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Q & A: Why are not all the solids hot?

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Most recent answer: 03/12/2015
Q:
We learnt that solid particles are in a state on motion also and that is vibrating, I think that would cause heat because they vibrate against each other. My question is why isnt all solids hot?
- sarah (age 15)
pmb
A:

You are absolutely right in that the particles of solids are in perpetual vibration, and indeed they are. You are also right in thinking that they should be hot (= not totally cold), and this is also totally correct. But the key point that you were missing is that the energy associated with these molecular motions already is exactly what we call "heat" or, more formally, "thermal ebergy". So it doesn't have to be converted to something else. 

In a metal slab, this thermal energy of the vibrations of metal irons is partly kinetc (they're moving) and partly potential (they're like springs strectching and compressing). In fluid phases something similar happens, including some roational jiggles of molecules.  So these vibrations do not cause generation of heat, but they constitute the heat itself.

The amplitude or speed of each of these modes is related to the concept of temperature, which you call hotness. Temperature is a parameter that is correlated with how energetic a material is. At room temperature, a dishwasher is at 25°C, but we prefer to use another scale called the Kelvin scale according which the room temperature is 298K. Unlike the Celsius scale, Kelvin scale has a minimum at 0, that is no negative temperatures. So absolutely cold means 0K (-273°C), at which everything is at the lowest possible energy state, meaning a cease of all these vibrations. The dishwasher is not cold, but 298 Kelvin units hot.

 may help you to understand better.

Tunc


(published on 03/12/2015)

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