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Author Topic: Could steel be burned using sunlight focused through a magnifying glass ?  (Read 37925 times)

Offline syhprum

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The power available depends on the area of the lens and temperature depends on the inverse of the f number, the problem of obtaining a sufficiently small f number can be overcome by using a two stage focusing system.
The melting of steel has been demonstrated both by this method and by mirrors.
References to follow when I hunt them down.
Whether or not 6000°K can be exceeded is an open question I would like to see comments.

http://gizmodo.com/5069043/solar-furnace-melts-steel-our-minds
« Last Edit: 03/02/2010 09:34:01 by syhprum »
 

Offline yor_on

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A
Why would a monochromatic LASER not have a colour temperature (it may not have a black body spectrum, but the light still has an energy associated with its frequency, and that energy equates to a temperature)?
Just because it doesn't have a blackbody spectrum. I can find a specific wavelength, let's say 550 nm, in the spectrum of a blackbody of any temperature. I can take a tungsten block at 20°C or at 3000°C, put a very good colour filter in front of it, which let pass only 550 nm light, and how could we establish this light comes from a 20°C or 3000°C block of metal? Certainly at 20°C the intensity of radiation at 550 is less, but this could also be ascribed to source's distance and surphace properties.
It's only the spectrum's shape (specifically: its maximum's position) which is related to temperature.

Lightarrow?
If I get it right you need a whole spectrum to make a decided temperature.

But if it comes to a lasers light, how would you go about to define a temperature for it, isn't it the wavelength/frequency you would use then?

They have an energy, they must have a temperature?
What am I missing?
==

Or should I ask about the heat instead?

"Temperature is a number that is related to the average kinetic energy of the molecules of a substance. If temperature is measured in Kelvin degrees, then this number is directly proportional to the average kinetic energy of the molecules.

Heat is a measurement of the total energy in a substance. That total energy is made up of not only of the kinetic energies of the molecules of the substance, but total energy is also made up of the potential energies of the molecules."
« Last Edit: 06/02/2010 18:21:50 by yor_on »
 

Offline lightarrow

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A
Why would a monochromatic LASER not have a colour temperature (it may not have a black body spectrum, but the light still has an energy associated with its frequency, and that energy equates to a temperature)?
Just because it doesn't have a blackbody spectrum. I can find a specific wavelength, let's say 550 nm, in the spectrum of a blackbody of any temperature. I can take a tungsten block at 20°C or at 3000°C, put a very good colour filter in front of it, which let pass only 550 nm light, and how could we establish this light comes from a 20°C or 3000°C block of metal? Certainly at 20°C the intensity of radiation at 550 is less, but this could also be ascribed to source's distance and surphace properties.
It's only the spectrum's shape (specifically: its maximum's position) which is related to temperature.
Lightarrow?
If I get it right you need a whole spectrum to make a decided temperature.
Not only this, you have to be sure that your spectrum is part of a blackbody spectrum.

Quote
But if it comes to a lasers light, how would you go about to define a temperature for it, isn't it the wavelength/frequency you would use then?
Absolutely not. As I wrote (3 years ago!) in the post you quoted, you could have two equal light beams, the same intensity, the same wavelenght and coherence, everything the same, but one coming from your laser and the other coming (after adequate filtering equipments) from a big bulb lamp. How do you establish the temperature?

Quote
They have an energy, they must have a temperature?
What am I missing?
To define the temperature of an EM radiation you must have a photon gas with a blackbody spectrum. Energy and temperature are two completely different concepts.

Quote
Or should I ask about the heat instead?

"Temperature is a number that is related to the average kinetic energy of the molecules of a substance. If temperature is measured in Kelvin degrees, then this number is directly proportional to the average kinetic energy of the molecules.

Heat is a measurement of the total energy in a substance.
No, heat is a way to transfer energy between two bodies: through a difference in their temperature.

Quote
That total energy is made up of not only of the kinetic energies of the molecules of the substance, but total energy is also made up of the potential energies of the molecules."
...at least. :) There is also another form of energy: mass of particles.
« Last Edit: 06/02/2010 23:32:52 by lightarrow »
 

Offline yor_on

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Thanks LA, one mystery less :)
 

Offline Geezer

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Everyone: Thank you for your contributions on this topic. I think it's clear that solar energy can be concentrated sufficiently to melt steel, and a lot of other things too. In the presence of oxygen, steel will burn at those temperatures.

I think this horse has been officially flogged and it's time to move on. Does anyone have violent objections to locking or abandoning this thread?

Of course, I think there are some very important questions and concepts associated with the original question, but it might be more productive if we were to initiate new topics that focus on those specific points.

Wadyafink?
 

Offline lightarrow

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...
The melting of steel has been demonstrated both by this method and by mirrors
....
http://gizmodo.com/5069043/solar-furnace-melts-steel-our-minds
In this video we can see a metal plate which is melting in a circular region and producing a hole. But why it doesn't become red, then orange, then yellow, then white? Is it steel or tin.... :)
(Then the image change, then it comes again on the melted plate with the hole, and this time it's orange-hot...strange :)).
« Last Edit: 07/02/2010 11:10:02 by lightarrow »
 

Offline Bored chemist

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My guess is that the filters they must have used in front of the camera (to stop the incredible brightness washing out the image) must have distorted the colour. Afterwards they show a conventional image as the metal cools down and you see it's still red hot.
 

Offline lightarrow

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It could be, but it doesn't convince me a lot. Why we don't see any smoke? If there were put filters for brightness, why we don't see the various colors (red, orange, yellow) in the different regions of the plate, even if the entire plate is perfectly visible?
 

Offline yor_on

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Are you thinking coherent (lasers) light versus incoherent (sun) light Lightarrow?
Could it be that the effect is so fast, the heat so instantaneous that the atoms around it doesn't have the time to react?
 

Offline Bored chemist

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The melting takes a few seconds so it can't be that.
It's  a good question. The molten metal doesn't seem to be glowing as it melts; and it should be.
I still think it's glowing, but you can't see it because of the glare of the multiplied Sun. We normally think of molten steel as bright but, in this context, it's quite dull compared to the incident light.
The same dark glass filters that protect the camera are hiding the glow.
 

Offline yor_on

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My link is so phreaking (G3) slow so i usually stay away from movies, but I guess i will need to see this one. Just guessing, I think you're right though Bored Chemist, If it takes seconds there is no possibility of the atoms not being able to react and then it seems as it must have something to do with the light caught in the camera.
==
Very cool effect and over quite a large area too, I was expecting it to be more like a laser, a needle sort of :) But I agree, I think it's the camera that gets 'overloaded' by the light even with the filter on, or possibly that it gets melted so quickly and over such an large area that the redness sort of gets hidden under it and . . Awh :) But now that I've seen it I got to admit that I don't know, you can retract the imagery and compare the plates as they melt and after and the plates color seems almost the same even though that the 'after' should be without any Sun-beam working?
« Last Edit: 08/02/2010 08:43:08 by yor_on »
 

Offline lightarrow

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How could we explain the absence of smokes?
 

Offline Geezer

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It heats up so rapidly there is little time for any oxidation. The steel is melting rather than burning. When you cut steel with oxygen, you are burning it in an exothermic reaction.

BTW - you can take a trip to France to see one in action (if it's still in operation.)

http://en.wikipedia.org/wiki/Solar_furnace

Edit: Come to think of it, the answer to the original question is, technically, no.

Unless you grind up the steel into small particles, it won't burn. A steel plate can only be burned in an atmosphere that is very rich in oxygen.
« Last Edit: 08/02/2010 17:18:49 by Geezer »
 

Offline Bored chemist

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How could we explain the absence of smokes?
Because the steel is only just melting,not burning.
 

Offline lightarrow

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How could we explain the absence of smokes?
Because the steel is only just melting,not burning.
There's no oxygen in the atmosphere there? :) At ~ 1400°C Iron, carbon, iron carbides do burns and generate smoke, IMHO.
« Last Edit: 08/02/2010 19:24:57 by lightarrow »
 

Offline lightarrow

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It heats up so rapidly there is little time for any oxidation. The steel is melting rather than burning. When you cut steel with oxygen, you are burning it in an exothermic reaction.

BTW - you can take a trip to France to see one in action (if it's still in operation.)

http://en.wikipedia.org/wiki/Solar_furnace

Edit: Come to think of it, the answer to the original question is, technically, no.

Unless you grind up the steel into small particles, it won't burn. A steel plate can only be burned in an atmosphere that is very rich in oxygen.
It doesn't have to burn completely to generate smoke, only a little part.
 

Offline Geezer

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It heats up so rapidly there is little time for any oxidation. The steel is melting rather than burning. When you cut steel with oxygen, you are burning it in an exothermic reaction.

BTW - you can take a trip to France to see one in action (if it's still in operation.)

http://en.wikipedia.org/wiki/Solar_furnace

Edit: Come to think of it, the answer to the original question is, technically, no.

Unless you grind up the steel into small particles, it won't burn. A steel plate can only be burned in an atmosphere that is very rich in oxygen.
It doesn't have to burn completely to generate smoke, only a little part.

I'm sure there is a small amount of smoke produced, but at that temperature any carbon particles (soot) will be converted into CO2 rather quickly.

BTW, despite the fact that this is quite an impressive demonstration of localized high temperatures, it's not really a very good demonstration of power production. I can burn a hole in a chunk of steel almost as quickly with my oxyacetylene cutter.
« Last Edit: 08/02/2010 20:16:45 by Geezer »
 

Offline Busky Dubbs

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So the answer is, not really with a lens, but yes with parabolic/concentrating mirrors. This article cited earlier in the thread talks of making experimental alloys and 6,000F newbielink:http://www.time.com/time/magazine/article/0,9171,909204-1,00.html [nonactive]  so certainly a steel sheet is not a problem. And yeah, smoke is from incompletely burned materials, so here they're raising it to its melting temperature, not 'burning' it with oxidation (not intentionally), like they'd do with a graphite electrode in a smelting crucible. Pours right out.

I'm still confused about the whole temperature / heating / light wavelength back-discussion.
I've wondered this for several years, and I just found out:
newbielink:http://www.3drender.com/glossary/colortemp.htm [nonactive]

My friend works in film and they're always concerned about their light source - Keno Flos, natural sunlight, filters, 10Ks, blah blah - and apparently it dates back to William Kelvin's observations of a heated block of carbon. It has nothing to do with the heat of the filament or radiant body's burning. And so now I get what people meant by "black body" - not as in a non-visible light radiation spectra, but  literally a black-bodied chunk of graphite.

The video is of an experimental parabolic concentrator, not a power-generating one; for a power generating one, they only heat a boiler as much as they want the steam temperature to be at for their turbine
otherwise they'd melt the tower..
 

Offline lightarrow

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I'm still confused about the whole temperature / heating / light wavelength back-discussion.
The concept of "Temperature" is not actually as simple as it would seem.
Temperature is a statistical concept, you can't define temperature for a single particle and not even for some. In this case the particles are photons; you must have a lot of them , and so a lot of frequencies, to define T.
In a recent discussion I had in another forum, a physics professor wrote that when a physical system A exchanges energy with another system B only by means of a perfectly monocromatic laser beam, it doesn't exchange heat...
 

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