In his first lecture about the nature of Matter and Atoms, Professor Feynman claims that the higher the temperature of the steam gets, the quicker the movement of Water molecules will be. I don't see however how these two phenomenas are related. It makes sense that the size of the atoms and thus the Water molecules will get bigger, but this doesn't imply that the velocity of the Water molecules will get higher. I just don't see the relationship. Could someone please elaborate further on that ? Thank you in advance.


2 Answers 2


It's not at all obvious that the two phenomena - the steam being hotter and its molecules moving faster – are related. Evidence piled up quite slowly until the nineteenth century, but is now overwhelming. Some of the first evidence was indirect: if you accept the idea that the pressure a gas exerts on its container is caused by gas molecules hitting the container walls, then it's hard to explain the experimental fact that the pressure increases with temperature, unless you accept that the molecules move faster (on average) and hit the wall harder as the temperature increases. More directly, we can see the increasingly rapid 'Brownian movement' of microscopic particles suspended in a gas, when we raise the gas temperature. And there's plenty more evidence.

  • $\begingroup$ Could you please explain in layman's words, how heat as energy that our steam receives, is in fact converted into a kinetic energy for the water molecules within ? $\endgroup$
    – ecdhe
    Mar 11, 2018 at 18:01
  • $\begingroup$ Suppose the steam is produced by an electric kettle. Electrons moving in the wire of the element bump into the atoms (strictly ions) in the wire and increase their vibration energy. We perceive this as the element getting hot. The extra vibration energy is passed on to the water whose molecules vibrate more (the water gets hotter) and some molecules get enough energy to leave the water as steam. This is increasingly the case as the water gets hotter. $\endgroup$ Mar 11, 2018 at 18:43
  • $\begingroup$ One thing more to add please, is that if I understood well : The kinetic energy that the water molecules gain after exposing the kettle to the "heat" is not actually a conversion of the "heat" into kinetic energy as I stated above but rather the kinetic energy of the electrons circulating in the wire is transfered directly to the water molecules. The sensed heat is just the materialization of the portion of lost energy, and it is not the direct cause of the motion of water molecules. Is that right? $\endgroup$
    – ecdhe
    Mar 11, 2018 at 20:08
  • $\begingroup$ Yes, I think you've understood. Note that we're talking about the $random$ kinetic energy of molecules. As a rough guide (pretty accurate for gases) temperature is a measure of the mean random KE of the molecules. [Strictly, physicists reserve the term 'heat' for energy $in\ transit$ from a higher temperature to a lower temperature. If you have, say, a cylinder of gas, you would call the random KE of its molecules its $internal\ energy$, rather than $heat$.] $\endgroup$ Mar 11, 2018 at 22:32

Because they are not different things, They are the same. Every mechanism we have for measuring temperature is in fact measuring the average speed (or, more accurately, the momentum, which is speed times mass), that the atoms/molecules are in the material possess. The word temperature is just a euphemism or surrogate we use when discussing the macroscopic properties of large amounts of material for average molecular momentum of the individual particles.

  • $\begingroup$ Still this does not explain how heat as energy that our steam receives, is in fact converted into a kinetic energy for the water molecules within. $\endgroup$
    – ecdhe
    Mar 11, 2018 at 14:49
  • $\begingroup$ No, that is not what temperature is. $\endgroup$
    – user137289
    Mar 11, 2018 at 17:21
  • $\begingroup$ @ecdhe, Because heat is nothing more than the kinetic energy of the molecules in the material that comes into "contact" - (actually, comes close enough for electromagnetic forces to interact), with the material you are heating. The word Heat is also a euphemism, in this case, for the kinetic energy of the molecular motion of the particles in the material. $\endgroup$ Mar 11, 2018 at 17:50
  • $\begingroup$ @Pieter, In the context of this op's question, it most certainly is. $\endgroup$ Mar 11, 2018 at 17:56
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    $\begingroup$ @Charles Bretana The "average molecular momentum of the individual particles" due to their random motion is zero, whatever the temperature. Remember that momentum is a vector! $\endgroup$ Mar 12, 2018 at 10:25

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