My book says work can be converted into heat but I want to know how work affects kinetic energy of molecules which, when transferred, is called heat?

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    $\begingroup$ What is work? Roughly speaking, it's when you move things around. Now, those things move smaller things, which in turn move something smaller yet, all the way down to molecules, and when you move those, you increase their kinetic energy, which is heat. $\endgroup$ Jun 29, 2017 at 18:56
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    $\begingroup$ Work is not always converted to heat. For example, if you expended 1000 calories moving a one liter thermos bottle of water up an incline the water would not change temperature. However, if you expended 1000 cal stirring the water, it would warm up by one Celsius. Also you could expend energy a certain way and end up cooling the water (less efficient!) $\endgroup$
    – iad22agp
    Jun 30, 2017 at 2:39
  • $\begingroup$ The disorderly motion of molecules is called thermal motion. Work is identified as energy transfer making use of the organized motion of atoms in the surroundings, and heat is identified as energy transfer making use of thermal motion in the surroundings. $\endgroup$ Feb 25, 2018 at 17:50

2 Answers 2


Work can be understood as a variation of energy during which the transporters (may they be molecules) are spatially aligned or bonded, meanwhile heat is a variation of energy in which the transporters aren't aligned (therefore heat dissipates energy). With this in mind, any machine that uses an ordered energy transfer (like your hand waving) can induce heat in a fluidic means, as particles in fluids are initially moving randomly and are not likely to move far on the orientation of your hand. In this example, the induced work is little and restricted to the analysis on the surface of the moving hand, but if the analysis is made from further on a wider system, we can assume you're doing work to generate heat alone.


Work is the change in kinetic energy, and in some cases, say you do some work to move a bunch of molecules; if it results in them colliding, thereby releasing heat in the process, due to their kinetic energy during collisions converting into heat.


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