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Heat flow through 2 conductors

  1. Jun 1, 2013 #1
    Say I have a metal object and a wooden object at 0 degrees and I touch them. The metal object would feel colder because it is a better conductor of heat and so it removes heat away more quickly. Would it be because each collision between the atoms of the two materials increases the kinetic of metal atom more than of the wooden atom?

    Also the metal would be able to transfer the heat away more quickly because the same principle works more the metal atom collision with another metal atom whereby the kinetic energy would be spread out more easily? But would this trait of allowing heat to transfer away more quickly cause the metal to feel colder?

    Because even if the heat stays at one atom, if it can absorb the KE from the hand better then it doesn't matter if the heat travels away right? Or does it?

    Because if the heat is unable to transfer away would that stop the heat transfer after a while? I don't think it should affect the heat transfer between the 2 materials because the heat from the hand should be able to continuously increase the KE of that metal atom even if its KEmetal>>KEhand because like in evaporation some liquid particles can have the a lot of KE while others no so much. Am I right about this?

    So the metal reduces the kinetic energy of the hand more quickly than of the wood?

    But if I had a same 2 set up at 50 degrees. When i touch them, the metal one should feel hotter too. How should we explain this? When the wooden and metal atoms collide with my hands would the metallic atom impart more kinetic energy than of the wooden one so the metal would feel hotter?

    And also the metal can transfer more heat to the metal atom in contact with the hand compared to the wooden object. But again would this affect how the metal or wood feels?

    In this case I would think it should because if the heat is unable to transfer within the material itself, the metal atom in the surface of the 2 materials KE would drop to 0. Then it would start absorbing energy instead. So here i feel that its important that the heat transfers within the material to ensure that heat continuously flows more the hotter to colder material.

    So I'm quite confused. In one case, I feel that the heat transfer within the material itself doesn't affect the heat transfer between the 2 materials but in another case, i feel that its important.

    Can someone help me clear this misconception? Thanks so much :)
     
  2. jcsd
  3. Jun 1, 2013 #2

    mfb

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    Collisions between atoms do not increase the kinetic energy, they just transfer it between atoms.

    There are no "wood atoms", just carbon, oxygen, hydrogen, nitrogen and smaller contributions from other elements.
    The molecular structure is important for heat conductivity.
    Electrons are important, too - metals have free electrons (free within the material), wood has not.
    I don't understand that question.
    Exactly.
    Are you refering to heat capacity? That is relevant, too - a larger heat capacity means that the energy of your finger does not have to travel so far to get dissipated. Heat capacity is not so different for most solid materials, however - at least not as different as heat conductivity.
    Heat can flow in the opposite direction (into your finger), too. The process will reach an equilibrium if the metal and your finger reach the same temperature.
    The temperature. Unordered kinetic energy in the material contributes to the energy associated to this temperature, but it is not the only contribution.
    Right, and it does. It is the same effect, just with a reversed direction of heat flow. The surface of the metal (touching your finger) stays hotter.
    Why should this be true?

    Heat conductivity is always important if you have different temperatures at different places.
     
  4. Jun 2, 2013 #3
    Hi thanks for the great reply. I was thinking for the first case even if the heat remains at the atoms just touching the hand (metal atoms or ions), should the heat continue to get transferred ovetr? Cos I thought in evaporation, certain atoms can have a lot higher kinetic energy to escape. So shouldn't this also mean that those atoms should be able to keep increasing their kinetic energy? So would it feel cold still?

    But in the second case, I was thinking if heat didn't keep getting supplied to those few atoms on the metal then eventually their kinetic would drop to 0. So that would stop heat flow from the metal to hand.

    So I'm having trouble concept wise here. In one case I feel that the best transfer within the metal is useless while in the other (second) case I feel that heat transfer within the metal is important.

    What's wrong with my cocept here?
     
  5. Jun 2, 2013 #4

    mfb

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    Some atoms have a higher than average energy for a brief moment of time (less than a picosecond), and lose this again quickly afterwards in collisions - unless they happen to be at the surface of a material, where they might escape, but that is not relevant for metal and wood at room temperature.

    Why should their energy drop to 0? The temperature cannot drop below the temperature of the coldest object (without heat engines), so the average energy is always positive. The metal in contact with your finger would lose some energy to your finger, and get energy from the remaining metal at the same time. You get some temperature distribution and a continuous heat flow until the whole metal and your finger have the same temperature.
     
  6. Jun 2, 2013 #5

    Oh i was just making a make-belief scenario where the kinetic energy retains and do not get conducted away within the material (http://postimg.org/image/7lv6oh6nl/ [Broken]) on the atoms on the surface of the metal where it is in contact with my hand. So can we say that generally collisions between the two atoms would give rise to the same kinetic energy between those 2 atoms? Because if energy can only pass between them then, then if there is excess energy on one atom it would equally distributed to the other atom?

    So in this make belief scenario (let's call it A now) where heat doesn't transfer in the metal. If if was colder (the first scenario), the heat would stop getting transferred when they both have the same kinetic energy? So its not really possible for those metal atoms (at the surface) to keep gaining more and more kinetic energy? Because initially, I thought that those atoms could just continuously increase in its kinetic energy.

    And for the second make belief scenario (let's call this B where the metal was hotter) and heat could not get conducted to those surface atoms. Then again, those surface atoms of the metal could only give out as much as energy such that the kinetic energy of the hand and metal remains the same right?

    So its essential for the conduction within the metal itself to bring away the heat to continue allowing more heat to be either absorbed (in A) or for it to give it out (in B)?

    Thanks for the help :)
     
    Last edited by a moderator: May 6, 2017
  7. Jun 2, 2013 #6

    mfb

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    A similar energy is the most likely outcome.

    No. Heat conductivity means the metal conducts heat - the atoms behind 1, 2 and 3 will increase their energy, too.

    Zero heat conductance? You would not feel any temperature effect then, as the first layer of atoms is way too thin to be notable.
     
  8. Jun 2, 2013 #7
    Ohh. I get it now. If in this make-belief scenario, once the KE of atoms 1, 2 and 3 are equal to of the hand (they will transfer heat in reality must let's just play with this scenario for a bit) then no more heat will be felt.

    So now going back into reality, if more kinetic energy can be transferred 1, 2 and 3 from the other metal atoms behind them, then there can be more heat transferred to the hand per unit time making the object feel hotter?
     
  9. Jun 2, 2013 #8

    mfb

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    Right. And this happens within something like picoseconds, so there is no way to feel it with a finger.
    If the metal is hotter, right.
     
  10. Jun 3, 2013 #9
    Ohh this helped me a lot thanks mfb :)

    1 last question regarding this topic. I read from an old forum post that the heat transfer between the two material is dependent on the lousier conductor of the 2 materials.

    So now in this hand-metal and hand-wood case, if the hand is a terrible conductor (worse than either of them) then shouldn't both the metal and wood feel the same? Because if the hand is a terrible conductor then once it gains a little energy and it takes a long time to transfer anything, it doesn't matter if the metal is superb at transferring energy the whole conduction process is slowed down right?

    So the fact that feeling metal is colder than wood in scenario A, would this mean that the hand is a better conductor than wood? Or how would there be a link here?

    Thanks again :)
     
  11. Jun 3, 2013 #10

    mfb

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    It depends on both (more on the worse conductor than on the better one), but the hand is still a better conductor than wood, I think.
     
  12. Jun 3, 2013 #11
    Oh but if we were to touch the metal, it being a better conductor than the hand then heat should flow slowly eventually because of the hand's low conductivity? But when we touch the wood since it's a lousier conductor than the hand so only a limited amount of heat can pass through it. So the metal ends up feeling colder?

    But what if now the hand is a lousier conductor than the wood. Then won't it feel the same as the metal as now only a limited amount of energy (which is the same in the metal) can pass into the hand per unit time?

    Because at first I thought it was simple (We just used the conductivity of the metal or wood prior to this) but now that i think about it. Won't the lousier conductor be the 'limiting' factor to the heat flow?

    At first we just used the non-constant material but can we just do that even when the hand is a lousier conductor than both of them? So actually what would be a good way to estimate the heat transfer between two materials?
     
  13. Jun 3, 2013 #12

    mfb

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    Hot metal would feel even hotter if your hand would be a better conductor.
    Right, therefore hot wood does not feel so hot.

    The conductivity of the other material is still interesting. In the limit of perfect conductivity of metal/wood, the surface is always at 50 degrees, but in reality the surface will be a bit below that - and this depends on the conductivity of both touching materials.
    Model both materials as blocks with a fixed temperature at one side, find the equilibrium temperature at the contact area.
     
  14. Jun 3, 2013 #13
    sorry I think I was a little vague. Thinking about this now I came up with another question. Say now both the metal and wood is cold. So now that the metal is a better conductor it would be able to gain more KE per collision with my hand right? So does the hand affect how much KE it would give up as well? Like if i were to change the material of my hand would it should affect how much energy it gives up? But its only that now both the wood and the metal case, that amount is constant as the hand is fixed right?

    So going back into the question, say i have two plastics of different conductivity at 0 degrees and my hand has a lower conductivity than either of them. X has a better conductivity while Y has a lousier conductivity.

    Both X and Y should they feel the same because the hand is a lousier conductor so it can only impart a maximum amount of energy per collision. So each collision with X and Y imparts the same amount of energy. So the amount of energy per unit time is the same for both of them making them feel the same?

    But if i were to say i have the same two plastics at 60 degrees, and i touch them. Both of them would feel the same right? Because since my hand is a lousier conductor it can only absorb the same amount energy per collision. So, the energy transfer per unit time would be the same for each of them right?
     
  15. Jun 3, 2013 #14

    mfb

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    This is just getting a repetition of previous questions and answers. I see nothing I could add for the general concept. Read a book if you are interested in more details.
     
  16. Jun 3, 2013 #15
    Okay! What would this topic lie under for conduction through 2 materials? The sources I found mainly are about conduction through 1 material.

    Thanks :)
     
  17. Jun 4, 2013 #16
    Actually now that I think about it, if A and B was at 0 degrees. A should still feel colder despite my hand having a lower conductivity right? Because now my hand can impact a specific amount of heat per collision depending on the KE of both atoms (hand and A or B). So the first collisions bring about more kinetic energy into A and B. But since A is a better conductor it would take away more kinetic energy to the rest of its atoms. As a result more kinetic energy can pass into A per unit time than B making it feel colder.

    And if now A or B is hotter than the hand, A being a better conductor of heat can transfer more kinetic energy for the first collision between the two different material's atoms. Also as heat is transferred away from the hand in both cases, more heat from A would be transferred to the hand per unit time than B. so again A should still feel hotter right?

    So actually it doesn't matter if my hand is a lousier conductor then either A or B right? A would still feel colder in a cold environment and hotter in a warm environment. Is this correct?
     
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