A Question for the Scientifically Versed.

[Martin Milner]

I think one possible source of Emm's lad's confusion is that although aluminium IS a good heat conductor like most metals, it's not as good as some, like gold or copper. Some aluminium pots & pans have a copper disk in the bottom, but covered over so it doesn't get in contact with the food. This helps heat conduction even more - but cheap aluminium pots still work without the copper disk.

You also might want to remind him about the three methods of heat transfer, conduction, convection, and radiation, and how these work together to rapidly cool the thin aluminium foil while the pizza, which is a poor conductor of heat, remains hot.

Ceramics like plates and crockpots are poor conductors of heat, and thus retain their heat longer.

And now for the scientifically minded - if you fill one ice cube tray with cold tap water and one with boiling water, and put them side by side in the freezer, which will form icecubes first, and why?

[chrisoff]

and thus the right to spell... jail ...the right way

wait, what?

[SteveShaw]

In their application on the outside of spaceships hurtling through the upper atmosphere it is not the function of the tiles to heat to the core. Were this to be the case the heat gained (from friction) would dissipate both ways, both back into space and also, fatally, into the ship. The whole point is that they heat up on their outer surface only, their cores remaining relatively cool, as a result of their being very poor conductors.

As for the thinness of aluminium foil and radiator fins, I'm at a loss as to why you omit the most crucial part of the explanation - in these instances, as with the elephant's ears, it is the very large surface area to volume ratio that ensures that heat can be dissipated rapidly. Sometimes it takes a biologist to get the physicists to the heart of the matter.

Actually, no - that really isn't correct Steve. The tiles DO in fact heat up in their cores, and not just superficially on the surface. You would be perfectly correct however stating that the outer surface (the one directly in contact with the ionized atmosphere during re-entry, or the one directly in contact with the intense flame from the torch we used) heats up more than the core. But we did in fact heat the core of those tiles to incandescence daily, as well as the exterior surfaces.

No, no, NO!!! Of course you can roast the damn things to perdition in an oven and they'll get hot in the middle eventually, but in their application on the space shuttles the whole point of them is that they heat on just the outer surface and not to the core. After all, the source of heat in space is unilateral, unlike in an oven. The situations are entirely different.

As to the surface area issue - this is a property of any matter that is heated. Spread it out, and the increased surface area will cool it faster than if the material is more dense and compact.

It is directly a function of the surface area to volume ratio. And it is not the increased surface area that cools it faster but the increased rate of heat loss by conduction and radiation from the larger surface area. Note, I do not say increased rate of conduction and radiation. Per unit area, that would remain the same, the only limitation being that heat has further to travel by conduction from the middle of a thicker mass of metal than a thinner one, but as aluminium is a very good conductor anyway the practical effect of this would be very small. And I will refrain, out of sheer kindness, from pointing out to the whole world your misuse of "dense" in this context.

You can spread the coals and embers of a campfire to help put it out right?

This is a confusing example to say the least. When you spread out the embers of a fire you indeed increase the surface area available for conduction, convection (relevant in this case) and radiation, but you are also increasing the surface area through which oxygen can access the coal, thereby causing the coals to burn more quickly and so burn out faster. Paradoxically, in some circumstances you may be disrupting the fire's structure by poking around at it in that you're coating the coals with ash and actually preventing oxygen reaching them. This will often be the case with a fire that is already quite low (only a little combustible material remaining and lots of ash). This smothering would put the fire out leaving some unburned coal. Happens all the time in my stoves at home. In any science class, there would be at least one intelligent kid (aka awkward pain in the arse) who would pick you up for giving such a shaky and ramshackle example. I know all about this because I taught science for 25 years and found I couldn't get away with pat explanations of things and because I've been burning coal in stoves to keep my house warm for 22 years.

My point if you check, was that the thinner the heat sink (aluminum in this case as that was the original question) the faster it will radiate it's stored heat energy. A heat sink made with very thick vanes, but the same surface area will not cool quite as quickly or efficiently in most cases - as a general rule.

It will not radiate faster. It will radiate at the same rate per unit area whatever the thickness, the only limitation being the one I mentioned above. It's just that a thinner vane will cool more quickly because it can't store as much heat as a thicker one in the first place. A heat sink with thicker vanes will cool more slowly, wait for it, because it has a smaller surface area to volume ratio.

Blasting off to supper! All the best...

Spread your food out on your plate and it will cool quicker...larger surface area to vo...aarrgh...

[SteveShaw]

and thus the right to spell... jail ...the right way

wait, what?

Gonzo said it, not me!

[Martin Milner]

In their application on the outside of spaceships hurtling through the upper atmosphere it is not the function of the tiles to heat to the core. Were this to be the case the heat gained (from friction) would dissipate both ways, both back into space and also, fatally, into the ship. The whole point is that they heat up on their outer surface only, their cores remaining relatively cool, as a result of their being very poor conductors.

As for the thinness of aluminium foil and radiator fins, I'm at a loss as to why you omit the most crucial part of the explanation - in these instances, as with the elephant's ears, it is the very large surface area to volume ratio that ensures that heat can be dissipated rapidly. Sometimes it takes a biologist to get the physicists to the heart of the matter.

Actually, no - that really isn't correct Steve. The tiles DO in fact heat up in their cores, and not just superficially on the surface. You would be perfectly correct however stating that the outer surface (the one directly in contact with the ionized atmosphere during re-entry, or the one directly in contact with the intense flame from the torch we used) heats up more than the core. But we did in fact heat the core of those tiles to incandescence daily, as well as the exterior surfaces.

No, no, NO!!! Of course you can roast the damn things to perdition in an oven and they'll get hot in the middle eventually, but in their application on the space shuttles the whole point of them is that they heat on just the outer surface and not to the core. After all, the source of heat in space is unilateral, unlike in an oven. The situations are entirely different.

I would think the situation Brian described is exactly the same - using a blowtorch on one face of the cube - as the unilateral heating from friction during re-entry. I don't see from where you get the idea that Brian uses an oven to conduct his heating experiments.

As to the thinness vs surface area, I think you're both on the same side of the argument. The same volume of material will cool quicker when spread out thinly than if rolled up as a sphere, which presents the smallest possible surface area to volume ratio.

Which leads to another great science experiment for Emm - take two identical sheets of aluminium foil, roll one in a ball and leave the other flat. Roast both in the oven for a while on a baking tray, take them out to cool for 1 minute, then get #1 son to hold the ball for a while, while you hold the sheet, and see whose eyes water most.


Dr. Karl - doing the Science.

[SteveShaw]

I would think the situation Brian described is exactly the same - using a blowtorch on one face of the cube - as the unilateral heating from friction during re-entry. I don't see from where you get the idea that Brian uses an oven to conduct his heating experiments.

Were the tiles to heat to the core, implying a degree of conductivity of heat from surface to centre, it follows that the dissipation of heat from them would be both outwards and inwards. The temperatures on re-entry on the outer surface of the tiles far exceed the melting point of aluminium, the metal the shuttle is predominantly made of. So you're saying in effect that the tiles dissipate a large amount of heat inwards, into the superstructure of the shuttle, which is itself an excellent heat conductor. This would be fatal to the shuttle and its occupants. In the instance of the shuttle re-entry the whole point of the tiles is that they mustn't heat to their cores to any significant degree ("incandescent," I believe Brian said). I should like to see the experimental evidence which reveals temperature profiles through the tiles whilst being subjected to intense unilateral heating. I'd be amazed if it were anything other than a steep temperature gradient from high to low, heated side to unheated side.

[Denny]

Brit Fight!!! Brit Fight!!!


ya know, it just doesn't have the same ring to it

[SteveShaw]

It'll be pistols at dawn, old boy. I'll make sure my bullets have a bigger surface area than his...

[Brian Lee]

Yeah Steve - I mean, I only worked directly with shuttle tiles in my hand for years and years. And we only knew a whole bunch of the techs from Nasa who were involved in their initial creation and refinement over the years. And we only worked on national education programs about the very tiles of which you speak. So I'm sure you know how hot the tiles were in the center while I was holding them in my own uncovered mitt. I understand the reasoning behind your thinking however, but I can promise you, the tiles did indeed heat in their core, and they indeed did not burn our skin on the opposite side. And I don't know how you British types do it over there, but my skin is definitely NOT as tough as the aluminum and titanium sub structures of the shuttle.

But I'll defer to you. You know more!

And Martin, I KNEW there was a reason they chose not to use pizza on the shuttle as a heat shield! LOL

[Denny]

Brian, the burning question "have ya ever made pizza with a blow torch"?

[SteveShaw]

Yeah Steve - I mean, I only worked directly with shuttle tiles in my hand for years and years. And we only knew a whole bunch of the techs from Nasa who were involved in their initial creation and refinement over the years. And we only worked on national education programs about the very tiles of which you speak. So I'm sure you know how hot the tiles were in the center while I was holding them in my own uncovered mitt. I understand the reasoning behind your thinking however, but I can promise you, the tiles did indeed heat in their core, and they indeed did not burn our skin on the opposite side. And I don't know how you British types do it over there, but my skin is definitely NOT as tough as the aluminum and titanium sub structures of the shuttle.

But I'll defer to you. You know more!

And Martin, I KNEW there was a reason they chose not to use pizza on the shuttle as a heat shield! LOL

Was there some sort of magic going on then? You could hold a tile that was red-hot in the middle and cold on the outside? How did the heat get to the middle, and how did the heat get back out again if not via your hand? Did it never cool down, like the sun staying so hot in the middle? I've flushed lavatories thousands of times for years and years yet I couldn't explain how they work. I am genuinely intrigued here. I'm lookin' for answers here!!

[buddhu]

It'll be pistols at dawn, old boy. I'll make sure my bullets have a bigger surface area than his...

Just hope that, as a target, you don't present the larger surface area...

[Brian Lee]

Was there some sort of magic going on then? You could hold a tile that was red-hot in the middle and cold on the outside? How did the heat get to the middle, and how did the heat get back out again if not via your hand? Did it never cool down, like the sun staying so hot in the middle? I've flushed lavatories thousands of times for years and years yet I couldn't explain how they work. I am genuinely intrigued here. I'm lookin' for answers here!!

None that I know of Mate - just physics. And while we played with it daily - there is still plenty I can't well explain properly. The tiles indeed were glowing white hot - in their cores, but we never were burned. If we left the torch on for about ten minutes, we could almost pretend we felt a slight increase in temp on the backside. But as the melting point of the alloys used in the shuttle are still much higher than human skin, it never got to a point as to even be remotely uncomfortable to the touch.

Now that being said - if you got in a rush with the demonstration, or were in any way careless - and let just a hint of that flame creep around the edges...please believe me when I tell you that you K-N-E-W it. IMMEDIATELY!!!! YOWZA!

We also had loads of fun burning things with concentrated liquid oxygen, freezing all manner of object with liquid Nitrogen, catching ceiling tiles on fire during our rocketry demonstrations, shocking the ever-living snot out of much deserving child types, trying not enjoy the loverly carcinogenic side-effects from our mini volcano and the burning ammonium dichromate TOO much, and who could forget the skin burning / sight threatening goodness of our krypton and argon lasers Baby! GOD I MISS LASER FLOYD!

But for whatever reason, the heat dissipation was gentle enough, that the backside of those darn tiles never got even remotely hot to the touch.

Sadly - we never had a single experiment involving beer. Not even one. Damn.

P.S. There was a show on the History channel the other night all about the mighty pooper. It showed how they work. I was enlightened. If I can find it somewhere on DVD, I'll post it to you for Christmas!

[izzarina]

personally, i'm wondering why foil has such excellent shielding properties against both government AND extraterrestrial mind-control apparati.

truly amazing stuff.

Yes, I agree totally!

[gonzo914]

And now for the scientifically minded - if you fill one ice cube tray with cold tap water and one with boiling water, and put them side by side in the freezer, which will form icecubes first, and why?

Alhough not scientifically minded, I would think the tray made from the cold tap water would freeze first.

Why?

Because it starts out colder.

And because the Mpemba effect only occurs in very specific conditions, and it is unlikely you will get those in your freezer.

If you want quick ice cubes, use cold water. (And if you want good chili, make it my way.)