I tries to search why heat flows from a hot body to a cold body and not vice versa. I could not get any satisfactory answer. So can someone please explain me why heat flows from a hot body to a cold and not vice versa in layman's term(without mathematical equations).

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    $\begingroup$ What is the level of your physics knowledge? Do you understand why water flows down hill and not up hill? $\endgroup$
    – anna v
    Jan 8, 2022 at 11:46

1 Answer 1


I will try explain it in the easiest way I can think of. First of all, what is heat? Heat can be taught of intuitively as a measure for the kinetic energy of all the particles in a substance. The faster they are moving, the hotter the object. That is why the lid of a pot starts vibrating when you would boil water, the molecules start moving more and more and bounce harder against the lid! If you then have a hot and a cold body, what happens in this reasoning? Well the fast molecules (the ones from the hot body), they will start running into the slow molecules (the ones from the cold body). By doing so, they will gives the slow molecules more momentum. Therefore, the cold body will heat up (they move faster), while the hot body cools down (they move slower). So you see that the heat (the fastness of the particles) travels from hot to cold!

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    $\begingroup$ You are describing internal energy not heat $\endgroup$
    – Bob D
    Jan 8, 2022 at 10:39
  • $\begingroup$ You are absolutely right, but heat is an energy transfer and I think that my explanation provides a good intuitive understanding of the concept $\endgroup$ Jan 8, 2022 at 10:48
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    $\begingroup$ I would not use the vibrations of the lid of a pot as an illustration of the relationship between temperature and molecular motion. The vibrations are due to convective motions that don't reflect the disordered molecular kinetic energy. Moreover, there is no explanation why the hot body cannot become even hotter! $\endgroup$ Jan 8, 2022 at 11:32
  • $\begingroup$ Well, the fact that the hot body becomes colder is a statistical thing. The fact that the momenta become distributed makes that hot body become colder. It could be that it becomes hotter, but the change this happens is extremely low! $\endgroup$ Jan 8, 2022 at 12:12
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    $\begingroup$ To quote Mark Zemansky’s plea to teachers on misusing the word heat “Teaching thermal physics is as easy as a song, you think you make it simpler when you make it slightly wrong “ $\endgroup$
    – Bob D
    Jan 8, 2022 at 13:20

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