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IDK anything about Physics and Heat, help!

  1. May 11, 2007 #1
    Hi... I dont even know if this is the right forum to ask about it but I did a project in school which we put heated water in a bottle and put some tinfoil around the bottle and put it in an air filled plastic bag to preserve the heat.
    It was still warm an hour later... Why?
    Thanks!
     
  2. jcsd
  3. May 11, 2007 #2

    mgb_phys

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    Something has heat - it will stay hot unless the heat leaks away to some colder.
    You can reduce the heat loss by wrapping the bottle in something that doesn't conduct heat very well - like air. This is way your clothes keep you warm, the clothes trap lots of air, air doesn't conduct heat very well and so the heat from your body doesn't escape.
    It's the same reason you put coffee in polystyrene cups, it doesn't conduct heat very well and so you don't burn yourself.
     
  4. May 11, 2007 #3

    Hootenanny

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    Something doesn't have heat, it has internal energy. Heat is the transfer of [thermal] energy. :grumpy:

    P.S. Any volunteers to write an FAQ on this?
     
  5. May 11, 2007 #4
    There are many ways for an object to lose energy to its environment.

    1) Conduction - This refers to direct contact, like putting your hand in a frying pan.

    2) Convection - Some substance, usually air, transports the energy from object A to object B. A hair dryer is an example.

    3) Radiation - This is usually not important on earth, but this is how heat travels from the sun to the earth.

    The heat gets trapped because the blastic bag does not conduct heat from the air well. Then the air in the bag with the water gets heated up, everything inside the bag is the same temperature, as the energy slowly gets transferred to the bag and then the outside air by conduction.
     
  6. May 11, 2007 #5
    Sorry, but this is exactly why I've questioned distinguishing those terms. In everyday lay-usage, the word heat is naturally understood in that way. Now a similar thing happens with "mass" and "weight" but in that example there are conceptual reasons for a distinction (astronauts can vary their weight, and since mass is still useful we distinguish inertial mass from the gravitational force acting on), whereas in the context of a sentence I can't see any ambiguity arising even if one word were to describe both "(contained) thermal energy" and "(transferred) thermal energy".

    If we are going to put it in a FAQ, surely it should start with a good example of a frequent conceptual mistake (leading to an incorrect result, rather than purely a difference of terminology) for motivation?
     
  7. May 11, 2007 #6

    mgb_phys

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    Yes I know but I assumed from the writing that the OP wanted a simple answer.
     
  8. May 12, 2007 #7

    Hootenanny

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    I refer you back to mine and crosson's post on the thread 'what is heat'. Saying something has heat is analogous to saying something has a change in kinetic energy or saying something has a flux. Work is the mechanical equivalent of heat so saying an object has heat is the same as saying an object has work, both are nonsensical. An object can do work or you can do work on it; but you can never say and object has work. Heat is a process rather than a property of matter; you can't describe the final state of a system with heat, but you can do that with internal energy.

    Does anyone else feel that I'm over emphasising the difference between heat and internal energy? Am I really just going off on a tangent or do people agree that we should maintain a clear distinction between heat and internal energy, even to a lay person?
     
    Last edited: May 12, 2007
  9. May 12, 2007 #8

    Hootenanny

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    Is a simple yet wrong answer preferable to a accurate, slightly more complex one?
     
  10. May 12, 2007 #9
    I agree there is a sensible distinction from internal energy, but I don't see why everyone keeps bringing that up (is this a straw man thing?). Everyone will agree that "internal energy" has a different meaning to "thermal energy", and it is only the distinction between heat and "thermal energy" that I am questioning.

    Is there any important reason to have a separate name ("heat") for thermal energy that is being transferred? I mean, to be a bit dirty about it, the distinguishing characteristic of thermal energy is that it is constantly being "randomly" transferred amongst the myriad internal degrees of freedom in a given material, so maybe even the historical definition of heat does technically apply to "internal" thermal energy.

    Can you give an example where calling all thermal energy by the shorter word "heat" would genuinely cause a problem?
     
  11. May 12, 2007 #10

    Hootenanny

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    Well yes, if you used heat and thermal energy interchangeably in under any circumstances you'd confuse the hell out of everyone! For better of worse, heat and thermal energy have distinct definitions, despite their popular usages. One can't go changing definitions as one wishes.

    In analogy to your question and in reference to my previous post, I would like to ask you a question;

    Can you give me an example where calling all kinetic energy by the shorter word "work" would genuinely cause a problem?
     
    Last edited: May 12, 2007
  12. May 12, 2007 #11

    cristo

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    What is the difference between "thermal" and "internal" energy? I've not studied thermodynamics (well, not since A level, when such terms were somewhat glossed over) so did a quick search and it seems to me that thermal energy is related to the kinetic energy of the particles of a body, and the internal energy is the sum of the kinetic and potential energies of the particles. However, I've also noticed some authors use the terms interchangably. Is this because the potential energy of particles in, say, a gas is neglibible?
     
  13. May 12, 2007 #12

    Hootenanny

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    You've got the right idea, although I'm not sure if thermal energy is used much in modern texts. My text refers to vibrational, rotational, and translational kinetic energies. Some authors do use thermal energy and internal energy interchangeably (since in ideal gases we assume non-interacting particles), but this really is bad form, and only leads to confusion.
     
    Last edited: May 12, 2007
  14. May 12, 2007 #13

    cristo

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    Thanks! :smile:
     
  15. May 12, 2007 #14
    But that is the evolution of the language.

    Regards your analogy: I do not think it simplifies anything to call all kinetic energy "work". The technical definition of work refers to energy deliberately put to (ordered) "useful purpose", which aligns fairly closely to the popular understanding of "work", and does not always involve any kinetic energy whatsoever (eg. lifting water jars, quasistatically up a hill, after breakfast). The technical definition of kinetic energy (the component associated with velocity, of the quantity that is conserved due to time symmetry of physical laws) may be "purposeful" or just chaotic (thermal). Therefore these two concepts are very different. Calling the concept of kinetic energy by the name "work" would create the problem of just requiring a new name for the concept from thermodynamics (since it is both important and different), and the additional problem of worse alignment with popular language usage/meaning.

    The difference with "heat" is that the popular understanding does not distinguish whether or not the relevent thermal energy is being transferred, and if we pick just one technical term for the concept of thermal energy then the context of its use easily disambiguates whether it is contained (by the particular place) or being transferred (by the particular direction).

    I'm not completely resolved in this, but on the face of it, it makes sense to talk (colloquially) about "preserving heat" even if we technically mean "preserving thermal energy", but the former does not demand knowledge of "energy". In fact, "preventing heat" might be a technically-correct equivalent expression, and it sounds completely counter-intuitive.
     
    Last edited: May 12, 2007
  16. May 12, 2007 #15

    Hootenanny

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    I can see what your saying, but this in not a colloquial discussion, this is a physics forum, and the OP ask a question relating to a school project, which should be a technical discussion. I hope the OP's teacher would make clear the distinction between heat and internal energy. I must say, I thought you were suggesting we merge internal energy with heat; now I see that this is not the case, you were talking about thermal energy. To some extent I agree with you here; I think thermal energy is a poor concept anyway. In my opinion, it is much better to talk of the kinetic energies of the molecules rather than thermal energy. By defining the internal energy simply in terms of the sum of the kinetic and potential energies between the molecules is very much preferable to the sum of the thermal and potential energies and then having to define thermal energy. Sometimes thermal energy can be rather vague, and the distinction between heat and thermal energy can become a little more difficult to define (as our discussion has proved). I think perhaps the term thermal energy is somewhat redundant, do you agree?
     
    Last edited: May 13, 2007
  17. May 12, 2007 #16
    Language doesn't evolve by arguing, and technical terms are not affected by ignorant usage.

    "Preventing heat" is incomplete, try "preventing heat loss" or "preventing heat gain".

    Better yet, prevent changes in temperature. If internal/thermal energy (interchangeable terms) is too much for you to say, then just talk about temperature, which is nearly a synonym for internal energy in most every situation.
     
  18. May 12, 2007 #17

    mgb_phys

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    The reason for describing something as having 'heat' is because the experiment will almost certainly be described as measuring 'heat loss' - rather than change in internal thermal energy.
    There isn't really a good general word for 'heat loss' if 'heat' is meant as the transfer of thermal energy.

    All of physics consists of wrong answers - it's just some of them are 'right' enough for the circumstances! :smile:
     
    Last edited: May 12, 2007
  19. May 13, 2007 #18

    Hootenanny

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    Heat loss is a perfectly acceptable technical term and is very much preferable to saying something "has heat". Furthermore, there is a difference between an approximation and an incorrect theory/answer.
     
    Last edited: May 13, 2007
  20. May 13, 2007 #19

    Astronuc

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    No! Hoot, you are right on. :approve: A good opportunity to clear up some misunderstanding.

    Language does evolve from mis-use. The facts or reality do not change, but the common understanding may, and differences in understanding will eventually lead to argument and disagreement, and misunderstanding.

    We need to be precise and accurate in our language in order to avoid misconception and misunderstanding, which are counter-productive.
     
  21. May 13, 2007 #20

    Hootenanny

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    Cheers Astro, thanks for the support, the silence was becoming a little unnerving. I totally agree, and as Crosson and I have already said, although the colloqiual meaning of some terms may change over time it is imperative that the actual technical defintions remain constant, despite their common usages.
     
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