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I know when a substance is heated the molecules in the substance begins to vibrate in random motion. What I want to know is, is it possible to vibrate the molecules without adding heat to the substance? If yes, does the vibration of the molecules cause the substance to heat up?

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  • $\begingroup$ with vibrating in random motion, do you really mean just higher mean velocity of the molecules, or the vibration of single molecules? And if you increase the mean velocity, it is called heating, you can doing withe any heatsourcel like for example microwave? $\endgroup$
    – trula
    Apr 10, 2020 at 11:13

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What I want to know is, is it possible to vibrate the molecules without adding heat to the substance?

Yes.

For example, the temperature of a gas in a cylinder can increase by compressing it (energy transfer by work) as well as by heating it (having it contact a higher temperature substance- energy transfer by heat). In each case the kinetic energy of the molecules increases.

If yes, does the vibration of the molecules cause the substance to heat up?

It's the other way around. Technically, substances don't "heat up". Their internal energy increases due to either energy by heat (energy transfer due to temperature difference) and/or energy transfer by work (energy transfer due to force times displacement). Each type of energy transfer can cause an increase in internal kinetic energy (molecular motion) as reflected by a temperature increase.

Hope this helps.

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  • $\begingroup$ @ChemEng I will update my answer in an attempt to answer your follow up question. $\endgroup$
    – Bob D
    Apr 12, 2020 at 17:44
  • $\begingroup$ @ChemEng I just realized you are not the person whose question I answered, so I will not be updating my answer and instead reply here. First, compression is not a heating process, it is energy transfer by work that can increase the internal kinetic energy (and temperature) of the gas. Second, viscous friction (again not "heating") can increase internal kinetic energy, but if the compression is carried out extremely slowly viscous friction doesn't play a role. Microscopically, when gas molecules collide with the moving piston they rebound with a greater velocity because of the piston motion. $\endgroup$
    – Bob D
    Apr 12, 2020 at 17:57
  • $\begingroup$ i was wondering what is the mechanism of heating during compression? is it due to viscous heating where the heat is dependent on the strain rate squared and the viscosity plus the partial of the natural log of density wrt the natural log of temperature at const Pressure multiplied by the material derivative of pressure and it is a problem of fluid dynamics or can this heating be explained on a static ideal gas manner? $\endgroup$
    – ChemEng
    Apr 14, 2020 at 4:14
  • $\begingroup$ @ChemEng I answered previously. Did you delete your previous comment and then repost it? $\endgroup$
    – Bob D
    Apr 14, 2020 at 14:14
  • $\begingroup$ yea i had to add the natural logarithm in there $\endgroup$
    – ChemEng
    Apr 14, 2020 at 16:16

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