Physics Q&A Game: Exploring Faraday's Law with Marcus' Polar Route Question

[nbo10]

It was close.
I have one more that is semi-interesting, a engineering proferssor asked this to my class. By the way his solution wasn't even close to being correct.

When you clap your hands, what keeps your hands from passing through each other? Or when anything touches anything, what keeps it from mergeing together?

Bonus if you can guess how the professor answered this question.

JMD

 

[Tyger]

If you don't mind a broad brush answer

I would say the Exclusion Principle. It prevents matter from being any more "crowded together" than it is.

The professor's answere might be electrical repulsion.

 

[nbo10]

The professors responce was Pauli's Exclusion Principle. but that is not correct. Pauli's Exclusion Principle imposses symmetry on the wavefunctions.

"electrical repulsion" is also incorrect. You might be thinking along the correct path.

Another bonus, restate the question in a scientific way.

JMD

 

[marcus]

Originally posted by nbo10
It was close.
...
When you clap your hands, what keeps your hands from passing through each other? Or when anything touches anything, what keeps it from merging together?

JMD

Fluids do merge together so the idea is what makes a solid solid
and related issues

So most solids have some lattice of bonds
or an effective system of intermolecular forces
that gives them integrity

cells in your hands are bags of fluid enclosed in membrane

if you ask why don't my hands merge when they press together
it must have to do with the strength of membranes

that is like crystal lattice bond-work in two dimensions.

these are amusing questions. i wish you were prepared to
come up with some more

 

[marcus]

Where is nbo? I want to know if I got the answer right or not.

He asks why don't things merge on contact and the answer is that some things (fluids) do tend to mix on contact so it is a question about solids---how they maintain integrity.

Having one solid pass thru another would involve breaking a lot of bonds. So the bond energy is the answer.

but also, given that the lattice is going to hold, solids are still mostly empty space so there is a further question of why one atom cannot just pass thru another atom.

Even when gasses mix, atom does not pass thru atom and that (which is not what the question is about) may have to do with the exclusion principle. So I stick with my original answer:

Originally posted by marcus
Fluids do merge together so the idea is what makes a solid solid
and related issues

So most solids have some lattice of bonds
or an effective system of intermolecular forces
that gives them integrity

cells in your hands are bags of fluid enclosed in membrane

if you ask why don't my hands merge when they press together
it must have to do with the strength of membranes

that is like crystal lattice bond-work in two dimensions.

these are amusing questions. i wish you were prepared to
come up with some more

 

[nbo10]

I have to sleep sometime

Thats good enough marcus. Your question

To restate the question

Why do solids have a bulk modulus?

Electrostatic potentials forming a lattice.

JMD

 

[marcus]

Originally posted by nbo10

Thats good enough marcus. Your question

It is more interesting for me when you ask the questions.
hope you answer this and return with a question.

One seldom sees the Einstein equation written in form
compatible with SI metric units, but suppose one did.
then the equation would be written this way:

G_mu,nu = 8pi (G/c⁴) T_mu,nu

In what SI units are the quantities on the left expressed?

In what SI unit would (c⁴/G) be expressed? I am intentionally looking at the reciprocal of what appears in the equation. Of course G is Newton's constant and c is the speed of light.

In what SI units are the T_mu,nu terms on the right expressed?

dont be offended if this is too trivial a question. the fact is that people usually see the equations written in units with c = G =1 so in my experience they don't think concretely about how things would look in familiar SI terms

 

[Alexander]

Momentum-energy stress T is in N(ewtons), curvature metrics g is unitless.

G/c⁴ unit is N^-1.

In some papers curvature g is expressed in m^-2 and momentum-energy tensor T in N/m²=J/m³

 

[Alexander]

Originally posted by nbo10
The professors responce was Pauli's Exclusion Principle. but that is not correct. Pauli's Exclusion Principle imposses symmetry on the wavefunctions.


JMD

Actually vise versa, the (anti)symmetry of wave function results in Pauli exclusion principle (repulsion of fermions) as a mathematical by-product.

 

[marcus]

Originally posted by Alexander
...
G/c⁴ unit is N^-1.

I was asking about (c⁴/G) which is a force and the answer I was looking for is that in SI it is expressed in Newtons, the SI force unit.

However you said something just as good! You said the reciprocal is expressed in reciprocal Newtons, or as you say N^-1.

Originally posted by Alexander
...

In some papers curvature [G_mu,nu, not g] is expressed in m^-2 and momentum-energy tensor T in N/m²=J/m³

The curvature G_mu,nu is expressed in 1/area units----in other words---and the T_mu,nu is expressed in energy density units----joules per cubic meter.

As you point out, joules per cubic meter is also equivalent to the unit of pressure, Newton per sq. meter.

And as you say this happens in some papers, while in other papers the curvature is made dimensionless, so it is not expressed in SI units or in any kind of units.

OK Alexander, your turn. Have a good one!

 

[Alexander]

Origin of blue sky?

 

[nbo10]

Rayleigh scattering

JMD

 

[Alexander]

On what?

 

[marcus]

Originally posted by Alexander
On what?

That's really a second question. So I will put in a thought----it will still be nbo's turn because he said Rayleigh scattering.

I've always been assuming that the Rayleigh scattering that causes the blue of the sky was mostly from the outer electrons of the N2 molecule, just because nitrogen is the most common.

But it is not specific to nitrogen. Oxygen would work just as well and would look blue.

In fact if the Mars CO2 atmosphere were not so full of reddish dust it would look blue also.

So the question "off what?" seems as if it could be answered
"whatever light-element gases the air is made of"
or maybe one could just reply "the air".

 

[Tyger]

Off what?

Even air has an index of refractions, and it isn't homogenous. To my recall it's scattered off the irregularities in the atmosphere. It's also the cause of the Cyrillian blue we see when we look in the distance.

 

[Alexander]

Originally posted by marcus
That's really a second question. So I will put in a thought----it will still be nbo's turn because he said Rayleigh scattering.

No, it is Tyger turn - he is correct.

Molecule of N2 is too small (d~0.3 nm) to give any essential contribution into blue sky color. Recall that intensity of scattered light is proportional to (d/[lamb])⁴, so it drops dramatically with size. It would take about hundred times more air to get essential contribution into scattering from individual molecules. But the distribution of molecules in air is fluctuating, and it turns out that most contribution into intensity of scattered light comes from fluctuating pockets of air of order of 10-100 nm in size. These pockets can have dramatically different number of molecules per unit volume.

Density of larger pockets does not fluctuates essentially, and there are less of those pockets, so despite their larger cross section their contribution is less.

Kinda interesting how interplay of math factors creates tangible phenomenon (blue sky) from something which is not very tangible itself (fluctuating due to thermal motion voids/concentration of certain size range).

 

[Alexander]

Tyger, your turn.

 

[On Radioactive Waves]

Whats going on? This was too much fun to let the thread die. If Tyger doesn't want to go, someone else should pose a question.

 

[Ivan Seeking]

Originally posted by On Radioactive Waves
Whats going on? This was too much fun to let the thread die. If Tyger doesn't want to go, someone else should pose a question.

Is that your question?

Why don't you go.

 

[Tyger]

Sorry, I didn't know

that I got this one. I'll come back with a question when I think of a good one, hopefully today.

 

[Tyger]

Here's my question.

Why are metals stronger and more ductile than non-metals?

 

[nbo10]

electrostatic forces from the lattice of atoms that make of the metal

JMD

 

[marcus]

Originally posted by Tyger
Why are metals stronger and more ductile than non-metals?

this is not an answer exactly. I am thinking out loud: metals have comparatively small incomplete outer shells
and give up their outer electrons more easily than non-metals.

in a metallic lattice the outer electrons tend to be owned *collectively* by all the atoms---they are shared in common

such a lattice will be easier to deform (less rigid and brittle) and it will retain its integrity even while being deformed

in a non-metal lattice the ownership of the electrons is not widely shared but instead there is only sharing between nearby neighbors.
so deforming the lattice becomes a complex and difficult negotiation where specific bonds must be broken between old neighbors and reformed between new neighbors.

it is easy to see how cracks could develop and propagate in a non-metallic lattice

but "strong" is a word with various meanings and I suppose sometimes a carbon (non-metal) fiber could be strong---tho not ductile!

I yield to nbo, if there is any question about this, because he said "electrostatic forces" which covers all this in general tho it is not very specific---maybe I was just elaborating on nbo's earlier answer.

 

[Tyger]

I'm holding out

for a more specific answer. There is a more precise way of stating why.

 

[KillaMarcilla]

Man, I'm clueless

Although this page was a real interestin' read:
http://matse1.mse.uiuc.edu/~tw/metals/prin.html

 

[Ivan Seeking]

Originally posted by Tyger
for a more specific answer. There is a more precise way of stating why.

Band Theory.
The energy separation between successive levels is extremely small - referring to the electron orbitals for each atom - due to sharing of unfilled orbitals.

 

[Tyger]

Now what to do?

The answer I was looking for was Surface Tension. The electron gas provides a powerful surface tension that holds the faces of the crystal latice together so that they slide over each other rather than break apart. We can see this surface tension at work in Mercury where the droplets resist deformation despite the high density.

 

[marcus]

Originally posted by Tyger
The answer I was looking for was Surface Tension. The electron gas provides a powerful surface tension that holds the faces of the crystal latice together so that they slide over each other rather than break apart. We can see this surface tension at work in Mercury where the droplets resist deformation despite the high density.

nice explanation. would not have occurred to me to think of
strength and ductility of metals due to surface tension of electron gas

unless there is some objection, why don't you ask the next question Tyger

 

[On Radioactive Waves]

I'll think of a question to post tommorow if someone dosn't beat me to it.

Once again, Tyger is lagging.

 

[marcus]

Originally posted by On Radioactive Waves
I'll think of a question to post tommorow if someone dosn't beat me to it...

that seems reasonable to me. I hope you do post a question.
The rules of the Q/A game are at the beginning of the "Astronomy" Q/A thread----I think "nicool" wrote them. The rules say something like it is up for grabs if the thread is cold for 3 days.
I forget exactly. But why stand on ceremony? Go for it.