Why do solids maintain their individual identity when placed together?

[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