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 Post subject: Physics question: why does metal feel colder than ambient?
PostPosted: Wed Nov 29, 2006 1:12 pm 
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This is a bit of a general question, as I am not terribly familiar with thermodynamics. Let's say we have an ambient temperature of 20c in a room. We have two 1kg masses of aluminum and wood. The wood block probably feels about ambient temperature. But the aluminum block feels colder than ambient.

Are both masses actually 20c, or is the aluminum block actually colder than 20c? Is this just a perception issue with the thermal conductivity difference between wood and aluminum? Is it possible for an object to be colder than its ambient temperature? If so, how?

Of course, this whole hypothetical situation assumes that a sufficient amount of time has passed to allow the room + aluminum + wood to reach equilibrium.


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PostPosted: Wed Nov 29, 2006 1:25 pm 
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I'm not sure if the following is correct, but it's the first thing that occured to me...

Wood is a thermal insulator. Aluminium is a thermal conductor. When the ambient temperature is 20*C, your body is ~17*C over ambient. By touching the aluminium, you will be transferring some of your body heat into the aluminium, theoretically until your hand and the spot you're touching are in equilibrium. Since wood is an insulator, it cannot draw heat as easily from you, so your hand stays warmer.


edit: thermal conductivity values, liberally adapted from wikipedia:

Air: ~0.025 W/mK
Wood: ~0.100
Aluminium: ~210

So, because the aluminium is drawing heat from you three orders of magnitude faster than either the air around you (your "ambient") or the wood you put your hand on, your brain (calibrated for air, since that's what surrounds us most often) sees it as colder.

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PostPosted: Wed Nov 29, 2006 1:47 pm 
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qviri wrote:
I'm not sure if the following is correct, but it's the first thing that occured to me...

Wood is a thermal insulator. Aluminium is a thermal conductor. When the ambient temperature is 20*C, your body is ~17*C over ambient. By touching the aluminium, you will be transferring some of your body heat into the aluminium, theoretically until your hand and the spot you're touching are in equilibrium. Since wood is an insulator, it cannot draw heat as easily from you, so your hand stays warmer.


edit: thermal conductivity values, liberally adapted from wikipedia:

Air: ~0.025 W/mK
Wood: ~0.100
Aluminium: ~210

So, because the aluminium is drawing heat from you three orders of magnitude faster than either the air around you or the wood you put your hand on, your brain (calibrated for air, since that's what surrounds us most often) sees it as colder.

Thank you, that is exactly what I was thinking for one of the possible explanations I considered to this question!

For my other thought, I don't know how much BS this is, but I'm wondering if the thermal capacity of materials plays into effect here? Would some meterials just absorb more ambient heat than others, causing a difference in temperatures at equilibrium?


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PostPosted: Wed Nov 29, 2006 2:59 pm 
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Your body generally senses changes in environmental conditions, not absolutes. For things like temperature that means things that feel cold are things that are rapidly conducting heat away from you while things that feel ambient are slowly or not conducting heat away from you. How fast something conducts heat away from you (or into you) is a function of its temperature delta with respect to you and its thermal conductivity. Diamonds feel very cool as they are incredibly thermally conductive-a full order of magnitude better conductors than aluminum. Yes, that does mean you want a diamond heatsink.


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PostPosted: Wed Nov 29, 2006 3:34 pm 
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Or a carbon nanotube heatsink! Very good conductor. The problem with carbon nanotubes is it is a one dimensional conductor. All the chemical bonds are in one direction.


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PostPosted: Wed Nov 29, 2006 4:24 pm 
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autoboy wrote:
Or a carbon nanotube heatsink! Very good conductor. The problem with carbon nanotubes is it is a one dimensional conductor. All the chemical bonds are in one direction.


This shouldn't be a problem as long as the nanotubes can all be aligned in the same direction, which it appears they can:

http://en.wikipedia.org/wiki/Carbon_nanotube#Synthesis

Quote:
Of all the various means for nanotube synthesis, CVD shows the most promise....no other growth methods have been developed which produce vertically aligned nanotubes.


Of course, there is the question of cost.... :lol:

disclaimer: I know nothing more about this subject than what is written in wikipedia; a computer w/ access to wikipedia would probably give you slightly more intelligent answers.


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PostPosted: Wed Nov 29, 2006 4:47 pm 
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autoboy wrote:
Or a carbon nanotube heatsink! Very good conductor. The problem with carbon nanotubes is it is a one dimensional conductor. All the chemical bonds are in one direction.


This depends upon how you arrange the carbon nanotubes (actually, carbon nanotubes are a bad choice for a computer heatsink as the thermal expansion coefficient is rather mismatched)

I was at a seminar[1] a month or two back where they were discussing the possibilities of using a Tungsten core CVD diamond fiber in a silver matrix to give excellent thermal conductivity and the right coefficient of thermal expansion. Admittedly this was with a p.f. of 0.8 which is arguably infeasibly high, but it shows excellent potential.


[1] As an interested observer; really it was nothing to do with me and I probably shouldn't even have got invited, but hey ho.

On the original question, a somewhat helpful analogy is to consider yourself as a battery (hear me out on this) the air around you to be a perfect insulator, and when you touch the wooden block (a fairly good insulator) not much current (energy) flows through it because it has a high resistance. Conversly aluminium being a good conductor when you touch that a large current (more energy) flows through it
(Assume that you, the lump of wood and the lump of aluminium have a common ground).

Ask yourself would a battery shorted by wood drain faster than a battery shorted by metal? And then you have a pretty good analogy.

The energy (heat) is leaving you faster to go into the aluminium block, so it feels colder when you touch it. The key point is that you are at a higher potential energy level -be that a higher voltage in my battery analogy, or a higher temperature than the two blocks, meaning that energy flows from you into them. If the block were at body temperature rather than 20 C then neither would feel cold.

According to CES (material selection software) wood has a typical thermal conductivity of 0.31-0.38 W/m.K along the grain and 0.15-0.19 across grain. And Aluminium ranges between 76 -235 W/m.K (this is such a broad range as we haven't specified which alloy we are refering to)

For the other question, if the two materials are at a thermal equilibrium, then they have the same temperature by definition. If on the other hand you expose two materials with different specific heat capacities to the same amount of energy the yes, the temperatures will rise by different amounts. In the case of wood versus aluminium it will actually take more energy to increase the temperature of the wood by a given amount than it will the temperature of the aluminium.

But as you practical experiences tell you, it is the rate at which the energy is leaving your body that defines how "cold" a material is.


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 Post subject:
PostPosted: Wed Nov 29, 2006 4:55 pm 
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jaganath wrote:
autoboy wrote:
Or a carbon nanotube heatsink! Very good conductor. The problem with carbon nanotubes is it is a one dimensional conductor. All the chemical bonds are in one direction.


This shouldn't be a problem as long as the nanotubes can all be aligned in the same direction


Noooo, you don't want them only in one direction. Think of each fiber as a heat pipe. If you get it right (and I don't think it's straight forward with a p.f. of 0.8) then the heat is going out to the fins on the heatsink far more effectively than with a noble metal :)


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