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Why exactly Is Iron cooler than it's environment at 20°c?

  1. Nov 5, 2012 #1
    I've been doing classical physics and specific heat so I started wondering why some materials cool and/or heat faster, I figured it probably had something to do with how close together the atoms were in the material or such... than i started thinking why are matters like Iron always so cold when you touch them, but not a pencil or wood? I was thinking it's specific heat is different but I figured they tought us that heat spreads evenly, just at different speeds..?

    One thing I guessed was that the iron atoms are closely packed so any kinetic energy can be transferred trough it easily and back out again the other end like a shock wave, but it didn't quite cut it... I just don't really get it... seems classical physics can be more complicated than quantum mechanics sometimes.

    Anybody care to elaborate why hard materials tend to be cold, or what the properties of cold materials tend to be.. is it just specific heat?
     
  2. jcsd
  3. Nov 5, 2012 #2
    The thermal conductivity is high so the heat gets sucked out of your hand quickly. So to the touch metal feels colder than other things when it is at room temperature. On the other hand metal also feels hotter when it is above the temperature of your hand. As you will notice when you touch a hot grid in the oven, that feels much hotter than the air that is at the same temperature.
     
  4. Nov 5, 2012 #3

    Thats even simpler than i though and makes perfect sense... thanks
     
  5. Nov 6, 2012 #4

    CWatters

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    There are two different properties of materials that give these effects..

    The heat capacity of a material determines the amount of energy stored in a block of material at any temperature. For example 1kg of water at say 373K (100C) holds more energy than 1kg of almost any other material. For example it holds roughly 10 times as much energy as a 1kg block of copper at the same temperature. If that stored energy were to leak away by conduction at the same rate (eg the same joules per second) it means the water would remain hotter for longer.

    When you touch different materials heat flows either from the material to your hand or in the other direction depending on the relaive temperature and the thermal conductivity of the material. If the thermal conductivity is very very low there is little heat flow regardless of the temperature.

    The combination of these two properties can give surprising effects..

     
    Last edited by a moderator: Sep 25, 2014
  6. Nov 6, 2012 #5

    sophiecentaur

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    Heat loss is quite complicated.
    It seems to me that you are assuming that the heat is conducted in the form of 'vibrations'. That's the way they tell you about conduction in solids at school. However, in metals (amongst the most common solids we meet), the energy is transferred in the same way as electric current. The (outer) electrons are extremely mobile and move at a very high average velocity. So they travel freely inside the metal and carry excess Kinetic Energy about very fast through the metal. Both electrical and thermal conductivity are high.
    The sensation of 'cold' when touching a cold metal is largely due to the fact that heat is transferred very fast and keeps the temperature of the surface that you are touching, low. Your hand just keeps losing heat. Touch a cold plastic object and its surface very soon starts to get to the same temperature as your hand because the conductivity is so low.

    Of course, Specific Heat also counts and the overall cooling effect will depend upon both factors. The SH of water, for instance, is extremely high (much higher than metals) and water is an excellent coolant but, unless you stir it round (convection) it will not transfer the heat away as fast (wet suits work this way).
     
  7. Nov 6, 2012 #6
    There does not seem to be a clear answer about the OP original question.Heat is transferred fast but is iron cooler than it's enviroment to start with.
    Stick a lump of iron in a room which is at 20c does the iron quickly attain the rooms temperature of 20c or does it become cooler than the room.
     
  8. Nov 6, 2012 #7

    sophiecentaur

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    Air is not a good conductor. It only feels cold when it is forced past you quickly. (Wind chill). Of course I assume we are talking in terms of everything having the same temperature. Iron from the oven could feel v hot.
     
  9. Nov 6, 2012 #8
    Irrespective whether or not if the iron came from an oven or a freezer.Would the iron quickly attain the same temperature of it's surroundings.In this specific case 20c.
    Sense says it should and the only reason it feels cooler is because you are warmer.
     
  10. Nov 7, 2012 #9

    CWatters

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    If you put anything (eg lump of iron or slice of beef) in a room that is maintained at 20C that object will eventually reach 20C as well. It doesn't matter if it started out hotter or colder than 20C. Predicting how long it will take to reach 20C is another matter.
     
  11. Nov 7, 2012 #10

    sophiecentaur

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    It depends what you mean by "quicker". I would only commit myself to the word "eventually" as you don't specify the conditions.
    I assume the OP specified iron and air initial temperatures to be the same, otherwise there is no answer to the question. If both iron and air are at the same temperature then how does that make sense?
     
  12. Nov 7, 2012 #11
    Yes I suppose the time it would take depends on the temperature of the iron when placed in the room and how quickly it cooled with regards to other materials at the same start temp.

    Your assumption could be true but to be honest the OP might actualy have believed that the iron is cooler if measured and in certain conditions it could well be.

    It makes sense when you touch something that is cooler than youself that you recognise it only in as much that you realise good conductiors of heat feel cooler than bad because they rob you more quickly of your body heat to try to equalise the temp.
     
    Last edited: Nov 7, 2012
  13. Nov 7, 2012 #12

    sophiecentaur

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    In equilibrium, the iron would not be at a lower temperature. (By definition).
    "Robbing" is not a scientific term. Thermal energy Flows according to temperature gradient.
     
  14. Nov 8, 2012 #13
    Quite so neither is pedant but both quite legitamate words when used as expressions.
     
  15. Nov 8, 2012 #14

    sophiecentaur

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    Fair enough when arm waving but isn't the thread trying to get to a 'scientific' explanation.that should be as unambiguous as possible? If the answer were to come out in mathematical terms then you wouldn't expect 'unofficial' jargon. We have contributors with a range of familiarity with science and English. Does it help them to use unclear terms?
    The word "robbing" has connotations of an 'active' process. It is not active.
     
    Last edited: Nov 8, 2012
  16. Nov 8, 2012 #15
    Yes I agree, that is why one posts in the first instance to give and get a clearer understanding of processes involved.How you skin that particular cat is subjective to the individual and the contents of the posts.If the answer had been in mathematical terms there may be no ambiguity but less of a captive audience.
    I am sorry if the word rob has connotations of an active process for you.
    Calm oneself and try to imagine Robin Hood a more passive type of thief.
     
  17. Nov 8, 2012 #16

    sophiecentaur

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    I would also try to avoid anthropomorphism wherever possible. So many people talk of electrons "wanting to" do something - as if they may change their minds on a whim. "Trying to equalise the temperature" is also a potentially hazardous description - particularly when there are alternative ways of describing processes which cannot be misinterpreted.
    We have been trying (since the Greeks) to avoid the 'Nature abhors a vacuum' type of description because it has long been realised that it is not helpful. At least, once we get into the Maths, that is less of a risk but purple passages and flowery phrases can get in the way of the Science. They may make it more cuddly but cuddly didn't sort out Quantum Mechanics.
     
  18. Nov 8, 2012 #17
    I have to take issue, what's wrong with nature abhors a vacuum type description there is no denying that you don't bump into many vacuums whilst treading the Earth, smart people those Greeks and not bad at Maths either.
    Aether got in the way once or twice and might once more but where would our knowledge be without it.
    Flowery phrases can get in the way but they can also be educational in how we arrive at our current understanding the history of science a subject in it's own right.
    Cuddly didn't sort out Quantum Mechanics.Well you know what they say if you understand Q.M you don't!
     
  19. Nov 9, 2012 #18

    sophiecentaur

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    It all depends what you require for your 'Science Education' and 'Science Progress'. I thought that Science progressed by reducing things to their simplest possible model. This excludes giving inanimate objects a will or the ability to choose.
    The history of Science is very interesting but we have had several paradigm shifts since the Greeks and The Enlightenment to help us with our progress.
    "Nature abhors a vacuum" is about as helpful a statement as "Things naturally fall down". Newton and subsequent Scientists have improved, somewhat, on that way of looking at things. I can't recall having read any recent text books or published papers in which we find things describes as "trying to do" anything.

    I guess it's basically down to personal preference and to how interested one is in an objective, rather than a subjective appreciation of Science.
     
  20. Nov 9, 2012 #19
    Taken as an isolated statement "Nature abhors a vacuum" is pretty useless the problem is when I see this statement I don't see nature doing anything what I imagine is two teams of horses and a large sphere.The horses are attached to the sphere by chains and trying to pull the sphere apart which is in two halves with the air sucked out of it.
    So sure it becomes subjective to a point but it also makes the subject alive and exciteing.
    I don't see no reason why there can't be a mixture of the preferences provided inanimate objects remain just that.
     
  21. Nov 10, 2012 #20

    sophiecentaur

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    Have you ever had to salvage some sense out of a statement made by a 14 year old who needs to get a Science education and has been exposed to 1. TV Science fiction, 2. Simulations in computer games, 3. Religious Fundamentalism 4. Anthropomorphic Primary School Science teaching?
    Of course, Science is 'ooh aah' exciting but getting it as near right as you can is even more 'ooh aah' exciting.
     
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