weight of the gas in a box is equal to the area of the box times the difference in the pressure at the top and bottom of the box.

That sentence makes sense as did your equation and gave me the right feel for how it works, but I am still baffled about how a single barometer reading is equivalent to the weight. You are saying that we need pressure at top and bottom to be able to derive the factor for our calculation. That is two barometric readings, surely? You can't come up with a pressure gradient from one reading at the bottom of the column can you?

Or does it follow that in the case of the atmosphere, the value at the top of the column is just zero? Still, for any other box of gas, say one 5000 feet in height, both A and B have non-zero values, so in those cases the lower barometric reading would not equal weight?

Or maybe I am still not getting 'weight'? Or 'pressure gradient'. Or 'weight density'.

Sigh... Back to school for me!

 

[Dale]

That is correct. You need two barometric readings in general. You can only get away with a single reading if for some reason you know what the other one is. For example, you know that the pressure at the top of the atmosphere is 0

 

[K1NS]

Why is a solid so solid? Because a solid is NOT SOLID! Normal matter (excluding special cases like neutron stars or black holes), even very dense materials like a block of metal, are actually mostly empty space. Subatomic particles, relative to their size, are farther apart than stars in the sky. Look up in the sky at night. You see mostly black, right? That's because stars and other objects are very far apart relative to their size. The same is true of that block of solid metal.

But the block of metal LOOKS solid because the wavelength of visible light is longer than the distance between particles, so the light reflects off the matter and it looks solid and smooth. It FEELS solid because if you try to push your hand through the block of metal, powerful forces repel your hand, and the block of metal feels solid when in fact it is mostly empty space.

Imagine for a moment you are a neutrino, tiny particles that fly around the universe and pass right through "solid" matter. This very moment, billions of neutrinos are passing through you, and me, and the "solid" core of the earth. Neutrinos are not susceptible to the forces that most matter is,, and the blissfully pass right through the empty space that is solid matter, ignoring the particles they come close to but never collide with. When you imagine the universe as a neutrino "sees" it, it becomes a very different place, and we see that we are actually all a part of the same immense field of particles and forces. There is no line between us. There is no surface to "solid" matter. The center of the earth, as dense as it is, is hardly different from intergalactic space.

When a spiritual mystic says that reality is illusion, he may be right, eh?

 

[Graeme M]

I guess that was partly my original question K1NS. Given a 'solid' object is composed of atoms/molecules, what forces actually bind it together in a form that has a certain integrity to it. Regardless of how it looks at the sub-atomic level, at our level solid objects are solid, they have boundaries, and they have properties that can be quantified. At our scale, solid objects certainly DO differ from intergalactic space. I for one am glad of that otherwise I shouldn't be writing this. I was simply wondering why things ARE like that, which led to an interesting (to me) discussion about gasses and weight in which I gained an understanding I didn't have before!

But I take your point, it is intriguing to visualise the universe as nothing but a field of particles and forces.

 

[jerromyjon]

When a spiritual mystic says that reality is illusion, he may be right, eh?

Reality is merely an illusion, albeit a very persistent one.- [URL='http://www.brainyquote.com/quotes/authors/a/Albert_einstein.html']Albert Einstein

 

[jerromyjon]

But I take your point, it is intriguing to visualize the universe as nothing but a field of particles and forces.

Or one step further, just electromagnetic radiation and its attraction.

 

[Albert Newton]

well...when taking about the properties of Liquids and Solids...we have to consider their structure and the types of bondings between the Molecules...
In Liquids.., if we take water., the molecules are connected through weak hydrogen bonds and also they don't have a definite Lattice Structure and the Molecules are free to Break bonds and move around due to weak Hydrogen bonds ...
While in Solids.., ..they have a Rigid Lattice Structure and are held Strongly by neighboring atoms and need some amount of Energy to break those Bonds which in case of metals / some other Solids is very High ...

also remember that when you are talking about merging of two portions or whatever .., its all about the Bonds between the consecutive Molecules...if the Molecules are mobile and can break their bonds easily..,.then they merge easily.. :)

 

[jerromyjon]

...If they have similar atomic structure or they conform to each other.

 

[Albert Newton]

even if they have similar atomic structure or conform each other.., they(Molecules) have to first break their bonds with neighboring Molecules...to merge with foreign Material .(be it similar or different material).

 

[Tom1996]

There is a interesting phenomenon that if sth stuck on the ground for too many years, it would be impossible for u to remove it,or clean away its color.this can be seen in some old house.I think this is because the molecules r in constant motion , and somehow run into each other .

 

[jerromyjon]

And as they merge the resulting ions freed attempt to find entropy around the surface of the new system.

 

[Albert Newton]

i think it maybe due to the perfect formed between the particles in the course of time...

like...all known surfaces have atleast some irregularities on the surface and there exists some air between the irregularities and hence a perfect physical contact is not maintained and when 'something' is left on the ground for a longg while.. these irregularities maybe break if necessary and due the course of time come into greater physical contact with each other and the air between them is eliminated...

also note that if there's nothing (vaccum) between two substances , then they stick to each other like Glue. .. :)

 

[jerromyjon]

Or in the case of Au and Ag they fuse perfectly with virtually no effort at all...

 

[Albert Newton]

Oh ..Cool..!...i didnt know that...and is it universal..? like...no required conditions at all..?

 

[jerromyjon]

A perfect vacuum and temperature of I'm not sure..

http://en.wikipedia.org/wiki/Cold_welding

 

[Albert Newton]

Lol...just read that like 10 min ago ... :P . ... well didnt read about that Au and Ag though...

 

[Astronuc]

Atoms from two metals in contact and diffuse into each other, and this is particularly the case the lower the melting point.

In the normal day to day experience (at what we call room temperature or in the temperature range we normally experience (excluding fire and other forms of heat)), solids tend to retain their shape, but we can load them such that they creep or flow. Flow usually implies internal stresses above the 'yield strength' (YS), while creep occurs below YS. Furthermore, solids become 'less' solid as temperature increases, especially as temperature goes beyond about 1/3 of the melting temperature. As temperature increases, YS decreases and the stress required to produce a given creep strain or flow becomes less.

Some metals for eutectics in which they melt each other at temperatures below the pure element melting temperature, but that's another matter.
http://en.wikipedia.org/wiki/Eutectic_system

Liquids, where interatomic/intermolecular bonds still exist, tend to 'readily' assume the shape of whatever vessel contains them.

 

[Chronos]

On earth, metal surfaces attract and bond to atmospheric molecules creating a barrier. This is not an issue in the vacuum of space, which makes what is called cold welding possible. Google on cold welding for further information. You can even do this on Earth with highly polished surfaces - e.g., wringing of gage blocks or optical flats. They can permanently fuse together if left wrung for too long.

 

[UncertaintyAjay]

Why solids are solid is explained by the Exclusion Principle. As I understand it, Graeme's question is something along the lines of " if atoms are mostly empty space, why do things feel solid. This is explained but he exclusion principle which was proposed by Wolfgang Pauli. It states that no two fermions of the same kind ( there are two kinds of particles- fermions, which are electrons, protons etc and bosons, which are things like photons and well, the Highs Boson) can occupy the same state.
In a solid, atoms are packed together. So, when we place another solid on it( say our hands) the electrons in our hands and the nuclei of the atins in our hands get close to those of the other solid. Because they can't be in the same state there is a repulsion between the two surfaces. So we feel them to be solid and distinct. In liquids, since the molecules are much further apart, new fluids and even solids can mix without feeling significant repulsion due to the exclusion principle.

 

[mfb]

Atoms from two metals in contact and diffuse into each other, and this is particularly the case the lower the melting point.

Gallium is very good at that, and there are videos how it can destroy the stiffness of other metals.

 

[sophiecentaur]

Gallium is very good at that, and there are videos how it can destroy the stiffness of other metals.

It's not only highfalutin gallium that produces that sort of effect. Mercury will eat holes in aluminium and, they say, will cause panic if spilled in an aeroplane. Apparently, it alloys with the aluminium, which then oxidises at the surface, releasing more mercury, to carry on the dirty work. Jewellers run a mile when lead turns up because it alloys with hot silver and just won't go away. Lead solder is, of course, banned.

I would take a bit of an issue with the PEP being the 'reason' for solids being solids. Isn't it more just a re-statement of the way they behave?

 

[UncertaintyAjay]

I would take a bit of an issue with the PEP being the 'reason' for solids being solids. Isn't it more just a re-statement of the way they behave?[/QUOTE]

In what sense is it a re-statement?

 

[sophiecentaur]

I would take a bit of an issue with the PEP being the 'reason' for solids being solids. Isn't it more just a re-statement of the way they behave?

In what sense is it a re-statement?[/QUOTE]
What I meant is that the "principle" is just that. It's an observation of the way things behave and the behaviour is observed to occur inside atoms and that behaviour extends to situations involving many atoms. PEP is 'only' (no value judgement here) a statement about the way a lot of particles behave. I guess I was being picky because there is always a temptation to seek and produce 'reasons' for things - which are really only shifting the crux of the explanation further away - and that is only the best we can hope for. This was a Feinman thing. Many of the 'explanations' we come up with are little more than 'bootstrap' arguments. Just why Fermions are like they are is not understood (?) yet.

 

[UncertaintyAjay]

Isn't that being a bit pedantic? I admit that yes, it isn't the "reason " its just explains it- but isn't that true of all physics? Of all sciences? And true - why fermions behave as they do isn't understood completely yet, but feynman mentions in the Feynman lectures that Pauli did work out a ( complicated) explanation.

 

[sophiecentaur]

Isn't that being a bit pedantic? I admit that yes, it isn't the "reason " its just explains it- but isn't that true of all physics? Of all sciences? And true - why fermions behave as they do isn't understood completely yet, but feynman mentions in the Feynman lectures that Pauli did work out a ( complicated) explanation.

I guess so. But my first instinct is to shy away from 'explanations' when the are presented as a 'deeper' truth - rather than what they usually (always?) are.

 

[UncertaintyAjay]

But sometimes explanations are deep. Look at the PEP- that basic concept- that no two fermions can occupy the same state underlies so many other things in physics and in chemistry.

 

[sophiecentaur]

Absolutely. The PEP applies everywhere. Perhaps I was thinking that the agency through which it manifests itself - i.e. the intermediate step in the explanation is to consider EM forces - is more what is required in the thread. I have to acknowledge that the link to PEP was well worth while stating here, though.
HAHA - I am getting more and more convinced that I was out of order with that comment I made.

 

[UncertaintyAjay]

No- being pedantic is fun sometimes

 

[Tom1996]

This has been used industrially in connecting two metal piece