**Ask a monkey a physics question thread**

[kingnat]

http://www.nfb.ca/film/how_to_build_an_igloo/

[kingnat]

I feel compelled to point out that an "igloo" is a cartoon invention, as far as I can tell. I would LOVE to see a link to any indigenous people using an igloo as a normal part of their daily lives. I found lots of variations on the practice of making a temporary shelter out of ice/snow, but none of them are what I'd consider an igloo.

Also, having made and used a snow shelter in Boy Scouts, I can tell you that if you can't keep the wind out, you've wasted your time. If you're going to sleep in it, you'll want a (at least mostly) sealed door, and a small roof vent so you can put a camping stove or small campfire inside. This will cause the walls to slowly melt, but so slowly that it's not relevant. However, the water runoff will ruin your comfort if you don't prepare for it. 1) The door to the igloo goes downhill. 2) carve a fist-sized groove in the floor around the wall on the inside and out the door. 3) Never put anything in the groove.

I was talking to friend about this exact issue of wind. I was thinking about making a wall on the interior to serve as a tunnel and then have a cover for the entrance. As it is, we made the exterior hole, WAY too big.

[MadMojoMonkey]

http://www.nfb.ca/film/how_to_build_an_igloo/

No tunnel entrance = not what you're making, which is what I think of when I think "igloo".

[kingnat]

No tunnel entrance = not what you're making, which is what I think of when I think "igloo".

I was posting that link in response to this:

I feel compelled to point out that an "igloo" is a cartoon invention, as far as I can tell. I would LOVE to see a link to any indigenous people using an igloo as a normal part of their daily lives. I found lots of variations on the practice of making a temporary shelter out of ice/snow, but none of them are what I'd consider an igloo.

We don't have such an igloo, we have the iconic cartoon like structure, but yes, i realize we need something to block up the door to provide some heat... and I think the heat source on the interior (assuming everything is sealed properly) shouldn't need to be dramatic to provide noticeably higher temp than the exterior.

[MadMojoMonkey]

... and I think the heat source on the interior (assuming everything is sealed properly) shouldn't need to be dramatic to provide noticeably higher temp than the exterior.

Yes, it doesn't take much.

You can put a few cold rocks in the center. Warm some other rocks in a campfire (make absolutely sure they're not flint, which will explode like a shrapnel grenade when heated in a fire). Place the hot rocks in a metal bucket, and place metal bucket on the cold rock-bed in the center of your shelter. You will want an extra set of rocks being heated at all times, and you'll switch them out. This is a very safe solution, but will cause you grief in the middle of the night when no one's tending the spare heat stones.

You can Google "flower pot space heater" and see some DIY ideas for a candle-powered solution.

It's hard to find a safe solution that will last for the 6 - 8 hours needed for uninterrupted sleep.

[Galapogos]

http://www.nfb.ca/film/how_to_build_an_igloo/

Wow, grade school flashback. I actually had to watch this in class in grade 5 or a year around it wayyyy back in the day.

[chemist]

An upmarket igloo or perhaps an eskimo's holiday resort.
http://www.icehotel.com/accommodation/winter/

Utterly amazing, but personally I would prefer a warm bed.

[chemist]

I feel compelled to point out that an "igloo" is a cartoon invention, as far as I can tell. I would LOVE to see a link to any indigenous people using an igloo as a normal part of their daily lives. I found lots of variations on the practice of making a temporary shelter out of ice/snow, but none of them are what I'd consider an igloo.

Also, having made and used a snow shelter in Boy Scouts, I can tell you that if you can't keep the wind out, you've wasted your time. If you're going to sleep in it, you'll want a (at least mostly) sealed door, and a small roof vent so you can put a camping stove or small campfire inside. This will cause the walls to slowly melt, but so slowly that it's not relevant. However, the water runoff will ruin your comfort if you don't prepare for it. 1) The door to the igloo goes downhill. 2) carve a fist-sized groove in the floor around the wall on the inside and out the door. 3) Never put anything in the groove.

Shelter
The Inuit were semi-nomadic. On winter hunting trips they built igloo out of compacted snow. Two men could build one igloo in an hour. Igloo let light in, kept warm inside and kept the wind out. In summer tents were built from walrus skins.

http://ecx.images-amazon.com/images/...L._SL1500_.jpg
Is this the link you would love to see?

[MadMojoMonkey]

Shelter
The Inuit were semi-nomadic. On winter hunting trips they built igloo out of compacted snow. Two men could build one igloo in an hour. Igloo let light in, kept warm inside and kept the wind out. In summer tents were built from walrus skins.

http://ecx.images-amazon.com/images/...L._SL1500_.jpg
Is this the link you would love to see?

It's a good pic, but really grainy. I can't tell if the shape(s) on the right are separate "iglooyas" or the tunnel entrance I'm not seeing. I'm specifically fixated on the tunnel entrance to an ice shelter... I'm willing to accept snow for ice. The reason is that, I think, whenever I say the word "igloo" to a random person, they imagine that tunnel entrance... not a snow/ice wigwam.

When I was young, someone told me the tunnel was where the sled dogs slept... but I've heard nothing similar in 20+ years, so I don't know if it's worth believing.

The caption explicitly states that an igloo is a permanent structure, made of stone.

[MadMojoMonkey]

Can you concisely explain states of matter and how boiling and freezing points work? Like taking water for example, it's freezing point is 0 degrees celsius. Does that mean that any water that reaches below 0 is automatically ice? Similarly is any water that reaches the boiling point steam instantly?

Preface:
I fear that the word concisely is going to give me some trouble. I am pressed to fully explain P-T diagrams (much easier to point at as a result than to teach you to make) as well as a thorough explanation of entropy, enthalpy, internal energy, and more. These concepts take a few hours (if not weeks) to wrap your head around. They are forms of energy which matter utilizes. If I don't explain them and account for them, then you'll never be happy with my description of phases of matter. Without them, I can't really teach you about steam tables... I can only do some hand waving and "trust me" teaching... and an understanding of steam tables is what you're really driving at.

Renton, I strongly encourage you to buy a Thermodynamics book. I feel confident that you'll be flipping the pages and reading it out of personal enrichment and curiosity of the subject matter. It'd be well worth the purchase price.
[/preface]

Freezing & boiling:
We touched on this before when you asked me about water's freezing point in #157. Here's my initial reply in #159.

I think the one thing I want you to know about this is that a freezing or melting point requires BOTH a temperature AND a pressure.

So the freezing point of water is 0 C at 1 atm (or 1 times the average pressure of the atmosphere at sea level, 14.7 psi or 101 kPa). If the pressure goes up or down, the freezing point will change, too.

The same is true for boiling point. At high altitudes, most recipes offer substitute cooking instructions. This is mostly due to the decreased atmospheric pressure causing water (and everything else) to boil at lower temperatures.


OK, so let's talk about the "all at once" part of your question. Let's start with a picture. When I was a freshman in HS, my Physical Science class did an experiment with ice and water.

We put ice into a beaker and put that beaker on a hot plate. We measured the temperature of the ice and then turned on the hot plate. We stirred the ice and monitored the temperature of the ice/water in the beaker until it was boiling for a few minutes.
Here's an image that shows the theoretical results:

The horizontal axis is time. The vertical axis is the temperature of the ice/water in the beaker. The rate of energy produced by the heat plate and absorbed by the beaker is assumed to be constant.

You can see that while the matter is changing phases, it doesn't change temperature. This is true as the ice melts and again as the water evaporates.

OK, so why?

Temperature is just a fancy way of measuring and talking about vibrations on the molecular and smaller scales. Temperature is vibration. Something cold is vibrating less than something hot.* Heat is the flow of temperature, or the translation (movement) of vibrations (temperature). In order to change temperature, one vibrating thing has to bump into another vibrating thing. They exchange vibrations and the result is that both particles have an amount of vibration that is closer to the average of their vibrations... that sounds so dense....
How about: 2 particles bump into each other and are more similar to each other in temperature afterward than before. The total vibrational energy is the same, but the low energy one gained some and the high energy one donated some.

OK, so now we understand that temperature is really really just kinetic energy in the form of oscillations on the micro scale of molecules and smaller, and that temperature is passed by collisions. We also know that the collisions tend toward an equilibrium between the involved things.** It is also true that temperature can be gained or lost by emitting/absorbing photons, and maybe even other mechanisms that I'm not thinking of at this moment, but the primary mode of thermal exchange in this example is collisions.

OK, we're getting close.

The inter-molecular bonds which hold the water molecules in a bond with their neighbors have a maximum range. Well, the attractive force decreases with distance, and at a certain distance is negligible. The magnetic dipole moment of H2O is the primary contributor to this bonding in the phase of liquid water. (It is the hydrogen bonds which dominate in ice; there are no dominating bonds in steam.) The dipole moment of each molecule makes it want to align with its neighbors, as, by doing so, it is in a lower energy state than if it weren't aligned.

Basically the material, in this case fluid, bond is like tying a thin thread to hold something together. If it's pulled on hard enough, it will break. When the temperature of a particle increases to a state where it is vibrating beyond the range of the bonding force, then that particle will break loose of it's bond. In this case, a water molecule absorbs a bit of energy, causing it to vibrate slightly more than it was before. However, just before, it was vibrating as much as it could while still maintaining the fluid bond. Once the energy was absorbed, it could no longer maintain the fluid bond, so it evaporated. In evaporating, it carries energy away with it, cooling the matter it leaves behind ever so slightly. (Sweat glands FTW)

This must happen to every molecule individually, so the total amount of heat energy needed to make this happen would have to come instantaneously for this to happen instantaneously.


***
Linking to some higher concepts:
*If that something is charged, like, say, a proton or electron, and it is vibrating, then it's creating waves in the electromagnetic fields, which are photons... blackbody radiation! If it has non-zero temperature, then it radiates!

**If the vibrations of 2 interacting particles approaches the average vibration between the 2 particles, then no finite number of interactions could ever bring a vibrating particle to be completely still... I.e. nothing can be cooled to absolute zero in any finite number of steps.

[MIKEovMANTON]

Cool thread.

Where is the centre of the universe? Science tells us that anywhere can be considered the centre, but that makes absolutely no sense. If the Earth is at the centre and the CMB is at the edge (of the visible universe at least) and I travelled 1billion light years towards the CMB, I would surely not still be able to class myself as being at the centre of the universe? or would I?

[MadMojoMonkey]

Cool thread.

Keep that in mind as I'm about to pick on your choice of words. (Note that I'm not picking on you, just the sentences.)

Where is the centre of the universe?

I don't even know if this is a reasonable question. Where is the center of the surface of the Earth?

Science tells us that anywhere can be considered the centre

No, I have to stop you there. Science tells us a procedure to answer a very specific category of questions, those with measurable answers. Science doesn't tell us anything about the universe. Science tells us how to check if someone's claims about the universe are in line with our own observations.

Scientists tell us about the universe. Science tells us how to evaluate those claims made by scientists. Science makes no claims (other than the implicit claim that science itself is a viable process); science provides a system by which claims are verified or refuted.

anywhere can be considered the centre, but that makes absolutely no sense. If the Earth is at the centre and the CMB is at the edge (of the visible universe at least) and I travelled 1billion light years towards the CMB, I would surely not still be able to class myself as being at the centre of the universe? or would I?

Yeah. I agree it makes no sense. It's difficult to imagine anything outside of space-time. Nonetheless, space-time is expanding. Does that mean that there is something besides space-time that space-time is expanding into? I don't know. Physics does not answer questions about unobserved things.

The CMB is not stationary, it is the photonic residue from when electrons transitioned from one spin state to another during the ridiculously early universe. Photons are light, and they travel at the speed of light. The CMB is expanding outwardly away from you at the speed of light. No conceivable travel you could do for any amount of time would change the fact that the CMB makes a sphere around you. No matter what you do, you will never be any closer to the CMB; it will always be receding away at the speed of light, and you can't go that fast.

More fundamentally, we have observed a miniscule fraction of the universe, both in scope and in duration, and we just don't know how to answer a question like "Where is the center of the universe?" We don't know the fundamental topology of the universe, so we don't know what "shape" space-time is. We don't know how many dimensions it truly needs to appear as it does to us in 3 spacial dimensions and 1 time dimension.

What if the universe as we perceive it is really the surface or a projection of some many-dimensional form? The notion of a center is meaningless, then.

***
Why does the idea of the center of the universe fascinate us? It seems to me that it would be just like any other part of the universe.

I think scientists think they're clever when they say the center is everywhere. I understand the logic. Big bang = everything is one place (never mind that no one knows what that really means), so if everything was in one place... and it's not everything that moved, but the place that grew... then everything exists exactly where it did in the beginning, and it's merely the growth of space-time that made the universe bigger. So everything is right where it started, which is the center.

I mean... come on, right? I find that a less comfortable answer than... well, we're too ignorant as a species to answer that question or even know if it's a reasonable question. Is it that hard to say "I don't know."?

***
Once again, please don't let my harsh treatment of this question put you off. It's a borderline-physics question, and it's one that I can't easily answer or explain why I can't answer or say "that's not physics"... it's a tough one, and a good one for all of those reasons.

[Renton]

MMM thanks a lot. I remember learning about heat of fusion/vaporization, I just always thought that occurred in transit from 0 degrees to 100, not while the whole liquid simmers at 100.

So this means when a pot of water has just started to boil, the contents are exactly 100 degrees (give or take a smidge)?

[MadMojoMonkey]

MMM thanks a lot. I remember learning about heat of fusion/vaporization, I just always thought that occurred in transit from 0 degrees to 100, not while the whole liquid simmers at 100.

So this means when a pot of water has just started to boil, the contents are exactly 100 degrees (give or take a smidge)?

I'm facepalming that you didn't mention pressure of 1 atm, but yes, you have it right.

Anything dissolved or suspended in the water will have an effect, but in general, these effects are not too very drastic. You have to add an insane amount of salt (as far as taste is concerned) to water to increase it's boiling point by even a degree.

[MadMojoMonkey]

Upon re-reading your post, Renton, I want to clarify something.

You said, "[...] when a pot of water has just started to boil, the contents are exactly 100 degrees (give or take a smidge)"

This is true the entire duration of the water boiling, regardless of how rapid the boiling, not just at the beginning. The water is 100 C just when it starts to boil, and stays 100 C up to the moment that ALL of the water has evaporated.

Water (pure), in the form of liquid, at 1 atm, can not be hotter than 100 C. The rate of boil does not affect the temperature of the water. A tediously slow simmer is 100 C and a tumultuously rapid boil is 100 C.

***
Now... I have gotten schooled when it comes to cooking. The above is fine for boiling pasta (very close approximation), but when it comes to soups and other dishes, the fact that the water has so much stuff in it changes things a bit.

When there are large chunks of food in the water, they absorb heat and restrict mixing. So the bottom of a large pot is likely hotter than the top. In general, this is not the case with pure water, which mixes rapidly, distributing the heat throughout the fluid.

Not to mention that a rapid boil can pulverize potatoes and other "soft" foods.

I feel like there is more going on in cooking, but I feel comfortable (jokingly) saying that cooking is magic, so clearly not physics.

[Renton]

OK that makes a lot of sense. So when you put broccoli in boiling water, it soaks in some of the heat that would ordinarily have contributed to the heat of vaporization of the water, thereby making the broccoli potentially much hotter than 100 celsius and retarding the rate that the water boils. I guess that's why it is so common to turn the heat down once something begins to boil, cause then you know you've got a fairly consistent temperature with which to cook the broccoli/whatever.

[MadMojoMonkey]

Nothing that you put into the water will be hotter than the water, provided it's floating in the water. (If it's resting on the bottom of the pan, where the heat is applied, then blah, blah, blah)

If the broccoli were hotter than the water, then heat would flow out of the broccoli into the water.

Heat never spontaneously flows from cold to hot. Kind of like saying water doesn't spontaneously flow uphill, or current doesn't spontaneously flow from low voltage to high. In each of these cases, you can drive it to do this "unnatural" flow, but it will always cost you more than you get out of it.

I.e., When you force 1 J of energy (in the form of heat) to flow from a cold thing to a hotter thing, then you will use more than 1 J of energy (in any form) to accomplish this feat.

[Renton]

So is the fact that nearly every instruction calls for turning down the heat once boiling start indicate a thermodynamic fallacy? Or is it just better to reduce the rate that the water boils off?

[MadMojoMonkey]

I don't understand the fallacy part.

I think it's primarily to reduce wasting heat since you can't change the temp, just the rate of evaporation, like you said.

Also, in cooking, a rapid boil can pulverize the food. Probably other stuff, too. Cooks know magic, and they will argue with me to no end when I try to tell them what's up. Bottom line is that they know their trade, and I know theories which overly simplify things when I'm applying my knowledge lazily.

[wufwugy]

I believe I have finally found the answer to the ultimate question -- you know, the one that goes like "who/what created the universe, what's the purpose to existence, etc?"

Find evidence for that sort of thing in the first place. Like, is there any evidence that creation happens? The answer is no. Everything that gets "created" is just some form change. Likewise, what evidence is there for the existence of ultimate purpose? There's none. Existence does what it does because that's what it does, and we're unsatisfied with that answer because we're unsatisfied with that answer

[wufwugy]

Horribly, terribly, genocidally interesting: http://www.reddit.com/r/science/comments/21cplp/time_is_an_emergent_phenomenon_that_is_a_side/

Not sure how accurate, but the concept of time as emergent instead of real just seems too reasonable

[jackvance]

That probably explains reality. We're seeing a 3-dimensional cut from a (say) 10-dimensional reality and we get the impression time is advancing while nothing is actually happening, we're just observing different states of the same thing. That's how I understand it at least.

[Savy]

Not sure how accurate, but the concept of time as emergent instead of real just seems too reasonable

Based on your deep understanding of physics? lol

That probably explains reality. We're seeing a 3-dimensional cut from a (say) 10-dimensional reality and we get the impression time is advancing while nothing is actually happening, we're just observing different states of the same thing. That's how I understand it at least.

tl;dr version - I don't have anything insightful to say

[wufwugy]

Based on your deep understanding of physics? lol

did you just wot m8 me?

[jackvance]

Shh he's really sensitive about time.

[MadMojoMonkey]

Quantum Mechanics explains what happens at the ridiculously small scales. The actual physics going on at that scale is so far removed from our daily observations and interactions with the universe that it's stupendously hard to make emotional sense of it.

This is what's going on here.

There are multiple interpretations of QM which are drastically different in concept. They are attempts to make psychological order appear in a universe of partially restricted randomness.

How can a thing exist equally (or unequally) in 2 states at once? How can a particle simultaneously have 2 different energies?

This kind of question makes our heads twitch. We want to exclaim, "This is a lie! The particle must have a single energy. Perhaps it's not clearly defined." However, the math does not support an unclearly defined energy, the math supports a simultaneity of energy states. Experiments bear out that it is a clearly defined property of particles to exist in multiple states.

So. The reality that is observed at this scale is at odds with our macroscopic reality. This drives people to come up with hand-waving explanations for why it is what it is... but the math already tells us what it is... it's just hard to accept and even harder to communicate without the math...

This makes interpretations which are approachable by laymen obtuse.

Ask yourself this:
If time is emergent... an illusion caused by some quirk of our human experience... What difference does it make?
What are the implications?
Any?

[wufwugy]

Question: what is impossible?

More specifically, if there are infinite universes with infinite timelines and infinite possibilities, doesn't that mean everything exists? Doesn't that mean there is some universe that's exactly like ours except instead of humans it has talking chairs that eat spaghetti phones? But if that is possible, then, um, how is it possible, because, um, that's not fucking possible

[Savy]

Something which has a 0% probability doesn't happen.

Unless you want to assume different laws of nature blah blah blah in which case not science.

[wufwugy]

What has 0% probability? If there are an infinite number of different universes/timelines, doesn't that mean nothing has 0% probability? If there are infinite unique universes, then by the time you have every universe with all the things you say are >0 probability, you have an additional infinite number to find

Doesn't the concept of infinity mean that nothing is impossible?

[OngBonga]

Doesn't the concept of infinity mean that nothing is impossible?

No. Try feeding a star to a dog while at the same time juggling fifteen 1982 model Ford Escorts. Do you think that's happening somewhere, based on a flawed understanding of infinity?

[wufwugy]

No. Try feeding a star to a dog while at the same time juggling fiftteen 1982 model Ford Escorts. Do you think that's happening somewhere, based on a flawed understanding of infinity?

I don't think that's happening anywhere, but I don't understand how that is reconciled with the concept of infinity.

In what way is my understanding of infinity flawed? I'm not saying it's not, I don't know if it is. If infinity means there is always more, then there is always more, and if there is always more, then a dog swallows a star. Not only does a dog swallow a star, but a dog swallows a star an infinite number of times

I never hear this: "there may be an infinite number of universes and they're all like ours". What I do hear is this: "there may be an infinite number of universes with an infinite number of possibilities."

[wufwugy]

I really would like to know if there is something I'm missing about the concept of infinity WRT math/physics. Maybe infinity doesn't mean infinity. Maybe when we say "infinite possibilities", we say it because that's the closest thing to how we can understand it, even though it isn't true that there are infinite possibilities. But then, how could we know that? I would think that if there is any infinity in any way whatsoever, it means everything is possible and there is an infinite number of examples of each possibility

[OngBonga]

I think I can prove to you very easily that not everything is possible... here are two statements...

This is the only universe in existence.

Other universes exist.

It is not possible for both these statements to be true. That is an example of something that has a zero percent probability of happening... multiple universes in a single universe model. It's pure contradiction. There must be an infinite amount of contradictions such as this, which means there will be an infinite number of impossibilities.

[OngBonga]

Ok my language wasn't the best. I can't know your understanding of infinity is flawed, it just seems that it is. Infinity doesn't mean everything is possible. From a mathematical pov, zero multiplied by infinity is zero. So something with a zero percent probability of happening will happen zero times in a sample of infinity.

Infinity is a strange thing. There's more than one kind of infinity, and some are bigger than others. I won't try and explain that to you, I'll just link you to someone who can explain it...

https://www.youtube.com/watch?v=elvOZm0d4H0

[wufwugy]

I think I can prove to you very easily that not everything is possible... here are two statements...





It is not possible for both these statements to be true. That is an example of something that has a zero percent probability of happening... multiple universes in a single universe model. It's pure contradiction. There must be an infinite amount of contradictions such as this, which means there will be an infinite number of impossibilities.

I thought current theoretical physics postulates that both (and neither?) are true.

[OngBonga]

I have no idea, but that doesn't make any sense to me. I thought Schrodinger's cat is either dead or alive, not both; I thought that whole thought experiment was designed to show that certain interpretations of quantum mechanics is flawed.

[a500lbgorilla]

No, it was to show the wave nature of things. How electrons take every possible path when they're going from A to B (or that really, to calculate the position of the electron, you have to sum up all possible paths). The cat takes every possible state until you 'collapse the waveform'.

[Savy]

What has 0% probability? If there are an infinite number of different universes/timelines, doesn't that mean nothing has 0% probability? If there are infinite unique universes, then by the time you have every universe with all the things you say are >0 probability, you have an additional infinite number to find

The probability of me rolling a dice numbered 1-6 and getting a 7 has 0 probability.

And once again if you want to start talking universes which don't observe our laws of nature then we aren't talking science.

Doesn't the concept of infinity mean that nothing is impossible?

no

From a mathematical pov, zero multiplied by infinity is zero. So something with a zero percent probability of happening will happen zero times in a sample of infinity.

No, the question just makes no sense. Just because you realise infinity has something to do with measure and number doesn't mean we can use it in calculations.

You wouldn't think 0 * ruler = 0 you'd think I was on crack.

I have no idea, but that doesn't make any sense to me. I thought Schrodinger's cat is either dead or alive, not both; I thought that whole thought experiment was designed to show that certain interpretations of quantum mechanics is flawed.

You're talking about different things. IF I showed you an experiment and said this happens. Some people think it's because A, some B, some C (A-C are explanations). You wouldn't assume that all those explanations make sense and then try to make them coherent with each other.

http://en.wikipedia.org/wiki/Schr%C3...the_experiment


Disclaimer - Not an expert on Quantum Dynamics nor on advanced statistics but a fairly decent idea what I'm going on about. Expect MMM/anyone else more qualified to correct any mistakes.

[OngBonga]

You wouldn't think 0 * ruler = 0 you'd think I was on crack.

I assume your point here is that multiplying a number by infinity is ridiculous. Which it is, because infinity is not a number. I knew after I posted that someone would pick up on that.

[boysingh]

I assume your point here is that multiplying a number by infinity is ridiculous. Which it is, because infinity is not a number. I knew after I posted that someone would pick up on that.

Nailed it!

[pantherhound]

Dunno if these are more chemistry questions, but I've got some questions about graphene and can't find the answers anywhere. So help would be most appreciated!
1. What does graphene feel like to touch? Could you cut yourself on the edge?
2. Can you spray paint it to make it visible to the naked eye? If not, why?
3. What can you do to make it visible? Are there any circumstances in which it reflects light?

Thanks

[chemist]

Dunno if these are more chemistry questions, but I've got some questions about graphene and can't find the answers anywhere. So help would be most appreciated!
1. What does graphene feel like to touch? Could you cut yourself on the edge?
2. Can you spray paint it to make it visible to the naked eye? If not, why?
3. What can you do to make it visible? Are there any circumstances in which it reflects light?

Thanks

1) It is one atom thick, so you can't feel it or be cut by it.
2) It is one atom thick, so you can't visibly spray it.
3) It is one atom thick, so you need an electron microscope to view it.

graphene monolayer monocrystals are harder to make than 22nm silicon wafers.

There is a lot of pseudo science (notably on you tube) about graphite claiming graphene properties which probably makes it hard to find the answers anywhere.
Though you might even have some unusable bits on your barbeque, top or bottom.

In the 22nd Century they will probably laugh at how little we knew about carbon.
(And all the things you want to do will probably be possible, when it cuts and reflects like diamonds, semi-conducts better than silicon, is thinner and stronger than carbon fibre, and will be as easy to use as spraying in any thickness or shape. And also make long lasting powerful small capacitors to replace batteries).

[pantherhound]

1) It is one atom thick, so you can't feel it or be cut by it.
2) It is one atom thick, so you can't visibly spray it.
3) It is one atom thick, so you need an electron microscope to view it.

graphene monolayer monocrystals are harder to make than 22nm silicon wafers.

There is a lot of pseudo science (notably on you tube) about graphite claiming graphene properties which probably makes it hard to find the answers anywhere.
Though you might even have some unusable bits on your barbeque, top or bottom.

In the 22nd Century they will probably laugh at how little we knew about carbon.
(And all the things you want to do will probably be possible, when it cuts and reflects like diamonds, semi-conducts better than silicon, is thinner and stronger than carbon fibre, and will be as easy to use as spraying in any thickness or shape. And also make long lasting powerful small capacitors to replace batteries).

Thanks. But I still don't get why you can't feel it. I read that theoretically a single sheet of graphene can support the weight of a cat over a big enough surface area. Does that mean the cat would look like it's floating in midair? If you can't feel it, can you put your hand straight through it? If so, how is it possible that it supports weight?

[chemist]

You actually wrote the keyword yourself: 'Theoretically'

A sheet big enough to support a cat hasn't in Reality been produced.

To give you an idea of current sizes, the best production methods make it on silicon wafers.

One imperfection would compromise the physical integrity.
Even at wafer sizes the sheets are not perfect.

Remember we are talking about a single atom thickness. You can't compare it to anything in the tangible world. Even a highly polished mirror that appears and feels smooth has more than a few atoms of bumpiness.

[pantherhound]

You actually wrote the keyword yourself: 'Theoretically'

A sheet big enough to support a cat hasn't in Reality been produced.

To give you an idea of current sizes, the best production methods make it on silicon wafers.

One imperfection would compromise the physical integrity.
Even at wafer sizes the sheets are not perfect.

Remember we are talking about a single atom thickness. You can't compare it to anything in the tangible world. Even a highly polished mirror that appears and feels smooth has more than a few atoms of bumpiness.

But if they know for sure it's 200 times stronger than steel, then they can surmise if it would be possible, no?

If it is correct (that it could support a cat) are you saying noone knows the answer yet to the questions I asked?

[MadMojoMonkey]

Hi guys! Maybe you wont mind if a monkey butts in? (monkey butts)

Chemist is right on the money when he cites current sizes and difficulty in manufacturing. (Read "difficulty" as "Even throwing millions of dollars at the worlds most intelligent and trained students and professionals isn't working that quickly.")

There are many physical problems with trying to create a crystalline structure that is 1 atom thick.

Once created, you have to deal with the fact that the exact same number of atoms NOT restricted to a plane has a lower energy. This means that even once you've isolated or grown graphene, it must be held in place to prevent it from curling, crinkling, wadding, etc. There are many 3D crystalline arrangements for the carbon atoms to form, so the graphene must be held flat everywhere... you can't just isolate certain nodes, you have to hold the whole sheet.

So even the notion of a "large" sheet of graphene holding a cat is a bit off the scale of practical. The closest you could get would be a carbon composite material, because what would be holding the cat is the worlds thinnest carbon-fiber sheet. Which is still quite cool.

Whether or not it would be transparent would depend mostly on the adhesive/substrate.

If you can touch a smooth lump of coal, then you can touch graphene. You'd destroy the surface crystal structure if you touched it with enough pressure to register on your nerves, so "feeling graphene" is a bit tricky to answer... some of the coal you crushed into your fingerprint might actually have a graphene surface... but on the whole, you're feeling coal dust.

[MadMojoMonkey]

Chemist is misleading that graphene can't be seen.

This one-atom-thick crystal can be seen with the naked eye because it absorbs approximately 2.3% of white light.
-Wikipedia "graphene"

Every atom (and molecule) absorbs and emits photons whose energies correspond to the energy gaps between available electron states. It can be the case that none of the frequencies which are absorbed or emitted are within the visible spectrum of human eyes. When this is the case, the object is transparent. However, even transparent objects can be "seen" when they have a different index of refraction than their surroundings.

Index of Refraction is the ratio of the speed of light in vacuum divided by speed of light within a material.

IoR = c_vacuum/c_material

Note that the speed of light in vacuum is the fastest possible speed in the universe. This means the denominator is always less than or equal to the numerator, so the index of refraction is always a number greater than or equal to 1.

IoR for air is ~1.0003
Ior for glass is ~1.5 (with a lot of variety in glass types)

Whenever a photon passes through from one IoR to another, it does a couple of things - reflection and refraction. Usually, it does them both, partially... meaning, based on probability, some otherwise identical photons will reflect and some will refract.

This is why sometimes when you look at a window, you see 2 images... one reflected image, and one that passed through the glass without reflecting from the other side. If the sides of the glass are not parallel, then you have a lens, in which the refraction effect is accentuated and a prism can produce a spectrum from a white beam of light.

Note that "normal" windows have parallel planes and they do also refract the light. The refraction is just less noticeable, because it results in a very subtle shift of the image to one side (depending on the angle which you view the window). Simplified: The light bends when it crosses into the window, and unbends in the same way when it crosses out of the window. The crossing in to the window resulted in a slight change in its direction of travel which was changed equally in the opposite way when it left the window. So objects are not distorted in size and shape, but the image as a whole is shifted slightly. The shift is more noticeable with thicker glass.

So even perfect glass, which is absolutely transparent in the visible spectrum, can still be seen under certain circumstances... namely, some ability to detect reflection and/or refraction.

[MadMojoMonkey]

On the whole "infinity" thing:

Whether or not there are other universes is purely speculation. Whatever they may be or whatever may go on in them is purely speculation. I can not speak as a physicist beyond this.

You've essentially said, "Imagine there is a place where physics as you know can be anything... anything at all."
"Now... can I have ABC, or XYZ?"
And my answer is, "Sure... I don't see why not. You just described cartoons."

The mere presence of infinity doesn't imply anything other than the ability for humans to imagine an unending number line... and to give a name to the unendingness of it.

Just because there is a name for a thing, that doesn't make the thing a thing. Pegasus


Sometimes in physics, infinity/infinity = 1, provided they're both "cooporative" infinities. However, in general, infinity/infinity is a nonsensical statement, because infinity is not a number, it's a description of unendingness.

It is common when solving integral equations to integrate over intervals which extend to +/- infinity... which is a bit abusive of the term, but yields good results... so... yeah.

Infinity: Handle with Care

[MadMojoMonkey]

Dunno if these are more chemistry questions, but I've got some questions about graphene and can't find the answers anywhere. So help would be most appreciated!
1. What does graphene feel like to touch? Could you cut yourself on the edge?
2. Can you spray paint it to make it visible to the naked eye? If not, why?
3. What can you do to make it visible? Are there any circumstances in which it reflects light?

Thanks

Oh... the original questions:

1) I covered this above. It is utterly lacking in the rigidity to cut you if you touched it on its edge. The substrate might cut you, though.

2) Umm... I guess the adhesive/substrate could be construed as "paint" but I don't think that's altogether accurate... it's more like the graphene is painted onto the substrate, but that's an abusively rude simplification of a very complicated task. It occurs to me that the spraying would be an indelicate way to apply a coating, but maybe I'm not thinking on the right scale.

Bottom line is that I think you were asking if graphene could be coated, and the answer is yes.

3) Light will reflect at the slightest provocation. See Snell's Law for more info. Snell's Law pontificates what I said about Index of Refraction - with math.
I think I'm safe to say... Yes, any surface reflects at least some frequencies of light from some angles.

Electrically conductive materials (most metals) tend to be shiny because they conduct electricity - and light is electricity. In that case, incoming waves stimulate movement in the electrons in the conductor which radiate new waves, with the same frequency as the incoming waves. This is a different kind of reflection that is not based on Snell's Law.

[OngBonga]

Are particles actually waves until we observe them?

[MadMojoMonkey]

Are particles actually waves until we observe them?

Particles are waves. Waves are particles.

It depends on how you observe them what you coax out of the observation.


from #67

There is no physical distinction between a particle and a wave in quantum mechanics.

In fact a man named J. J. Thompson won the Nobel prize in physics in 1906 for his work demonstrating that electrons are particles. Then in 1937, his son, G. P. Thompson received the Nobel prize in physics for his work demonstrating that electrons are waves.

I, personally, have recreated experiments that demonstrate both the wavelike and particle-like behavior of photons. I speak with firsthand knowledge that photons are BOTH waves AND particles.

Here is a very brief summary of the experiment I mentioned.

It that experiment I performed 8 variations on a theme.
I separately performed a single slit experiment and a double slit experiment
using separately a red laser and a green filtered incandescent bulb
using separately a photon particle counter and an electromagnetic transducer.

The wave-like cancellation was evident in both detection cases, and in both cases the frequency of the light was able to be shown using wave theory and a bit of trig. with some careful measurements in all cases.

So regardless of the fact that it's a laser, it's true for all light,
it acts in wavelike fashion when propagating over space and time,
the effects of which can be observed whether the detector is carefully designed to only observe waves
or if the detector is carefully designed to only observe particles.

[OngBonga]

Interesting. So a single photon fired through a lens will show evidence of refraction?

[pantherhound]

You'd destroy the surface crystal structure if you touched it with enough pressure to register on your nerves,

Wow thanks monkey for the responses If the above is true though, are you saying my finger would go straight through it? in which case, would I effectively be breaking the bonds between the carbon atoms, and so why did whoever it was say it could support the weight of a cat?

[MadMojoMonkey]

Interesting. So a single photon fired through a lens will show evidence of refraction?

Yes. Each photon interacts individually.

A single photon passing close to an atom or molecule will show evidence of refraction.

A single photon passing close to any edge will refract.


Observe: the sky is blue.
This is because the atmosphere is mostly Nitrogen molecules. The size of diatomic Nitrogen is such that it refracts violet to blue light far more than green through red light.
Since blue light is scattered, it bounces around the sky and seems to come from all angles... the sky looks blue.
The sun looks yellow because the blue light is spread throughout the whole sky, while the green, yellow, orange, red light tend to move straight through the atmosphere.
The sun is white... it just looks yellow in our atmosphere.
At sunset, you view the sun through more atmosphere, and the fact that the more reddish colors are also refracting through the atmosphere is evident. It just takes more atmosphere. If our sky was thicker, it would be a different color.
If you've ever been up a mountain and noticed the sky seemed more blue, or even indigo, that's because the atmosphere is thinner.

[MadMojoMonkey]

Wow thanks monkey for the responses If the above is true though, are you saying my finger would go straight through it? in which case, would I effectively be breaking the bonds between the carbon atoms, and so why did whoever it was say it could support the weight of a cat?

I'm saying that there is no sheet of graphene that exists unsupported.

I'm saying that any smooth surface on a lump of coal probably has small sections of it that are graphene. The thing is that if you touched those smooth sections, you'd break the bonds that were holding those atoms in place.

In reality, a sheet of graphene is held by something. If you could somehow magically create a "large sheet of graphene" and you didn't hold each molecule in a sheet, then it would just crinkle up into ordinary coal.

My point is that NO, graphene can't support anything, not even itself. However, if it's held in place by some substrate, then it could be strong. I don't know. My point is that by itself it is weak, but as a carbon composite material, I don't know.

No one anywhere in the world has produced a sheet of graphene anywhere near the size of a cat, so it's probably just someone trying to phenomenalize something for attention.

***
Also, a lot of people say a lot of things about cats.

[pantherhound]

I'm saying that there is no sheet of graphene that exists unsupported.

I'm saying that any smooth surface on a lump of coal probably has small sections of it that are graphene. The thing is that if you touched those smooth sections, you'd break the bonds that were holding those atoms in place.

In reality, a sheet of graphene is held by something. If you could somehow magically create a "large sheet of graphene" and you didn't hold each molecule in a sheet, then it would just crinkle up into ordinary coal.

My point is that NO, graphene can't support anything, not even itself. However, if it's held in place by some substrate, then it could be strong. I don't know. My point is that by itself it is weak, but as a carbon composite material, I don't know.

No one anywhere in the world has produced a sheet of graphene anywhere near the size of a cat, so it's probably just someone trying to phenomenalize something for attention.

***
Also, a lot of people say a lot of things about cats.

OK cool.

If I as a human being was the size of a neutrino and I was suspended in mid air by some sort of sub-atomic structure commensurate to my size, and I was in a normal human-sized office, would I be able to see anything? If so, what?

What about if I was the size of a carbon atom?

[OngBonga]

Yes. Each photon interacts individually...

Thanks! Interesting stuff.

[MadMojoMonkey]

OK cool.

If I as a human being was the size of a neutrino and I was suspended in mid air by some sort of sub-atomic structure commensurate to my size, and I was in a normal human-sized office, would I be able to see anything? If so, what?

What about if I was the size of a carbon atom?

A) this makes no sense. The complexity of a human being can not be expressed by any known particles on such a scale.

If you had a mass that tiny, a photon would hit you like a bus. Photons carry momentum proportional to their energy, and if your mass is small enough, then the absorption of that momentum will noticeably accelerate you.

Even if we could magically shrink a human to that size... we have all kinds of other considerations in how quantum mechanics deals with the very tiny... assuming we could somehow break physics for only you to be so small, with all your normal particles masses and charges... then we can't at all talk about how something from outside that physics-space interacts with you.

What happens to the "normal" photon when it enters your "tiny" eye? Does the normal photon become tiny, so that you can see it, or does the normal photon basically wash right past you, since you are far too small for it to interact with, and you have no significant conductive properties... and even if you were electrically charged, your mass is such that it can't easily impart it's momentum to you by waving you violently in a transverse manner... so it might not interact with you at all.

It's a dead end.

Things that are so tiny can not have "eyes", much less a mind capable of interpreting chemical stimulus as vision.

Can you reformulate the question into something that doesn't break physics in the assumption?

[MadMojoMonkey]

If something that tiny could see, it could only see very short wavelengths... like gamma rays and higher. The trouble is that gamma rays have so much energy that they tear apart atomic nuclei... causing nuclear fission.

[a500lbgorilla]

Talk about how the loudest sound possible on Earth is 1 atmosphere of pressure of loudness.

And then talk about the loudness of a sonic boom.

Thank you.

[pantherhound]

Can you reformulate the question into something that doesn't break physics in the assumption?

No
Explain string theory to me, but in a poem.

[MadMojoMonkey]

Talk about how the loudest sound possible on Earth is 1 atmosphere of pressure of loudness.

And then talk about the loudness of a sonic boom.

Thank you.

This sounds fun.

I don't know where to start. Loudness is generally expressed in terms of decibels, which is really a measure of how much RMS power is being delivered to a membrane.

Power is not pressure.

There is Sound Pressure Level, which I can look into and see if that corresponds to what you're saying.

I'll do a bit of digging on this one.

[MadMojoMonkey]

No
Explain string theory to me, but in a poem.

No

There are a bunch of posts about string theory ITT. The thing to remember about string theory is that there are a ton of them and none of them is a theory... they're all hypotheses. In order to be a theory, it has to be widely accepted and tested.

I'm not digging through the thread to find exact posts, but if you just ctrl-f and search for my posts, it wont take you too long.

TL;DR - there are a lot of string theorists, none of whom has yet to produce a string theory. String theory is a hypothetical way of making sense of quantum mechanics and general relativity with a unified mathematical framework.

[chemist]

This sounds fun.

I don't know where to start. Loudness is generally expressed in terms of decibels, which is really a measure of how much RMS power is being delivered to a membrane.

Power is not pressure.

There is Sound Pressure Level, which I can look into and see if that corresponds to what you're saying.

I'll do a bit of digging on this one.

But what if it left that membrane and traveled through another brane and then returned, couldn't it travel not only faster than the speed of sound but even faster than the speed of light. Could String Theory Booms be bigger than sonic booms, but which brane would boom.

A brain half understanding string hypotheories can postulate strangely.

[MadMojoMonkey]

OBV. a dead rillawolf shows up with challenging questions when MMM in the heat of battle lynching his wolf buddy JKDS.
rilla again, FFS.

(I'm still gonna answer the question, but a Monkey's gotta have priorities, ya dig.)

[MadMojoMonkey]

Sorry, rilla. Stupid fake-howl from JKDS. I suck at ww, btw.

First off, there is no such thing as negative absolute pressure. Absolute pressure is strictly non-negative.

Contrast with gauge pressure, which is more commonly used in affordable pressure gauges. Gauge pressure is relative to some non-zero pressure (typically 14.7 psi, or 101,325 Pa). A typical gauge can read out a negative value, because it's measuring a pressure difference.

These gauges are cheaper to manufacture because they to not need a robust pressure vessel built into them which maintains a vacuum. They just need a way to vent and reseal a (much weaker) pressure vessel, allowing it to come into equilibrium with whatever the ambient pressure is. This allows for accurate readings on cloudy days.

This negative value on the gauge is not indicative of a negative absolute pressure. It can only be at most as negative as the ambient pressure at the time the vent was sealed on the pressure vessel. This corresponds to an absolute pressure of 0 Pa.

***
What is pressure?
Pressure is caused by particles bouncing off of each other. If the particles are held in some vessel, they will bounce off the vessel, too. The higher the particle density, the higher the pressure. The higher the temperature, the more rapid the vibrations of the particles (as I've discussed before), which results in collisions which impart more energy transfer, and more rapidly moving particles, which in turn causes more frequent collisions, and higher pressure.

All that boils down to the ideal gas law. As per usual, this is an approximation, but it's good enough to illustrate my point.

PV = nRT

Pressure, P, times Volume, V, equals the number of particles, n, times {some positive constant}, R, times the Temperature, T.

Reformed to match what I said:

P = {R} * n/V * T

Pressure goes up when particle density (number of particles, n / volume, V) goes up. Pressure goes up when temperature goes up.
For the sake of talking about sound, we can ignore temperature, and, since it's always positive, too, we can just lump it in with R.

P = {blah} * n/V

***
What is sound?
Sound is a waving of particles in a medium.
When a particle moves, the space it enters gains higher particle-density, thus higher pressure. The space it leaves has lower particle density, and thus lower pressure.

This change in particle density is self-interacting. The particle collisions are going to favor bounces which send particles to the lower pressure region, so they move there... but now their moving leaves a lower particle density region behind them. So now the collisions favor moving them back. Back and forth; back and forth. This is particles waving.

It's not the particle-waves / wave-particles of QM... it's macroscopic particles moving in non-QM ways... this is classical physics. Sure the wave-particle stuff is going on, but at a much smaller length scale and a much shorter time scale.

***
Sound is pressure waves moving though the air. The pressure waves oscillate back and forth around some equilibrium pressure (typically close to 14.7 psi or 101,325 Pa). This equilibrium pressure imposes a maximum wave size, for exactly the reason that pressure can not go negative.

I found this exerpt from a really excellent article about Krakatoa:

[...] there’s a limit to how loud a sound can get. At some point, the fluctuations in air pressure are so large that the low pressure regions hit zero pressure—a vacuum—and you can’t get any lower than that. This limit happens to be about 194 decibels for a sound in Earth’s atmosphere. Any louder, and the sound is no longer just passing through the air, it’s actually pushing the air along with it, creating a pressurized burst of moving air known as a shock wave.

Taken from this article about the volcanic eruption of Krakatoa in 1883.

Physically, a shock wave is a region, or volume, characterized by a dramatic change in pressure, temperature and density.

More on shock waves later.

[chemist]

...
All that boils down to the ideal gas law.
...

Shouldn't that be: 'All that Boyle's down to the ideal gas law'?

[MadMojoMonkey]

no love for my use of {blah}? It's one of my favorite variables.

[chemist]

{blah} is better than {some positive constant}*T
What is the [positive constant] ?

Even Boyle and Marriotte couldn't be bothered to define it.




where P is the pressure of the gas, V is the volume of the gas, and k is a constant.

But it does mean more usefully:


My favorite irrelevant variable is {pink elephant} I don't know why it is just the first ridiculous thing that comes to mind but OngBonga is becoming a close second.

[MadMojoMonkey]

{blah} is better than {some positive constant}*T
What is the [positive constant] ?

My favorite irrelevant variable is {pink elephant} I don't know why it is just the first ridiculous thing that comes to mind but OngBonga is becoming a close second.

You and I both know that {some constant} is the Gas Constant. It's enough for this discussion to know that the value is non-negative and non-zero.

I avoided this before because the Gas Constant can be stated in so appallingly many different combination of variables, that I didn't want to have to define it. If you bother to click the link above, it's just wikipedia, but the table on the right shows what I mean.

Since you can express the equation PV = nRT in so many ways, broken up in different ways, the value of R could be ... not anything ... but the specific value it has in any given equation might be different based on units.

E.g. in fluid dynamics, it's commonly stated PV = mRT, where m is the mass of particles of a given average density. So the value of R has to accommodate that.

I will be sure to use {ongbonga} as soon as possible.

[MadMojoMonkey]

Shock waves are tough.

Shock waves are formed when the speed of the upstream flow is greater than the speed of the downstream flow. This causes increased pressure on the wave front, causing it to increase in density and accelerate.

Shock waves propagate faster than the speed of sound, and decrease speed as they propagate. When the speed of the shock wave slows to the speed of sound, the shock wave dissipates into a sound wave.

The wave speed tends to slow at an inverse square of distance covered. This is the same rate of energy dissipation as the decrease in volume over distance as a sound wave. However, it is in fact the minimum rate of energy dissipation of a shock wave, since a shock wave creates heat and turbulence in the flow after it passes.

So that's a start.

***
It gets complicated because a shock wave in air is different from a lot of practical examples.

If a faucet is running in an empty, smooth-bottomed sink, after a moment, there will be a ring where the depth of the water increases. This zone where the depth of the water changes is a shock wave. This shock wave is "moving" at a speed equal and opposite to the speed of the incoming flow, and appears stationary.

If you had a thin layer of honey on a plate, then you let a stream of honey trickle onto the plate, there will be a shock wave. That shock wave is visible on the surface of the honey as an expanding circle which is thicker than the underlying layer. The leading edge from this expanding circle is a shock wave, characterized by dramatic increase in density and speed over a small length.

When ocean waves break on a beach, the top of the wave, which breaks, does so because it is a shock wave. The top of the wave is moving faster than the bottom of the wave, and it spills forward, exchanging forward energy into heat and turbulence.

***
It gets even more complicated because of the fact that waves of different wavelengths travel at slightly different speeds. This is called dispersion.

"Complicated" waves are superpositions (additive combinations) of "simple" sine waves. That means that the complicated wave's shape is composed of some number of simple sine waves added together. Those sine waves may propagate at different "speeds of sound" within the medium. So the shape of the wave will change as it propagates.

(Water waves ripple and spread and combine because they are a superposition of simple waves which move at different speeds.)

This is pertinent, because some shock waves "leak" information in the direction of flow. E.g. in some viscous fluids, you can indent a mark on the leading edge of the shock wave. That mark will flow forward, ahead of the shock wave.

This all brings on a discussion of phase speed vs. group speed of the wave's propagation.

Each frequency of sine wave that composes the main wave moves at its own phase speed. The overall wave moves at the group speed.

The "center" of the wave moves as the group speed, but the width of the wave may spread or contract based on the dispersion characteristics of the medium. If the medium is non-dispersive, then the wave will move without changing shape.

So you can have a wave, which has portions of it's composition waves acting in a shock wave, while other portions are free to pass through the shock wave. This is true, even though the shock waves move faster than the speed of sound, because the speed of sound for the frequencies in the shock wave is different than the speed of sound for frequencies beyond that range.

[OngBonga]

If pressure is simply particle density, why does a large body of an incompressible medium, such as an ocean, have immense pressure at its depths? How are particles more dense when water in incompressible?

[Renton]

Pretty sure pressure has no effect on density unless the material is a gas or plasma. Pressure is just the force acting on a material. [/layman]

[OngBonga]

I always assumed pressure was the collective weight of what's above you, so water pressure is the weight of all the water directly above you, plus 1 atmosphere of air pressure.

If it's particle density, well that explains why pressure increases with temperature, and it explains atmpospheric pressure because gas is compressible.

If pressure has no effect on the density of a liquid, then how does particle density increase? That's why I'm confused, because I agree with what you said renton and that doesn't tally with the idea that pressure is simply a measure of particle density.

[MadMojoMonkey]

If pressure is simply particle density, why does a large body of an incompressible medium, such as an ocean, have immense pressure at its depths? How are particles more dense when water in incompressible?

TL;DR

AFAIK: Neither water, nor any known thing is actually incompressible.

The reason we say that water is incompressible is because it takes immense pressure to exhibit significant density change.
As ever, being able to set a variable to 0 or "no change" is a great simplification, and if it works... use it.

The particles are more dense (ever so slightly), and water is compressible (if you measure very accurately).

***
Pressure is particle density and temperature, this is an important factor.

PV = nRT

Pressure, P, times volume, V, equals number of particles, n, times the gas constant, R, times the temperature, T.

P = nRT/V

P = {R}*(n/V)*T

Pressure equals {ongbonga} times particle density, (n/V) times temperature.

***
You may have noticed that {ongbonga} is the GAS constant*, and water is not a gas. Additionally, the equation above is called the Ideal Gas Law, and it's a good approximation for many gasses, but no gas is "ideal". Steam is nowhere ideal, so this equation is useless when it comes to water, or even water vapor.

***

(I almost deleted this bit a couple of times, but I think it's really the closest to answering where your questions were coming from.)

Mechanically, particle collisions result in equal and opposite reaction forces. When a force is applied over an area, it is a pressure.
E.g. pounds per square inch or Newtons per meter squared.

Hydraulic pressure is a way of talking about the unbelievably countless amount of tiny bouncy interaction forces between particles by statistically averaging them out over an area.


Since the pressure is capable of expressing that force (since it can push/pull), it must be able to do so over some distance; you can't feel pressure unless it pushes you. The equal and opposite is true for what you're pushing. The application of a force over a distance is one form of energy.

E = F*d

Energy, E, equals force, F, times the distance over which the force is applied, d, (again a simplification, but I need to invoke calculus to be more accurate.)

The energy of the pressure, comes from elastic distortions.

An elastic distortion is one which resumes its original shape when the pressure is removed.
A plastic distortion is one which results in permanent deformation after the pressure is removed.

Plastic deformations transform the energy into a physical distortion, whereas elastic deformations store the energy of the deformation in a spring-like manner.

Drop a sand bag on the floor: no bounce: plastic deformation
Drop a rubber ball on the floor: bounces: elastic deformation

The bounce back comes from the rubber ball undergoing an elastic deformation that acts like a spring to return to its original shape. The "return" causes it to de-deform.

Most materials react in an elastic way when "small" pressures are applied, but will yield to plastic deformation when "large" pressures are applied.



/--------------/
* cause he's full of hot air. (OK, we can snuggle now, but only if I can call you Ongie during.)