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Heat Transfer

  1. Apr 2, 2006 #1

    danago

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    Gold Member

    Hey. I wasnt sure where to post this, since it isnt an actual question i need to complete for school, but it relates to it.

    When my year 11 physics class was solving the final temperature of an ice/steam mixture, my teacher kept talking about how energy is transferred too AND from the ice. Now, originally, i thought energy was transferred from a hotter substance to a colder substance, until their temperatures were equal. So in the ice/steam case, i would have thought that energy was only transferred from the steam to the ice.

    I tried getting him to explain the transfer of energy from the ice, but he just couldnt give me a straigh forward answer. He even referred to it as 'cold energy' at one stage.

    If anyone could bring shed some light on this topic, id greatly appreciate it.

    Thanks alot,
    Dan.
     
  2. jcsd
  3. Apr 2, 2006 #2

    Doc Al

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    I'm not sure what point your teacher was trying to express. Under normal conditions, energy spontaneosly flows from the hotter object to the colder object. (To reverse this natural direction of flow--like in a refrigerator--requires work to be done.)

    Can heat flow from ice? Sure, if it flows to something even colder. (Ice itelf can be any temperature from 0 degrees C on down. Ice at 0 degrees is hot compared to ice at -20 degrees.)
     
  4. Apr 2, 2006 #3

    danago

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    Thats exacly what i thought. I guess ill have to ask him again about this one.

    Thanks for the reply,
    Dan.
     
  5. Apr 2, 2006 #4

    Andrew Mason

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    Unless the ice is at absolute 0, the water molecules in the ice have kinetic energy (mostly vibrational). Since temperature is a measure of the average kinetic energy of the water molecules, we know that they do not all have the same energy. Some are vibrating faster and some more slowly than average. Some may even be moving around and some may have sufficient energy to leave the ice surface. Similarly, in steam the molecules have a range of kinetic energies. At any given moment, there may be some ice molecules that have more energy than some of the steam molecules.

    So when the hot steam (water molecules with higher kinetic energy) contacts the ice, what happens? The average energy of the ice molecules increases and the average energy of the steam molecules decreases. Some original energy from the ice molecules may actually increase the energy of some steam molecules, but overall, the heat energy flows from the steam to the ice.

    AM
     
  6. Apr 2, 2006 #5

    danago

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    oh ok. I see now. Thanks for clearing that up.

    So the energy tranferred from the steam to the ice is much greater still than the amount from the ice to the steam though?
     
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