how does isolation work ?

garfield1729

hi everybody,

how does isolation work ?

some things dont conduct heat , whereas other dont conduct cold.

what gives these properties ?

and in what proportion ?

im pretty sure mass density of the objects is sometimes related , but there must be more factors to it since light materials can isolate too.

and why are there things that only isolate against cold whereas others only isolate against heat , some dont isolate and others isolate for both cold and heat ?

greetings
garfield1729

gel

So what conducts heat but doesn't "conduct cold"?
What isolates against cold but not heat?

John Creighto

A greenhouse?

gel

hehe.
Not really conduction though.

John Creighto

I think that lattice structures are good at conducting heat because vibrational waves can travel easily trough the medium. Gases are bad at conducting heat because of the low molecular density. As a consequence materials with lots of air pockets like foams are bad conductors. I would conjecure that inprefection in lattices can impede the conduction of heat because they would scatter the phonons.

For lattice structures I would presume that the rate of heat transfer would be proportional to the number of phonons, the velocity of the phonons and the energy carried by each phonon. Also important would be how well heat is transfered at various boundaries in the medium and between the meadum and the soundings.

Also note that the number of phonos is realated to temperature. In classical physics each vibrational mode would carry an energy of [tex]\frac{1}{2}k_b T[/tex]

ZapperZ

This really does not answer the OP.

I suggest we all wait for the OP to come back and explain him/herself and give an example of something that conducts heat but not "cold". I am with gel in expressing my puzzlement at this question.

Zz.

russ_watters

The simple answer to the OP is that there is only one kind of heat conduction, and it is drawing heat from a warm area to a cold area. So the presumptin of the question is incorrect.

John Creighto

Well, I was presuming that one of his questions was, "how does insulation work?". I was presuming a spelling error.

garfield1729

why dont you guys believe me ?

isolation materials are used all the time.

what about greenhouses , isolation material to keep out the cold in buildings , or to keep out the heat in warm countries.

so you are claming all materials either both conduct cold and heat or neither ???

Riogho

Do you mean INSULATION?

ZapperZ

Look in a simple thermos container. It keeps warm drinks warm, but it also keeps cold drinks cold!

An insulator insulates against both heat AND cold. It simply means that it keeps heat in one side and cooler region on the other side without a lot of heat conduction. "cold" simply means less heat. There's no common material that can only conducts heat, but not "cold", because that phrase "conducts cold" makes no sense. You remove heat from something. You don't add cold to it.

Zz.

garfield1729

yes a thermos is a nice example of something that keeps things hot when hot and cool when cool.

so you claim no thermos of " exotic material " can be made to only keep cool ?

what about those boxes you put ice in to keep the food cold ?

they dont heat up much when the envirement out of the box is warm;

but they can still cool down more if the envirement is very cold.



suppose you are right , can you explain it ; prove it ?

if i am right , would it lead to free energy or perpetuum mobile perhaps ?

im now talking about heat , would the same apply if i wrote electricity , gravity , magnetism I.E. is the concept of isolation/conduction universal in physics or are there exceptions ?


regards
garfield1729

Riogho

The universe desires to be at an equilibrium. The energy wants to diffuse in such a way that it permeates all space equally. The only thing you can do is slow down the diffusion of the energy by putting something there to block it. Your 'isolator'.

And if I were to put a heat source in there as hot (compared to the outside) as ice is cold (compared to the outside) it would have the exact same effect.
Thermodynamics

ZapperZ

What?

Those "boxes" also keep stuff warm, the same thing with those thermos. There's no difference. All you have is a thermal insulation!

Thermodynamics. Study it.

Please read the PF Guidelines before you go off into your "perpetuum mobile" dreamland. We don't allow crackpottery, and we certainly have put such perpetual machine in our banned topics.

You need to study a bit more physics before putting out statements like this, especially when you still have a lot of misinformation, misconception, and misunderstanding of basic phenomena such as heat and thermodynamics.

Zz.

wysard

The second law of thermodynamics lets us know that insulation will never stop heat flow, just slow it down. Heat mechanics are no where near my strong suit, so I am sure that they will elucidate the matter shortly, but suffice it to say that my understanding is that insulation mostly works by slowing the transfer of energy from one side of an object (call it the "hot" side) to the other (the "cold" side) by some combination of minimizing the number of paths for energy transfer and making the remaining paths as energetically expensive as possible. A simple example of the first is a vaccum. Less particles means less opportunity to transfer energy through the medium, but energy still flows through the connecting structures that maintain the vacuum. A simple example of the second are the tiles that make up the exoskin of the shuttle where the centre can get white hot but you can still pick up the tile by the edges.

BTW OP, the tiles will still get hot eventually, the premise is that when the shuttle gets past the height of it's deceleration the heat spills back off the tiles to the cooler air of the atmosphere flowing over the shuttle and is dissipated. There is NO solution in physics to make energy flow "dissapear" magical or otherwise, that's just crackpot theories that have no place in physics.

garfield1729

sorry if i annoye you.

im sorry , im no expert in physics , dont intend to be a crakpot.
im not here to promote or believe in perpetuum mobile.

in fact i dont believe in perpetuum mobile or free energy.

but does it make sence to say " garfield is wrong ; because if he were right , he could make a perpetuum mobile " ??

regards
garfield1729

Hootenanny

I don't want this deteriorate into an argument and I'm sure that Zz would like to rebut you comments, but as an observer I have read through the thread and I cannot find a single line where Zz either, implicitly or otherwise, makes the following statement.
I am not taking sides, but you should realise that many people work hard to keep this site as respectable and scientifically accurate as possible. You should believe me that I mean no offense by this, but it is often very frustrating when someone with very little or no formal physics education posts ludicrous theories that violate fundamental laws. You say in you previous post that you do not advocate perpetual motion machines, and although I do believe you, you mention that if your opinions are correct then perpetual motion is possible and then challenge Zz to prove you wrong. It should be the other way round! It is you that is saying that if you are right perpetual motion would be permitted, which is contrary to mainstream science, it should be you that should be proving your comments. If you cannot prove them for whatever reason, you should take the advice you are being given and ask constructive questions.

Anyway, back to the problem in hand, I'll try to explain this in layman's terms.

Firstly if you have a cold drink in a glass, the reason the liquid doesn't stay cold forever is because energy is being transfered to it from the environment, via conduction (through the walls of the glass) and by thermal radiation (light from the sun). Now, if you want to keep your drink nice and cold you will want to put it in a thermos flask. A thermos flask basically consists of two opaque cylinders with a partial vacuum between them. This partial vacuum prevents energy being transfered to by conduction (via contact); the opaque material also prevents the thermal radiation from transferring energy to your drink. In this way, the liquid is said to be thermally isolated, it cannot exchange energy with the environment.

Equally, the opposite is true if you want to keep a hot drink warm. A hot liquid will transfer energy to the environment via conduction (contact) or convection and thus it's temperature will decrease. Therefore, if we thermally isolate the hot drink as above, no energy will be transfered to the environment and therefore your drink will remain warm.

I hope this makes more sense.