MadSci Network: Physics

Re: Why do the molecules of a substance move faster when heat is added?

Date: Wed Feb 16 07:53:00 2000
Posted By: Andreas Kieron P. Bender, Grad student, Chemistry, Trinity College Dublin
Area of science: Physics
ID: 950576196.Ph

Hi Keith,

the question you are asking is actually quite a fundamental one, and with this I mean the equivalence of heat and mechanical work.
I would prefer another formulation: Heat is nothing different than mechanical work - except for one aspect, and this is the direction in which the motion of molecules occurs, whether they all move in one direction or all move in different directions.

I want to give you an example - imagine a car braking. Before that event you have all particles (atoms/molecules) of the car moving in one direction, that's quite obvious. What happens if you brake? On the one hand the car stands still - but what happend to the energy connected to the motion? If you touch the street or your tyres you can feel that they are quite hot - because the atoms move quite fast. But now they are moving in all directions at the same time, so the tyre as a whole stands still.

Do you see the difference? Before you had a concerted motion, all in one direction, thereafter all the particles are still moving quite quickly, but in different directions.

If that's not enough, just ask again or have a look at some physics-resources in the web (like or in the yahho-directory)


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