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Heat Transfer

  1. Feb 1, 2014 #1
    Hello Everyone,

    The question I am about to pose concerns something I may possibly already have an answer; however, I would like to proceed with posing the query, that I might solidify my understanding.

    If a system transfers heat to its environment, does this necessarily imply that the system's temperature must reduce, or is just sufficient for that occurring? It would seem as though it were a necessary cause. Heat is defined as the spontaneous flow of energy from one object to another. So, if heat were flowing from the system, then the system's energy must be decreasing. Because temperature is a measure of the average energy, then if the energy is decreasing, then a decrease in temperature should be measured.

    Does this sound reasonable; am I missing anything?
  2. jcsd
  3. Feb 1, 2014 #2


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    You have this somewhat backwards. Temperature is the fundamental principle here. If two objects are at different temperatures, then heat will flow from the hot object to the cold one. I would modify your statement to read "Heat is defined as the spontaneous flow of energy from one object to another due to a difference in temperature."

    Your overall thinking is valid only under the condition that the only energy available is thermal energy. As a counter-example, a system could use chemical energy to maintain its temperature while transferring heat to its environment.
  4. Feb 1, 2014 #3
    Or work could be getting done on the system?
  5. Feb 1, 2014 #4


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    Could you change the word "heat" to the word "energy"?
  6. Feb 2, 2014 #5
    Heat content of a body is defined as the sum of kinetic energy of all its molecules; average kinetic energy indicates the temperature.

    If heat is transferred, some of the molecules kinetic energy is transferred. So, the kinetic energy of all the molecules will get lowered a bit. So, the average of kinetic energy of all the molecules must reduce and thus the temperature.

    Edit: Response to some of the recent posts.


    The above definitions for temperature and heat was extracted from elementary books for the laymen like me. From the recent posts, it seems to be arguing it as wrong. It might be.

    I can't even declare those definitions to be wrong or correct, because of lack of experience.

    Seeing those definitions it seems that, as hotness of the system increases, kinetic energy increases. And thus definitions from that particular text might be claiming heat to be sum of kinetic energy of all the molecules, and temperature to be average of kinetic energy of all the molecules.

    I feel it will be more exact, if heat to be defined as the form of energy which is proportional to sum of the kinetic energy of all the molecules and even temperature to be measure of hotness which is proportional to average of kinetic energy of all the molecules.

    And even I think the definitions mean kinetic energy produced due to thermal velocity.

    There might be more exact explanation some others might give. I will be waiting to see those exact and quality answers. If there is mistake here, I will be happy in knowing and making corrections myself.
    Last edited: Feb 2, 2014
  7. Feb 2, 2014 #6

    Vanadium 50

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    Nothing that Curie Willey said is correct. Throwing a baseball increases its kinetic energy but not its heat content or temperature, and, as pointed out above, there are other sources of energy that can be used to maintain temperature during heat transfer.
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