noob question, not even sure if this is the right forum.

H2O, water can take the form of gas, liquid and ice.
What makes that possible?
is it something with the oxygen or the hydrogen? Or is it because of the mixture?
Then my other question is, can heavy water take the form of ice (solid) ?

Temperature is the key into turning anything into some other form.

yes, i know that :D

But how? i was wondering a little about the chemical process

Maybe i should had written (upper noob question) :)

yes, i know that :D

But how? i was wondering a little about the chemical process

Maybe i should had written (upper noob question) :)

It's more on physical process than chemical process. Start with understanding that matter is composed of atoms/molecules. First step to your question concerning the state vapour, water and ice for H₂O would be:

1) in vapour or gas: H₂O molecules move freely
2) in water or liquid: H₂O molecules move but not so freely
3) in ice or solid: H₂O molecules do not move from place to place

Then my other question is, can heavy water take the form of ice (solid) ?

If I am not mistaken, heavy water contains hydrogen that has neutron (one or two) in it. The one with one extra neutron is called deuterium and the one with two extra neutrons is called triton. So, rather than having melocule weight number of 18 (16+2) for normal water, heavy water has 20 or 22 (or may be 19 or 21).

I don't see any reason for heavy water not to take the solid form or ice.

Paul and cosmic, that much i understand. Temperature controls how fast/slow the molecules move.

But i cannot seem to grasp how the molecules can create the "shapes". Water is a liquid form and i can just not understand how molecules can form something so complex.

Well, new question(s).
what is the molecular composion of Gasoline?
H H And something like that?
| | /
H - C - C - O
| | \
H H H

How many C? How many H? How many O?
Or am i on the wrong track here?


Other question,
Ligth (sun), visible light, IR, UV and all the other invisble ligth that the sun emmits.
Do they lose "power" the longer they travel? (Given that they do not enter any atmosphere).
I reckon that it does not in vaccum (space) but in Oxygen?
Does the forces on earth affect it (gravity and pressure for instance)?

oops, the PHP system apperently removed the spaces. Hope you can read it.

Baal: its not a continuum like everyone seems to be implying. When water reaches a certain temperature it forms a solid (or crystalises) which will happen instantly to all water at that temperature. This structure it forms is specific to the molecule (water) and is the preferential structure for the molecule. Liquid and vapour can have any structure they like (or no structure!)

k, how about the others?
Especially interested in the "light question".

Other question,
Ligth (sun), visible light, IR, UV and all the other invisble ligth that the sun emmits.
Do they lose "power" the longer they travel? (Given that they do not enter any atmosphere).
I reckon that it does not in vaccum (space) but in Oxygen?
Does the forces on earth affect it (gravity and pressure for instance)?

And i almost forgot, Does the light lose any "power" when it hits a 100% reflective surface?

Light will lose energy, intensity and power as it passes through space and this is generally caused by absorption with atoms along the way. I beieve gravity only affects the frequency of light but our own planet would have very little affect on light.

100% relection should theoretically not draw energy from light in itself :)

k, how "strong" (forgive me for my choice of words) is the light when "hits the earth"?

How much heat is "created" when the sun-rays hits a black surface? (alternative word for created? I know that it cannot be created, just ... ? )
Or maybe i am probing for information that no one has here.

And, does the light produce more heat if the surface is already "hot"

The intensity of sunlight is what u want to get and it is about 1600 Wm²

Generated is a better word than created (because of the latters conotations) but it is not wrong. It depends upon how big the black surface is!

And, does the light produce more heat if the surface is already "hot"

No, the light will add to the heat but the amount of heat ultimately generated by light will depend on how long the surface stays in the light and how strong the light is :)

if the light source is constant will it be an exponential "heat" then?

That intensity, is that per second or per sun ray?

btw, the "size" of the black surface cannot matter, right since then the more ligth will just be spread out on a larger surface. Or?

btw, the "size" of the black surface cannot matter, right since then the more ligth will just be spread out on a larger surface. Or?

U asked about how much heat will be generated. U DO need to know the size of the generating surface to work this out!

if the light source is constant will it be an exponential "heat" then?

I have no idea what u mkean by "an exponential heat"!

That intensity, is that per second or per sun ray?

Intensity can be thought of as the magnitude of the energy in the light.

one last question, will it be preferebly to have a constant light source giving a constant energy flow, or will it be better to have intense amounts of light at one moment?

Preferable for what? For reading a book at night on a train i would prefer the former :)

:p

For producing the optimal amount of kWh
(given that it is stored in power station)

And how do u propose to generate electrcity from the light?

Off the top of my head, ice forms when the temperature of the water drops far enough that each molecule has energy lower than the energy necessary to overcome the attarctive force between the molecules, ice will form. This will not happen at exactly the same time all through the liquid, and is one reason you get all these nice crystals. Crystallisation is a nice interesting area of study.
Then water is unusual in that it has strong hydrogen bonding, ie H to O bonds form that involve distortion of the electron clouds, rather than the formation of a full covalent bond. So this bonding increases the boiling point of water, as it means the molecules are more attracted to each other. When I say increase, I mean increase relative to other similar sized molecules that have hydrogen an oxygen on them.

As for the power station stuff, that depends upon whether or not your power station is more efficient at energy collection when you have a big flash, ie all in one, or whether it works better by slowly collecting.

John, very much like solar power.

Thank you, i have gotten replies on what i wanted. Now i will start doing some research and math on it. Just wanted to check if my idea was feasible and you have been more than helpfull.

Light will lose energy, intensity and power as it passes through space and this is generally caused by absorption with atoms along the way. I beieve gravity only affects the frequency of light but our own planet would have very little affect on light.

100% relection should theoretically not draw energy from light in itself :)

Except that photons have momentum, and the mirror has to recoil - the photons changes direction, so there is 2E/c imparted to the mirror. If you impart momentum, you impart kinetic energy as well. The photons should have a small frequency, and thus energy, shift.

k, how "strong" (forgive me for my choice of words) is the light when "hits the earth"?

How much heat is "created" when the sun-rays hits a black surface? (alternative word for created? I know that it cannot be created, just ... ? )
Or maybe i am probing for information that no one has here.

The amount of sunlight that reaches the earth has a power density of ~1.4 kW/m^2. I think that's perpendicular to the rays, at the upper atmosphere. On the surface it's a little less than 1 kW/m^2.

what i had in mind was a object (probably a sphere)
It would be made like a police station mirror. Where one side lets through the light and the other side reflects it.
Within it would be vaccum.

Since the mirror is not 100% reflective there would be a series of mirrors that would limit the energy losses.

But then i looked at google and others had also thought about it. I talked to James in my team and he had thought about it long time ago but he saw no value in it.
Storage of energy is not a problem.
I am not sure if it could be used as a weapon but i doubt it.

I came up with it about half a year ago but did not think it was anything special, it needs something more. Any suggestions to make it something worth building?

Except that photons have momentum, and the mirror has to recoil - the photons changes direction, so there is 2E/c imparted to the mirror. If you impart momentum, you impart kinetic energy as well. The photons should have a small frequency, and thus energy, shift.

Swansont: mirrors CANNOT impart 100% reflection! How about that?!

of course they don't.
Since the mirror is not 100% reflective there would be a series of mirrors that would limit the energy losses.

however, James said something about Cobalt mirrors that are close to perfect.

But u asked does light lose any power when it reflects a 100% surface so I just don't know why Swansont presumed I was talking about mirrors which are OBVIOUSLY not 100%!

I HATE PRESUMPTIONS more than anything else.
Just a little tip :)

I think swansont perhaps "read between the lines"

I am known for being vauge in my explenations.

But u asked does light lose any power when it reflects a 100% surface so I just don't know why Swansont presumed I was talking about mirrors which are OBVIOUSLY not 100%!

I HATE PRESUMPTIONS more than anything else.
Just a little tip :)

:confused: :confused:

And yet, you had written, (and I had quoted)


100% relection should theoretically not draw energy from light in itself

So it was not at all obvious that you were not talking about 100% reflectivity, since you were responding to a question about a 100% reflective surface.

Yes I was talking about a 100% reflective surface. The keyword in my last post was mirror.

Yes I was talking about a 100% reflective surface. The keyword in my last post was mirror.

OK, so you have a 100% reflective surface which is not a mirror. (semantics :rolleyes: ). The note about the photons imparting momentum still holds.

NO! Well, I'm not sure what ur trying to say but in 100% reflection, NO energy is lost from the light at all!

NO! Well, I'm not sure what ur trying to say but in 100% reflection, NO energy is lost from the light at all!

Then how are momentum and energy conserved?

They are still conserved. There is no energy lost or gained. This is why it is called 100%! 100% of the lights intensity remains after reflection :)

i think both of you are right.
Swansont, Momentum requires energy but as John says, a 100% reflective surface should not draw any more energy than as if it was traveling in a straight line.

They are still conserved. There is no energy lost or gained. This is why it is called 100%! 100% of the lights intensity remains after reflection :)

But the momentum of each photon has changed. So the "reflective surface" must now possess momentum of equal magnitude, but in the opposite direction. Thus it has energy. Where did that energy come from? The only source is the photon.

Even assuming an idealized situation of no photon loss doesn't allow you to ignore conservation of momentum and energy. "100% reflective" can't be the same as no energy loss by the photons. They don't mean the same thing. One is idealized, the other is sci-fi.

But the momentum of each photon has changed. So the "reflective surface" must now possess momentum of equal magnitude, but in the opposite direction. Thus it has energy. Where did that energy come from? The only source is the photon.

Even assuming an idealized situation of no photon loss doesn't allow you to ignore conservation of momentum and energy. "100% reflective" can't be the same as no energy loss by the photons. They don't mean the same thing. One is idealized, the other is sci-fi.

The trick is........................NO momentum or energy is lost by all the photons. The only way this could happen is if NO photon imparts momentum on the reflective surface (hence 100% reflectivity). Such a phenomenon occurs in nature :)

The trick is........................NO momentum or energy is lost by all the photons. The only way this could happen is if NO photon imparts momentum on the reflective surface (hence 100% reflectivity). Such a phenomenon occurs in nature :)

Okay, the question is....can this 100% reflective surface move? Since, you guys have already assumed that there is such thing called 100% reflective, there should be okay to assume that there is stuff that doesn't move (earth can be considered doesn't move after hit by a few photons :D ). If this 100% reflective surface remain unmoved after reflecting the photon(s), then the photon suffer no energy lose...but, if it move the photon must supply energy.

The trick is........................NO momentum or energy is lost by all the photons. The only way this could happen is if NO photon imparts momentum on the reflective surface (hence 100% reflectivity). Such a phenomenon occurs in nature :)

Momentum is a vector. The momentum before the reflection is E/c. The momentum after the reflection is -E/c. The change in momentum of the photon is 2E/c.

You can't just wave your hands and say, "There is no momentum change." There is an inherent change in momentum. Ignoring conservtion of momentum isn't "idealizing" the system. The phenomenon you describe does not happen in nature. What you describe is magic, not science.

swansont is right. the theoretically maximal reflection is a reflection where all momentum is conserved, and nothing goes to heat.

I do not know if any of the photons goes to "waste" in vaccum but i think one of you said that it does so i will go from that.

As i said before, i agree with swansont, a 100% reflective surface should not draw any energy and convert it to heat, However, the loss in vaccum will still be static.

I think you got me wrong, Baal. there is no loss in vaccum, but the photons do have momentum that has to be transferred to the mirror, so that momentum is conserved. Photons DO have momentum, even though they DON'T have mass. the mirror gains energy in the form of kinetic energy, not heat, at least if it is a perfect mirror.

In order for the photons to move momentum is required, are you telling me that it has no momentum (energy) transformation in space? (I think it has so i must be reading what you are saying wrong)

A 100% reflective mirror should not drain any more energy than a "normal" straight line path in space. (are you saying that this is wrong)
All that changes is the direction of the photons, not the photons themselves, correct?

When a photon bounces off a mirror, it loses a small amount of energy and momentum. These are transferred to the mirror, as shoffsta said.

but not if it is 100% reflective, how can that be possible?

I know that a mirror is not 100% reflective, but in theory.

btw, a photon, what exactly is it?

but not if it is 100% reflective, how can that be possible?

The reflectivity of the mirror doesn't actually make any difference, as long as the photon actually hits the mirror and is reflected. For a 5% reflective mirror, or a 100% reflective mirror, the photon will transfer some momentum to the mirror when it hits, and therefore will lose a small amount of energy on reflection.

oh, when i think about it. You are right.

Im sorry and i withdraw my statement.

James, the question is: do u believe that 100% reflection can occur in nature? I believe so.

swansont is right. the theoretically maximal reflection is a reflection where all momentum is conserved, and nothing goes to heat.

Sorry, editing this to say that although momentum is not conserved, energy is still conserved which is what we are talking about. Energy is a scalar quantity.

Everyone is still talking about mirrors even though I have categorically stated ages ago that there are no mirror surfaces in nature which can 100% reflect light!

Sorry, editing this to say that although momentum is not conserved, energy is still conserved which is what we are talking about. Energy is a scalar quantity.

If momentum isn't conserved, then it's not something that occurs in nature.

You said you think this occurs in nature. Do you have an example?

If momentum isn't conserved, then it's not something that occurs in nature.

You said you think this occurs in nature. Do you have an example?

Sorry I didn't mean "not conserved" I meant that its magnitude is not lost (which is where conservation is applied).

Do you have an example?

I just want to wait a while to see if James or others agree with me. Meanwhile, is there any other objections to what I have said?

"Sorry I didn't mean "not conserved" I meant that its magnitude is not lost (which is where conservation is applied)." - John Connellan
uh? momentum is a vector, which is only conserved if magnitude AND direction stay the same!
btw: have you heard of light sails, where they try to push spaceships reflecting sunlight, just the same way as normal ships do with wind?

momentum is a vector, which is only conserved if magnitude AND direction stay the same!

Think of throwing a ball at a wall. The ball does impart some momentum to the wall but think of throwing a lighter and lighter ball until it is infinitely light. The ball retains the same momentum but changes direction. NOBODY says that momentum is not conserved here just because the ball changed direction!!!

Now maybe u are using a different (more constrained) definition of what reflection actually is but I'm almost certain that the phenomenon I speak of IS reflection (albeit not necessarily including a mirrored surface).

btw: have you heard of light sails, where they try to push spaceships reflecting sunlight, just the same way as normal ships do with wind?

Yes.

Think of throwing a ball at a wall. The ball does impart some momentum to the wall but think of throwing a lighter and lighter ball until it is infinitely light. The ball retains the same momentum but changes direction. NOBODY says that momentum is not conserved here just because the ball changed direction!!!

Now maybe u are using a different (more constrained) definition of what reflection actually is but I'm almost certain that the phenomenon I speak of IS reflection (albeit not necessarily including a mirrored surface).


In your example, even with 100% elasticity, the ball loses a little energy to the wall/earth system.

"The ball retains the same momentum but changes direction." is an oxymoron. Momentum is a vector. If it changes direction, the momentum changes.

Hum,
sry to butt in...

[on topic]
Yes, deuterium can form ice, which fortunately for those who take `whisky on the rocks` is slightly heavier (10%) than normal h₂0...

The reasons that they exist in different forms is to do with the energy levels of the `individual` molecules (as answered in the previous posts)

[/on topic]


er, carry on...

In your example, even with 100% elasticity, the ball loses a little energy to the wall/earth system.

"The ball retains the same momentum but changes direction." is an oxymoron. Momentum is a vector. If it changes direction, the momentum changes.

What I'm saying is that conservation does not apply to direction. It only applies to the sum of magnitudes. If u keep thinking of the ball getting lighter, then at infinity, the momentum of the system will be completely within the ball even though it has bounced off the wall. There is no reason to suggest that at some point this changes.

There is a case in nature that when light hits a surface, it changes direction without loss of momentum. This is 100% reflection.

What I'm saying is that conservation does not apply to direction. It only applies to the sum of magnitudes. If u keep thinking of the ball getting lighter, then at infinity, the momentum of the system will be completely within the ball even though it has bounced off the wall. There is no reason to suggest that at some point this changes.

There is a case in nature that when light hits a surface, it changes direction without loss of momentum. This is 100% reflection.

Actually, momentum conservation must apply to both its magnitude and direction, cannot be just to its magnitude. There is no such thing as 100% reflection or reversal of motion direction of ball (or photon) hitting a wall without imposing momentum to the wall. Whatever minute the ball, it must give the wall momentum (however small), which if you take into account shall ensure the momentum conservation.

So how then, is direction conserved in your example?

So how then, is direction conserved in your example?

The wall recoils in one direction, while the ball recoils in the opposite direction. The wall, being much more massive, doesn't recoil very much.

"Direction" is not a conserved property, per se, but momentum is, and it's a vector - it has a direction.

Exactly, the only number conserved is the magnitude, so there's nothing in the conservation laws which state that momentum must be imparted on another object. Now there are other laws which state that momentum will be imparted if two objects touch each other but i still believe I have an example of reflection which does not impart momentum.

I will definitely tell u before the end of this week if nobody cares to comment whether I could be right or wrong :)

Exactly, the only number conserved is the magnitude, so there's nothing in the conservation laws which state that momentum must be imparted on another object. Now there are other laws which state that momentum will be imparted if two objects touch each other but i still believe I have an example of reflection which does not impart momentum.


No, sorry, but this is wrong. This is physics 101 material. If you have this wrong you have no hope of discussing more advanced concepts.

In the absence of an external force, momentum of a system will not change. An object that changes direction has changed its momentum. Therefore, something else has to have changed momentum as well, so that the sum remains constant. Since it's a vector, direction matters.

But go ahead and give your example.

OK we're going off topic a little again. I think I have said before in this thread that the important thing here is energy, not momentum so there's no point bringing it up. Energy is not a vector and is related to the scalar quantity of momentum. The energy of the light will not be affected by 100% reflection.

No, sorry, but this is wrong. This is physics 101 material. If you have this wrong you have no hope of discussing more advanced concepts.

Ur right, what i said there was wrong but it was not what i meant. When I talk about momentum from now on, I am talking about the scalar quantity which is the important thing here. The term is not so much conserved here (since we are excluding the vector part) but it STAYS THE SAME (i.e. without losses).

In the absence of an external force, momentum of a system will not change. An object that changes direction has changed its momentum. Therefore, something else has to have changed momentum as well, so that the sum remains constant. Since it's a vector, direction matters.

Now I could still be wrong, but is it not the case that light is bent by gravitational fields (which act perpendicular) but will not lose energy if the force remains perpendicular at all stages of its journey?

I would like u to agree with at least some of what i said before I tell u where (I think) light could be reflected as such. :)

Ur starting to make me doubt myself now though (which is a good thing I guess :D )!!!

You cannot simultaneously ignore conservation of momentum and be talking about a real system or even an idealized situation that represents a real system.

Now I could still be wrong, but is it not the case that light is bent by gravitational fields (which act perpendicular) but will not lose energy if the force remains perpendicular at all stages of its journey?

The situation you describe, but applied to massive particles, is exactly how you "slingshot" a spacecraft to give it more energy, so this doesn't help your case. You can't keep the force perpendicular at all points, so work will be done and energy transferred.


I would like u to agree with at least some of what i said before I tell u where (I think) light could be reflected as such.

This isn't politics, where compromise is expected. You are ignoring/violating a basic law of physics, so I can't agree with it.

You cannot simultaneously ignore conservation of momentum and be talking about a real system or even an idealized situation that represents a real system.

I just said, I am not ignoring conservation of momentum! Momentum IS conserved AND direction changes. Whats wrong with that?

I just said, I am not ignoring conservation of momentum! Momentum IS conserved AND direction changes. Whats wrong with that?

Nothing, other than it's inconsistent with what you actually said:

When I talk about momentum from now on, I am talking about the scalar quantity which is the important thing here. The term is not so much conserved here (since we are excluding the vector part) but it STAYS THE SAME (i.e. without losses).

"Momentum IS conserved AND direction changes. Whats wrong with that?" -John Connellan
Momentum is a VECTOR!!!
A VECTOR IS ONLY CONSERVED, IF MAGNITUDE _AND_ DIRECTION STAY THE SAME!!!
THAT's why what you're saying is wrong!!!

Yeah I see what u mean! Basically, if something is changing direction then it must have done so by some external force which would have changed the momentum of that external force (source).

In that case, what I read about a certain type of reflection must be wrong wrong if taken literally. Maybe they meant "effectively 100%" reflection or something!

"The phenomenon of Total Internal Reflection causes 100% reflection. In no other situation in nature, where light is reflected, does 100% reflection occur. So TIR is unique and very useful."

Internal reflection (http://floti.bell.ac.uk/MathsPhysics/1total.htm)

If the the angle in medium 2 is larger than the critical angle then the sine of that angle is larger than the sine of the critical angle. Thus the sign of the angle in medium 1 is greater than 1 which is impossible. Thus the light cannot be refracted at all, it is 100% reflected.

http://www.phys.ufl.edu/~avery/course/2049/f2003/lectures/lecture_critical_angle.doc

Above the critical angle, the field reflectivities have modulus 1; they are in the form of pure phase shifts. The power reflectivity is 100%.

http://www.cvilaser.com/static/tech_refltransphase.asp

Could these really be wrong? Could they be just approximating "no losses" of energy?!

100% reflection refers to no loss of photons - there is no transmission. The energy loss is small and is usually ignored, and this is typically a good approximation. But it's there.

100% reflection refers to no loss of photons - there is no transmission. The energy loss is small and is usually ignored, and this is typically a good approximation. But it's there.

Here are 2 more references I have found.

At incident angles greater than these values there can be no energy flow across the boundary and the wave becomes totally internally reflected.

http://www.irfilters.reading.ac.uk/library/technical_data/substrate_optical_properties/page07.htm

When one speaks of total reflection, it is really total in the sense that no energy is lost upon reflection. In other words, the angle of incidence is equal to the angle of reflection on the same side of the normal. This is the optimal path of light in fiber optic waveguides. The reflectance of most mirrors is around 95 to 99%. This means that at each reflection around 1 to 5% of the power is lost. If you have a waveguide 5km long this adds up to a lot of reflections and hence a lot of power loss. The advantage of TIR is that no power is lost in the reflection, hence the term total internal reflection.

http://www.incomusa.com/education.html

I actually tend to agree with u the more I think about it and that these sources are wrong. This should be pointed out to other people I think. Your explanation through the conservation of momentum law makes much more sense.

To think: physics education sources such as these made me forget the law of conservation of energy for a while there :D