Why do solids maintain their individual identity when placed together?

[Graeme M]

I was wondering this question the other day and have read a few websites but a lot of it goes over my head. I have two questions, can anyone explain in relatively simple terms? I am happy to read the more detailed science behind the concepts, but need the concepts to be stated pretty plainly.

What makes a solid, solid?
Why, if I place one solid on another, do they not merge? In other words, what maintains the individual identity of two blocks of lead even when placed together? A fluid of the same kind of course does merge.

 

[russ_watters]

Solids are held together by chemical bonds.

 

[Graeme M]

Yes, but why do the molecules in one lump of lead bond to each other, but not to the adjacent lump of lead? What makes the boundary so distinct and persistent?

 

[A.T.]

Yes, but why do the molecules in one lump of lead bond to each other, but not to the adjacent lump of lead?

They might, given a clean smooth surface:
http://en.wikipedia.org/wiki/Cold_welding

What makes the boundary so distinct and persistent?

Surface impurities and lack of consistent contact due to uneven surfaces.

 

[Graeme M]

Well, I'll be... I was actually going to pose the question how my lumps would go if placed on one on top of the other in a vacuum. I hadn't thought of the surface irregularities angle, but I figured a vacuum would remove any possibility for air/liquid obstruction. I like this bit: "The reason for this unexpected behavior is that when the atoms in contact are all of the same kind, there is no way for the atoms to “know” that they are in different pieces of copper. When there are other atoms, in the oxides and greases and more complicated thin surface layers of contaminants in between, the atoms “know” when they are not on the same part."

 

[DrDu]

The merging of solids is a problem in outer space as metals tend to merge which may destroy joints and the like. The point is that here on Earth there are oxide layers which inhibit the fusing of metal surfaces.
In soldering you typically apply some acids to remove these oxide layers.
Similar changes occur on other surfaces like that of glass. But there are some surfaces which merge easily, like organic materials (wax),

 

[sophiecentaur]

What makes a solid, solid?

We usually know what we are referring to as long as we don't push the definition too far.
If you use the naive definition of a solid then there are very few materials that are true solids. The classification of materials into the 'three' states, as taught in elementary Science doesn't take you very far. Most materials will exhibit 'liquid properties' - take the rocks of the Earth's Mantel, for instance, which behave like a viscous liquid under the great pressures exerted on them.
Classification can lead you up the garden path, if you are not careful.

 

[paulina]

I was wondering this question the other day and have read a few websites but a lot of it goes over my head. I have two questions, can anyone explain in relatively simple terms? I am happy to read the more detailed science behind the concepts, but need the concepts to be stated pretty plainly.

What makes a solid, solid?
Why, if I place one solid on another, do they not merge? In other words, what maintains the individual identity of two blocks of lead even when placed together? A fluid of the same kind of course does merge.

the intermolecular force of attraction is the cause for a solid to retain its rigidity... the intermolecular force acts mostly between same material... when you keep two different or same objects they do not merge because to create any bond you need to do some external work...this is the basic reason when you pile up solid they do not merge... and the little work done by you while piling is not enough for them to have the bonds and the forces of attractions...and so retains individual property.

 

[russ_watters]

Yes, but why do the molecules in one lump of lead bond to each other, but not to the adjacent lump of lead? What makes the boundary so distinct and persistent?

The fact that each has its own chemical bonds holding itself together but they don't share chemical bonds.

 

[sophiecentaur]

Two clean surfaces of the same metal could stick together very easily. Metallic bonding is due to the dissociated outer electrons that are attracted to all the positive ion cores in their vicinity. If you start with clean surfaces in a vacuum and can be sure that they are flat enough and under enough pressure to bring a majority of the areas into contact then there is no reason why bonds couldn't exist between both sides. Wrought Iron is made without actually melting the iron; it is just folded and folded and hammered together so that adjacent surfaces actually stick together. That process even works in the presence of a fair amount of Carbon, too.
It would be a different matter to try to get two crystals of a non metal to stick together unless they were brought very near to melting (or by using a solvent).

 

[Graeme M]

sophiecentaur, elementary science is about as far as my education went... :)

Your comment about pressures in the Earth's mantle is interesting and one I was thinking about after reading the earlier comments. If an object is 'solid' it is because the bonds constrain the atoms from being free to move? So with temperature/pressure, as an object heats (forgive my words here, I don't know the right language) those bonds weaken somehow permitting some range of movement and a solid can change to a liquid? So your viscous rocks are in a liquid state, aren't they? I read somewhere that atoms can vibrate, though I don't know what that means. So with increasing heat/pressure increasing the energy potential, does that make the vibrational intensity increase nand overcome the bond in some way?

 

[sophiecentaur]

sophiecentaur, elementary science is about as far as my education went... :)

Reading around can take you way beyond that. Good news - you found PF:). My point is that crude classification can let you down terribly. There's no such thing as an irresistible force or an immovable object but elementary Science often uses such concepts.

 

[Graeme M]

Talking of rocks becoming viscous under great pressure... That pressure is the weight of the crust above. Given that a solid, say my lump of lead, is composed of tightly bound atoms, how does gravitational force get expressed through that medium? The top layer of atoms/molecules will have weight which bears on the next layer and so on cumulatively, but what is it that bears and transmits the weight? Wikipedia says "Metallic solids are held together by a high density of shared, delocalized electrons, known as "metallic bonding". In a metal, atoms readily lose their outermost ("valence") electrons, forming positive ions. The free electrons are spread over the entire solid, which is held together firmly by electrostatic interactions between the ions and the electron cloud."

So if at the submicroscopic level we have atoms bound together by electrostatic interactions, what transmits the gravitational force? Or is the effects of this force a macro level effect for large objects?

In this case I don't mind how detailed you get, I can chase down the meaning of that by doing some searching and reading if I have the right pointers.

 

[enorbet]

It might be worthwhile to note that "solid, liquid, gas" are legacy terms coined before suspensions, colloids, amorphous solids, etc. were understood. Additionally the concept of solid originally assumed physical contact of solid components rather than electrical field repulsion. Dragging legacy into the present is a large, annoying problem of "inertia".

 

[Graeme M]

enorbet, can you expand on what you mean by "Additionally the concept of solid originally assumed physical contact of solid components rather than electrical field repulsion"?

 

[sophiecentaur]

Talking of rocks becoming viscous under great pressure... That pressure is the weight of the crust above. Given that a solid, say my lump of lead, is composed of tightly bound atoms, how does gravitational force get expressed through that medium? The top layer of atoms/molecules will have weight which bears on the next layer and so on cumulatively, but what is it that bears and transmits the weight? Wikipedia says "Metallic solids are held together by a high density of shared, delocalized electrons, known as "metallic bonding". In a metal, atoms readily lose their outermost ("valence") electrons, forming positive ions. The free electrons are spread over the entire solid, which is held together firmly by electrostatic interactions between the ions and the electron cloud."

So if at the submicroscopic level we have atoms bound together by electrostatic interactions, what transmits the gravitational force? Or is the effects of this force a macro level effect for large objects?

In this case I don't mind how detailed you get, I can chase down the meaning of that by doing some searching and reading if I have the right pointers.

Chemistry and the properties of material is all to do with electric Forces. This is because the Electric force is far stronger than the gravitational force. Electric forces are always fairly localised because there are always equal numbers of positive and negative charges around and the forces 'cancel out' once you are a short way from each electron or proton in a material. But there are no 'negative' gravitational forces so they just add up and add up, all over an object, pulling the whole thing downwards,. When the Electric forces on a molecule in a substance can manage to withstand the gravitational forces, it will stay solid. If they are too weak, it will break or 'flow (liquid or plastic behaviour).

Electric forces connect all the particles together and 'transmit a force on one part to another (if the object is rigid.)

 

[enorbet]

enorbet, can you expand on what you mean by "Additionally the concept of solid originally assumed physical contact of solid components rather than electrical field repulsion"?

Sure. My apologies if I was vague but trying to generalize about many cultures over many centuries of time tends to get that way. Without going into what each and every culture thought things were made of, before humans understood anything about the real nature of matter and even well after the concept of atoms was conceived by some and well up into the Middle Ages, people assumed, because of intuition, that whatever elements they believed in (such as Earth, Air, Fire, Water and Aether) could actually come into physical contact with each other. They didn't realize that all matter is well over 99% empty space.

In actuality what we feel is the repulsive field of electrons. This is one way that we can "see" how gravity is so weak by comparison. As is often joked, "it isn't the fall that kills you, it's the sudden stop" :D

 

[Graeme M]

OK, I can see that some kind of electrostatic bonding (again sorry if my terminology is incorrect) binds the atoms and molecules together. But this is an attractive force, and a force is not a thing. If gravity is acting upon, presumably, the atoms, and they are bonded by an attractive force, what actually propagates the gravitational force on each atom such that the object weighs cumulatively more as we progress from 'top' to 'bottom' of the object? And how does that work for gasses which presumably do not enjoy the same attractive qualities (or at least, their repulsive qualities overcome what attractive qualities they have).

 

[Graeme M]

Anyone? or even a pointer where I'd go or what to search on. How does the weight of an object accumulate through the object? Take my lump of lead resting on a table. Presumably the first layer of atoms bears on the next layer and so on, until the force is expressed on the table's surface. But how exactly does the 'weight' propagate through the molecular structure? Given that the lump of lead is largely space, and that the atoms are joined together by electrostatic bonds, what is the conduit for the expression of weight cumulatively? Or is that effect only a macroscale effect? Or am I just completely failing to understand how mass and gravity interact?

 

[sophiecentaur]

OK, I can see that some kind of electrostatic bonding (again sorry if my terminology is incorrect) binds the atoms and molecules together. But this is an attractive force, and a force is not a thing. If gravity is acting upon, presumably, the atoms, and they are bonded by an attractive force, what actually propagates the gravitational force on each atom such that the object weighs cumulatively more as we progress from 'top' to 'bottom' of the object? And how does that work for gasses which presumably do not enjoy the same attractive qualities (or at least, their repulsive qualities overcome what attractive qualities they have).

There's your problem. The electric forces will both attract and repel, depending on how you stress the object. Remember, the electrons are attracted to the positive nuclei but they will repel each other and the nuclei will also mutually repel, if you try to push them together. The equilibrium shape of the object is it shape when the internal forces, alone. are at work. At the bottom of your object on a table, the gravitational forces all add up to the total weight and the object will have distorted a finite amount, as a result.
Imagine, as a model for this idea, a heavy and weak coil spring, which can only just support its own weight on a table. The coils at the top will only have a small weight acting on them from above, so they will be widely spaced. The ones at the bottom will have more weight on them so they will be close together. The spring will not actually be uniform.

 

[Graeme M]

Thanks Sophiecentaur. I am not sure that clarifies things for me. Electric forces attracting and repelling are all very well, but it seems to me neither effect is a gravitational one? The gravitational force is a separate force. Are you saying (in simplified form) that as gravity pulls the topmost atom down, it exerts a repelling force on the one below it, which now has both the repelling force pushing down and gravity pulling it down. Those summed forces are then applied to the one below that and so on?

I can see that, sort of, but that depends on the relationship between the atoms in the solid. That is, they are bound by an attractive electric force but their electric forces repelling each other are how gravity expresses through the solid. I am not sure I see how that works for a gas.

A gas is not a bound thing is it? One of the properties of a gas is that it expands to fill its container hence I assume you can have a very low density gas. A low density gas in a large container would have its atoms relatively homogeneous at equilibrium wouldn't it? And the distance between atoms relatively large. Do those electric forces work over those scale? I thought the idea was that the electric forces are strong but only locally.

Given our gas molecules are free to zip around in that container according to their temperature and density, there is no actual 'bond' between them. So how does gravity affect the gas using the model of weight you describe for a solid?

 

[sophiecentaur]

Thanks Sophiecentaur. I am not sure that clarifies things for me. Electric forces attracting and repelling are all very well, but it seems to me neither effect is a gravitational one? The gravitational force is a separate force. Are you saying (in simplified form) that as gravity pulls the topmost atom down, it exerts a repelling force on the one below it, which now has both the repelling force pushing down and gravity pulling it down. Those summed forces are then applied to the one below that and so on?

I can see that, sort of, but that depends on the relationship between the atoms in the solid. That is, they are bound by an attractive electric force but their electric forces repelling each other are how gravity expresses through the solid. I am not sure I see how that works for a gas.

A gas is not a bound thing is it? One of the properties of a gas is that it expands to fill its container hence I assume you can have a very low density gas. A low density gas in a large container would have its atoms relatively homogeneous at equilibrium wouldn't it? And the distance between atoms relatively large. Do those electric forces work over those scale? I thought the idea was that the electric forces are strong but only locally.

Given our gas molecules are free to zip around in that container according to their temperature and density, there is no actual 'bond' between them. So how does gravity affect the gas using the model of weight you describe for a solid?

Why should they be gravitational? The gravitational forces are between each of the atoms, individually and the nearby huge mass of the Earth. The g forces between the individual atoms are more or less zero. If you took the thing out into deep space, the same internal forces would be there. If you stretched it or compressed it with your hands, it would still distort with no measurable external gravitational influence. You have asked why a solid ' resists' the effects of gravitational forces. It is because it has other forces at work inside it. Both sets of forces are involved. The electric forces are strong but 'local' and the gravitational forces are very weak but all add together because there is only one sign of the force. With a solid, the forces all establish equilibrium by the object taking up a modified shape.
BTW, this all happens for an object that's suspended - but the other way round, with the maximum stretch being at the top and zero stretch right at the bottom.

The gas thing is another issue but the basics must apply. The molecules at the bottom of a container will be ever so slightly closer together because of the fact that pressure is greater - due to the weight of gas on top. Think of the atmosphere getting lower and lower pressure as you go up. The AP grips to a half, every 5000m of altitude or so (very roughly and ignoring all the other effects, such as temperature) due to the 'weight of gas above'. The g field hardly changes at all between ground level and 100k but the AP drops very low by that height. Have you read about the Gas Laws yet?

 

[Graeme M]

OK, I've had a read of the gas laws, and while I generally grasp the concepts, the detail is well beyond me. It hasn't really helped me with my question though.

With the solid, it retains its shape because the local forces overcome the gravitational forces. I understand - speaking very simplisticly - these local forces are composed of several forces. The intramolecular forces which holds the atoms together, and the intermolecular forces which hold the molecules together. The intermolecular forces are weaker than the intramolecular forces. And some kind of 'repulsive' force that stops them getting too close together (or is this repulsion an aspect of the inter and intra molecular forces?).

So that's why a solid is solid.

But I still unclear on the propagation of gravity. If gravity attracts the atoms, then each atom is pulled 'down'. I assume it then pushes on the atom below it which must push on the one below it etc. This 'push' must be via either of the attractive forces I describe above with the repulsive force acting to retain the solid's shape under this pressure. Obviously the pressure can overcome the local forces and cause the solid to deform.

Now, given all the atoms/molecules are bonded together, the force of gravity must propagate sideways as well, internally? While gravity is acting in a downwards direction, the bonds are between all atoms/molecules and thus the force would seem to me to spread throughout the structure.

In the same way that gas pressure inside a container is equal at all points, would not the pressure inside our solid be equal at all points?

This still doesn't help with the gas though. If the bonds are how the gravity or 'weight' is propagated, whatever happens with a gas? The molecules of a gas are not bound to each other. The intermolecular bonds are too weak to overcome the rapid motion of the molecules, which are free to move more or less at random.

I understand that the effect of gravity on gasses is to change the average distribution - more molecules lower in the atmosphere, or alternatively fewer as you go higher and urther from the earth.

The effect is then higher pressure at lower altitudes with the highest pressure at sea level (ish).

But that pressure is not actually 'weight' in the way that weight is expressed by a solid, IF my description above is correct.

What's really going on?

 

[Graeme M]

I've come back to this one and it reminds me that I generated a long thread here a while back about atmospheric pressure. The end result was that I learned that the weight of the atmosphere is essentially equal to surface pressure and that 'weight' was transferred through the vertical column by way of the energetic collisions of the air molecules. That still bothers me though.

Now, given what I have learned on this topic, I offer the following pretty simplistic explanation of how a solid is solid. As I summarised above from what other commenters have offered, a solid is solid because attractive forces hold the molecules/atoms together. I assume that the resistant forces of the particular composition of the solid determine the density of the object - that is, the closer together and more molecules/atoms there are per volume then the denser the object.

Liquids are different and deform more readily than a solid. I think the reason is that the bonds are less strong and so the molecules in the liquid tend to be bonding and unbonding rapidly as the molecules move around.

A gas is much more energetic and the molecules/atoms do not bond at all. As a result a gas tends to expand to fill its container in contrast to a solid or a liquid which are more tightly bound.

I am still left with the question of weight. Is weight propagated in a solid as I imagined above? That is, gravity pulls the atoms down and each atom then pushes on the atom below it which must push on the one below it etc. This 'push' must be via either of the attractive forces I describe above with the repulsive force acting to retain the solid's shape under this pressure. Obviously this accumulating pressure can overcome the local forces and cause the solid to deform.

But from my earlier discussion around gasses, I learn that weight in a gas is propagated via the collisions of the molecules applying force cumulatively from top to bottom. But I still struggle to grasp how these collisions constitute both pressure and weight. I posed the question last time of a safe in a vacuum, and I don't think this was ever resolved as the conversation branched off to other details.

The equation PV=nRT describes the derivation of pressure, but this makes no use of mass or gravity. This equation tells me that pressure depends on number of molecules, volume and temperature to give rise to pressure, and my reading tells me that pressure in a container is equal on all walls. And the pressure on the walls is the impact of molecules per unit area of surface. So what particular component of that pressure is just pressure arising from temperature, volume and number of molecules and which is from weight?

Going back to my safe in a vacuum - if I transport a safe containing normal old air at 1 atmosphere into a vacuum and maintain internal temperature, will the pressure change or stay the same? I assume the same, but if so, what component of that pressure is derived from the 'weight' of the overlying atmospheric column which is no longer present?

I just cannot get my head around this question of weight of an object versus a gas. And a liquid too, come to think of it.

 

[sophiecentaur]

I just cannot get my head around this question of weight of an object versus a gas. And a liquid too, come to think of it.

Where is the difference? The weight of the air in a column happens to be spread evenly over the floor but you could achieve this with care, if the column were solid. The force on a piece of the floor is due to the sum of weight forces of sections of the stuff above, transmitted by direct action via the inter molecular forces in a solid or by change of momentum during the collisions of the molecules in an ideal gas. The force on a horizontal element of a vertical bar / column is due to the weight above and the same goes for gas in a vertical cylinder. It increases as you go down because there is more stuff above. This force plus the weight of the section acts downwards on the next lower section and so on . . .

Btw, in an ideal gas, it is assumed that there are no 'bonds' between the molecules - or, in reality the attractive or repulsive energy involved is significantly less than the kinetic energy of the individual molecules. When they collide, they lose no energy (elastic collisions) and their Momentum is exchanged. There is increasing pressure as you go down (density increases) so there are more collisions per second. That's a circular statement, I know, but if you think of the effect of gravity over a thin horizontal slice will be to accelerate the molecules downwards so they will transfer more momentum downwards than upwards, resulting in a net upwards reaction force on the slice. This difference will just balance the total weight of the molecules in the slice, acting downwards.

 

[Graeme M]

I am not trying to be deliberately dim here. I get what you are saying, just as I got it last time. I can see how the molecules collide etc. But where I run into trouble is the idea that pressure equals weight. You are arguing for an accumulation of weight in addition to pressure when we talk of a gas.

The weight of a solid is something tangible - on a scale it can be measured directly. But only 'down'. That is, a solid has weight when I measure it from beneath. Not to the side, and not above.

A gas though has a pressure which is equal on all sides. If I have a container of gas, its pressure appears to be purely an outcome of three properties, none of which is weight directly. The force I measure from beneath is the same as from the sides and above, which is quite different to a solid. There is no 'downward' component for a gas which differs from its sidewards or upwards component.

As I said above, wouldn't a sealed container of air at ground level have the same pressure at 20,000 feet, or at 10 miles or out in interstellar space, all other factors (ie V, N, r, T) being equal (excluding gravity).

That's not gravity at work creating that pressure. So pressure cannot equal weight for a gas.

Edit: Further to my container in space. V, N and T are the deciding factors. If I take a container from ground level into space and keep T the same, but ADD molecules, I am changing N. So pressure must increase. or, if I just make my container larger, I am increasing V which reduces pressure. What part does gravity, or weight, play there?

 

[sophiecentaur]

Gravity is pulling every air molecule down. If there were no gravity, the air would just disperse into space. The pressure would drop and drop. The fact that the molecules don't all drop to the ground on Earth is because the pressure (due to thermal motion and collisions) is the effect of each molecule against its neighbours. When a substance is below its boiling point and particularly its freezing point, the molecules are very close together and thermal motion is not enough to break bonds. Their spacing then is mainly set at the equilibrium spacing (when attraction =repulsion). But a tall solid column gets compressed near the bottom so attraction and repulsion are also influenced by weight. In an ideal gas there is no attraction - just elastic collisions when molecules collide and the pressure is due to the frequency and speed of collisions on any real or virtual wall.
You have a problem with the downward and upward pressures in a gas. Yes, they are considered the same and that's the mantra we are taught at school but they are only the same for an infinitely thin slice (i.e. at exactly the same level). Like I said before, as soon as the slice has any thickness, molecules are accelerated downwards so they would actually be going downwards faster at the bottom than molecules even just 1cm above them are going upwards. They are all subjected to acceleration g. At any particular level you are getting the effect of all the slices above plus the small addition of the pressure difference across the slice. Here comes another factor, though. If the cylinder is thermally insulated, the temperature will even out over the column so, rather than having the molecules going faster at the bottom, they will be going at the same speed and their density will increase. (Even over a thin slice)
In a container, under pressure but out in deep space, there would be no pressure difference over the extent of the gas; the gas laws would give the same answer everywhere - but, of course, you need a container to provide the pressure from its walls to stop the gas from getting away. On Earth, there is gravity pulling the molecules towards it. If you bring the container to Earth, there will be greater pressure at the bottom of the container than at the top because, as before, they will be hitting the floor faster then the ceiling and they will be closer together at the bottom than at the top. After an initial settling time, the bottom will no longer be hotter but there will still be a density gradient
Re-reading what you have written I suspect that you think people have got this wrong and that you have exposed a fallacy. You haven't. If you rigorously follow the reason and consider everything involved, you will come to the accepted view.

You wrote: "That's not gravity at work creating that pressure. So pressure cannot equal weight for a gas."

This is false reasoning. Yes you can create pressure by using a container but that does not imply that its the only way of creating pressure. The alternative way of creating gas pressure is by attracting each and every molecule to a central point (gravity). In both cases, the molecules are constrained into a finite region. The constraining influence is just different.

 

[Graeme M]

Thank you for your reply sophiecentaur. A quick disclaimer - I am not trying to expose a fallacy. I apologise if my style sounds like that. Clearly, air is matter, matter has mass, mass weighs something, so the atmosphere has weight.

This discussion is, like the earlier one I referred to, very interesting and illuminating. I have learned a lot through both. In particular, this current one led me to read up on matters of inter and intramolecular forces, bonds, and gas laws. I now have a fair but basic idea of why a solid is what it is.

My education was pretty basic when it comes to science, and in my everyday life I certainly never run into deeply complex matters around the physical world. And of course, there are a million such things that I can't hope to understand. I just wanted to have a go at understanding this one.

If you tell me X, then I have a fact. But I don't necessarily understand that fact. Having a fact in mind is not really having understanding in mind.

In this case, years ago I happened across Miles Mathis' site and read a few of his articles. I have since read a lot of stuff debunking him. I can't hope to understand how right or wrong he is, but I did read his article on atmospheric pressure. That is something I have everyday experience of, so I thought about what he said and it led to that earlier thread about air pressure.

Where I am at now is that I think Mathis' article was utter rubbish - he seemed entirely ignorant of pretty much everything that you and other commenters here have explained. But I am still left with the central mystery of why pressure equals weight.

This is just one fact that I hope to unravel enough to understand, for no other reason than it is an interesting thing to grapple with. My discussion style is I suppose argumentative - I want to put my opposition or misunderstanding forcefully and have it solidly demolished. As that happens, I gain insight. I don't gain insight by just saying "Oh is that how it works".

So my apologies if it seems like I am trying to prove something. I'm not. I am seeking understanding in one tiny matter of no real consequence to anyone else. It is exercising my aging brain, if you like.

So if you or anyone else is willing to tolerate what must seem a tedious discussion, I'd like to continue.

Now, your reply above seems to me full of contradictions. I will try to articulate those later when I have the time to sit down and put them together. Again, I am not trying to argue with you, I am just trying to expose the weaknesses in my own mental construction...

 

[Graeme M]

Thanks again for your most recent reply sophiecentaur. It has actually been a help - I spent some time this evening thinking it through. I feel that you have some contradictory statements in there, but I suspect that just arises from my own limited understanding of some of the detail. Every time I dig into this stuff I uncover another layer of complexity and pretty soon I get well out of my depth...

Anyways, here's a couple of things that I thought seemed inconsistent.

You suggest that in fact there IS a downward force for any slice of atmosphere larger than a very thin slice. That implies a variation in air pressure over small scales, yet in the earlier thread, yoshtov claimed that there is almost no difference in air pressure over distances of 1 or 2 metres.
What is air pressure?

I guess 'almost' might be the operative word here? But 1 metre is a lot more than an 'infinitely thin slice'.

regardless, I still don't understand where this downwards force is coming from if the pressure responds to PV=nRT. A very small slice, or a larger slice, has a particular volume. According to my limited take on that equation, pressure is fixed on each surface, real or virtual. I still can't see weight in there anywhere.

I can see that gravity is pulling molecules down, hence on average density decreases with height. My 'n' is getting smaller, hence pressure reduces with altitude, or in reverse, pressure increases as we get lower. Again, this seems to be purely a function of volume, number of molecules and temperature. Gravity might on average increase my number of molecules, but where is this force we can call weight?

Next you suggest that for anything more than a very small slice of atmosphere, the molecules going down will go faster than those going up. That seems at odds with the idea of all molecules exerting the same pressure on a given surface area. Take my sealed container - the molecules respond to the fixed pressure (if n, V and T remain fixed) by bounding around energetically. If the ones going down have a greater force due to gravity, that suggests that a barometer in there will detect that effect. Yet it does not.

But then you seem to contradict yourself in the next sentence where you argue that for a thermally insulated container, the temperature will even out and all molecules will now go at the same speed over the whole column. But what happened to gravity? It was accelerating the molecules down a moment ago, but now with an even temperature profile it has no effect? What exactly did you mean here?

Your next paragraph actually gets right to the nub of my conceptual difficulty. You describe the effect of my container in space - a homogenous distribution of gas and pressure. On return to earth, we now have a pressure gradient. But this is what I said earlier - the effect of gravity is to increase density at lower levels, which introduces a pressure gradient. But that's not weight, at least it isn't what I understand by the term 'weight'.

Of course the molecules have 'weight', so too the parcel of air in our container. I am not arguing against that. But I mean weight in a Newtonian sense - that is, something I can measure as weight.

I am not convinced that the pressure we are measuring in my container is weight.

 

[sophiecentaur]

Sorry for sounding grumpy and if my last post seemed full of contradictions. But it is a convoluted business and there have been many argumentative threads about the situation in the atmosphere because of that.
You seem to accept what I have written about the pressure gradient so you are almost there.
You wrote:
"But that's not weight, at least it isn't what I understand by the term 'weight'."

I have to ask what you actually do understand by the term. Your 'weight' as a person, is normally described in terms of the force your two feet will apply to the bathroom scales. The 'weight' of the atmosphere above you is precisely the same - with one exception and that is in how you would measure it. The fact is that you also have air pushing upwards on the bottom of the scale pan, which will balance out any displacement of the pan. If you fit (seal) your scale pan into the top of a cylinder and remove the air in it you will have no upward pressure and only the downward pressure will give a force on the scale pan. The scales are measuring weight, just the same as if they are measuring your body weight - by the definition we normally accept as weight. Ignoring the non uniform gravitational field effect, if you took all the molecules in that very tall vertical cylinder and squashed them into a small volume container, they would 'weigh' exactly the same amount. There is no pressure out in space so there would be no additional forces acting on that tall cylinder (at the upper, open end). Consequently, there can be no more or less force on the scale pan, however you present it with all those molecules.
If you can accept that the addition of all the vertical forces (weights) in a gas is just the same as the addition of the vertical forces in a vertical solid then you will have cracked it.

All these discussions of gas pressure are based on an assumed 'virtual' surface which could be placed between one volume of gas and another adjacent volume. You see diagrams of cylinders with gas either side and a piston in the middle. The piston does not necessarily need to be there, in many cases - it just helps in the calculations. Likewise, a bag of air will behave as if there were no bag present (until diffusion sets in) but when we talk about pressure in the air, we are, to some extent, assuming it's in a container which has no pressure difference across the membrane. (Not a rubber balloon, which introduces its own forces.)

PS If you come across any website or book that picks a fight with conventional Science on such a broad front as Miles Mathis' site then it just has to be cranky. People moan about school textbooks 'getting things wrong'. There are small errors all over the place in school books but the overall picture is fine. Also, you need to compare books against books and not what you 'learned' at school with what you get in some random person's ravings. Likewise, don't just take my word for all this. Check it out against Hyperphysics and other reputable websites.

 

[russ_watters]

I am not convinced that the pressure we are measuring in my container is weight.

I think the main problem here is that you are bouncing around between many different scenarios, then mixing and matching answers that may or may not apply to each.

If you want to discuss a closed container on or near earth, then it will have two sources of pressure:
1. The momentum of the molecules.
2. The weight of the molecules.

Depending on the specifics of the scenario - which you haven't specified and keep changing - one may be much more relevant than the other.

Try this: pick one scenario, make it very specific, and mathematically identify the sources of pressure.

 

[Dale]

On return to earth, we now have a pressure gradient. But this is what I said earlier - the effect of gravity is to increase density at lower levels, which introduces a pressure gradient. But that's not weight, at least it isn't what I understand by the term 'weight'.

You seem to have this backwards. Gravity introduces a pressure gradient as a primary effect. The density changes only as a consequence of the material response to the pressure. Many materials are incompressible and their density changes very little if at all in response to the pressure gradient. But there is a pressure gradient in any static material whether is compressible or not. That pressure gradient is equal to the weight density of the material.

 

[A.T.]

You seem to have this backwards. Gravity introduces a pressure gradient as a primary effect.

Or maybe the whole cause-effect thinking is pointless here?

 

[Graeme M]

sophiecentaur, thank you for your comments throughout this thread. The paragraph in your earlier post regarding sources of pressure has helped me crack this. As I said earlier, I have limited science education and I think I use terms that have a specific meaning to you which they don't have to me, so there is perhaps some level of missed communication.

That said, in tussling with this overnight, I came up with a way to explain the whole thing that makes sense and answers all my objections (well most of them anyway). I will try to form this model into a succinct model to share with you, but I need to craft my words carefully.

What helped most were these words:
"Yes you can create pressure by using a container but that does not imply that its the only way of creating pressure. The alternative way of creating gas pressure is by attracting each and every molecule to a central point (gravity). In both cases, the molecules are constrained into a finite region. The constraining influence is just different."

I had been worrying over why a container of gas on the Earth's surface would have the same pressure in space without the 'weight' of the atmosphere. What you have made clear to me is that the pressure is 'set' on Earth's surface by the fact that the parcel of air in my container has a particular density of molecules at a particular temperature. This remains in my container no matter where it is - the container walls act to constrain my molecules and the conditions in the container then remain the same. Hence the pressure is the same. The weight of the atmosphere led to the pressure in the first place, but the walls of my container act to preserve it.

DaleSpam, I cannot quite follow your comment, i think it's that terminology thing again. You seem to mean something different by density to what I understand that to mean.

All, I will return later with my mental model that explains this to my satisfaction while incorporating the learning you have given me. I'll be most interested in your opinions.

 

[Graeme M]

Oops, one more thing. Sophiecentaur, what I mean by 'weight' is indeed central to this. In your last post you make the point that we could measure the weight of the atmosphere with a special sealed scale. But we already have such a thing and it's called a barometer. I would argue that your sealed scale would read the same whether I placed it up down or sideways in my loungeroom, because it is measuring air pressure. The weight of the atmosphere gives rise to the pressure, but the pressure is not the weight. Or so it seems to me. And I have an idea to explain why it works that way, I just have to come up with a way to put that into words.