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Brain-stroming discussion questions

  1. Sep 1, 2010 #1
    all are open to those who like to brain storming like me in physics question...i'm a physics degree student...and you are please welcome and try and comment to me if any...

    1. inside an just used oven the temperature is 200*c....
    you can put your hand into the oven without and injury you got as long as you didn't touch anything inside the oven...
    but the questions is since the air inside the oven also have a 200*c of temperature,but why you hand wouldn't get hurt by the vaporisation of heat of those air....???

    2. the sun surface has about 5800k temperature...
    according to the zeroth law of thermodynamics,objects always wanted to comes to a point at which is thermal equilibrium...but why the sun do not have thermal equilibrium process with earth ???

    3. a person pours the hot cofee to a cup and because she was so damn busy and she intend to drink it after 6 minute...so to keeps the hot coffee as hot as possible should her put some of the cream on top of the hot coffee or not ???...explain why or why not ???

    this is very good brain storming questions for me...i found on books...hopefully you would shared your answer to me...and of course i would like to have shared with you all if you wanted...thanks for trying...welcome...
     
  2. jcsd
  3. Sep 1, 2010 #2

    Dale

    Staff: Mentor

    Hi chinkitkit, welcome to PF

    In the future it may be a good idea to use a more descriptive title for your thread. E.g. this one should have mentioned something to do with heat transfer since that is what all of your questions are about.

    1) There are three mechanisms of heat transfer, conduction, convection, and radiation. When you stick your hand in the oven the primary mechanism is radiation, when you touch something the primary mechanism becomes conduction. Conduction is much faster than radiation in this case.

    2) The earth is also in thermal contact with the cosmic microwave background radiation which is about 3K. So the earth's temperature is in a kind of dynamic equilibrium between the 5800K of the sun and the 3K of deep space.
     
  4. Sep 1, 2010 #3
    thanks...and sorry to the mistake i did...i apologize for that...because i'm new to here to this physics forum...and thanks for you discussion too...

    but for the explaination 1...shouldn't the hot air gas will be attack our hand first...of course here i know that it won't be any hurt but i ignore all those common sense and trying to using physics law to explain this.......???
     
  5. Sep 1, 2010 #4

    Dale

    Staff: Mentor

    Remember the distinction between heat and temperature. Heat is a transfer of energy from a body at a higher temperature to a body at a lower temperature. Heat is measured in Joules, not Kelvin. For a given difference in temperature, different mechanisms of heat transfer will be faster or slower. For example, you can die by hypothermia after one hour in 10ºC water, but 10ºC air for one hour will only make you uncomfortable. The temperature difference between body temperature and the air or water is the same, but the heat transfer is much more rapid in the water.

    Similarly with your example of the oven, the air is high temperature, but the heat transfer rate to your hand is low. The metal in the oven is also a high temperature, but upon contact the heat transfer rate is very high. On the other hand, if you were to stick your head into the oven and inhale a lung-full of the hot gas you would greatly increase the heat transfer rate of the air and quickly damage yourself.
     
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