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Why does a metal door knob feel colder that a wooden one on a cold day?

  1. Jan 27, 2006 #1
    I disagreed to the responce my teacher gave to this question

    "Why does a metal door knob feel colder that a wooden one on a cold day?"

    He said it was because the metal one has a lower specific heat, and the wooden one has a higher specific heat. He gave the example of tile and carpet on the floor. Saying that tile also has a lower specific heat than carpet.

    THis doesnt make sence though....If the tile had a higher specific heat...that would mearly mean it heats up faster, and whan a peson touched it, it would feel warmer faster right?

    any helps appreciated
     
  2. jcsd
  3. Jan 27, 2006 #2

    Meir Achuz

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    The main property that affects how the door knob feels is
    coefficient of heat conduction. The higher that is, the colder it feels.
    Specific heat is part of the effect, but it is not as important as heat conductivity. Your textbook shoul have a table of heat conductivities.
    Sometimes teachers give oversimplified answers because they are trying to avoid more detail than they think the class would want.
     
  4. Jan 27, 2006 #3

    ZapperZ

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    You have misunderstood the meaning of "specific heat". The definition of a specific heat is the amount of heat needed to raise the temperature of an object of 1 kg by 1 degree kelvin. If you look at the units of specific heat alone, you can tell this.

    What this means is that if something has a high specific heat, you will need A LOT of heat to raise its temperature by 1 K. Something with a lower specific heat will not require as much heat. So no, if you supply the same amount of heat, something with a higher specific heat doesn't heat up faster than something with a lower specific heat. It really is the opposite.

    Zz.
     
  5. Jan 27, 2006 #4
    Yes. i know an object with a higher specific heat takes more energy to raise its temperature. So, if the tile has a lower specific heat (which is what he said) then when you step on it, the temperature should raise faster than when you step on the carpet. But when you step on the carpet, it feels warmer. So it makes sense that the specific heat of the carpet is lower, meaning it will heat up really fast when you step on it, making it feel warmer, than the tile (which has a higher specific heat, meaning it takes longer for it to warm up when you step on it, and therefor feels cold).

    Am i right?

    thanks
     
  6. Jan 27, 2006 #5

    ZapperZ

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    But Meir Achuz has given you a good answer. It isn't just a matter of specific heat, but rather how quickly heat is conducted away from your touch, or your skin. That is why metals tend to feel cooler on a normal day, because it can conduct heat more efficiently than an insulator.

    My contribution in all of this is simply to make sure you didn't get the wrong concept for specific heat.

    So yes, your teacher may have given you an incomplete answer if it is attributed ONLY to the material's specific heat. However, if we want to go into great details, the two properties of "specific heat" and "heat conduction" are often related, in general (meaning it is true most of the time, but NOT all of the time). Materials that have low specific heats tend to be good heat conductors like metals.

    Zz.
     
  7. Jan 27, 2006 #6

    Danger

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    Just my $.02 as a simplification; the metal sucks the heat out of you faster than the wood does, and so appears colder. It's not a matter of it heating to body temperature.
     
  8. Jan 27, 2006 #7
    how can a metal suck heat from you faster...your body should give off heat just as fast to all objects...right? and that doesnt even have to do with specific heat
     
  9. Jan 27, 2006 #8

    Integral

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    The amount of heat transfered between 2 objects depends upon the difference in temperatures. Because of wood's HIGHER specific heat and the lower value of thermal conductivity the surface of the wood will increase in temperature faster then that of the metal. Therefore the wood will carry thermal energy away from your hand at a lower rate. In comparison the metal "sucks" the heat out of your hand.

    The parameter which is a direct measure of this property is the Thermal Diffusivity.


    Late edit:
    I had an "oops" in the original post. I have edited it to be correct.

    The effect must be seen as a result of a combination of the parameters, density, Specific heat, and thermal conductivity. The wood's much lower values of density and thermal conductivity mean that the heat from your hand heats a lower mass of material. So even thou the wood requires more energy to cause a 1 deg change in temp, since you are working on a much smaller mass the temp is able to increase faster. The low TC of wood means that the volume being warmed is smaller, the lower density means the heated volume has a lower mass. This double edge sword out weighs the higher specific heat.
     
    Last edited: Jan 28, 2006
  10. Jan 27, 2006 #9
    well if the wood sucks the heat faster, then shouldnt it make your hand colder faster.....but this is not true. The metal feels colder, meaning that its specific heat is high, and the woods is low.

    right? is that what your saying
     
  11. Jan 28, 2006 #10

    Integral

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    You need to reread that last post. It is the METAL which draws heat away faster.
     
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