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What is happening at atomic Level between two bodies when there is friction force acting between them?

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When you are dealing with the surface of macroscopic objects, it is very important to learn about asperities. These describe the high spots on the surfaces, which are very important when understanding friction between these surfaces.

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When you touch two macroscopic surfaces together the asperities (high spots) touch and the surfaces cold weld at these contact points (the article I've linked has a good illustration of this). As you slide the surfaces the asperities deform then fracture apart. The friction is mainly due to the energy required for the fracture.

Friction on atomic scale

At the atomic/molecular level, these contact layers are not sliding but rather fracturing the contact path. As you slide these surfaces, these asperities deform, then fracture apart. Friction is manly due to the energy required to deform and fracture these asperities.

Árpád Szendrei
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  • What is the meaning of high spots? –  Oct 09 '20 at 17:09
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    @robert patrick when you zoom in on any material, no matter how smooth it appears to humans, you will find jagged hills and valleys at the molecular level. One of these "high spots" could be composed of thousands or millions of molecules. – electronpusher Oct 09 '20 at 17:37
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Friction is still a mysterious effect and it surely involves electrical charges. When two dielectric material gets into contact for some short time they develop charge accumulation in the contact areas. Charge accumulation has an important aspect of creating attractive and repulsive forces between the surfaces. This charge accumulation between surfaces has some complex pattern (that depends on the material and surface itself) and charge domains in the pattern can be thought as a result of contact electrification. Rest is basically the interaction between charges of the surfaces that causes some of the friction. As reference The Mosaic of Surface Charge in Contact Electrification.

asd.123
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The kinetic energy of their atoms' collective motion transforming into non-collective (unordered, entropic) kinetic energy (heat).

RobertSzili
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Atoms are held together in molecules by electromagnetic forces. Molecules form structures by electromagnetic forces. When friction is happening, molecules from one structure are interacting with molecules from other structures. Sometimes (e.g. sandpaper on wood) parts of one or both structures are separated from the structure. That requires force which does work.

Friction moves molecules around, but rarely, if ever, breaks them down directly. If enough internal energy is deposited in one of the structures to increase the temperature, the molecules might react with others or decompose. Individual atoms in compound molecules aren't changed by friction.

Electrons on molecules can be transferred by friction.

Basically, friction is an electromagnetic interaction of molecules which get in each other's way.

Bill N
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  • How is the temperature increased and why? –  Oct 09 '20 at 17:11
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    How can friction force break down molecules? –  Oct 09 '20 at 17:15
  • @robertpatrick "if ever" -- I don't know of any situations where it does, but there might be some researcher doing exotic things ... It's really difficult to say never. :) – Bill N Oct 09 '20 at 18:15
  • @robertpatrick Temperature is increased by doing work on an object: $|\Delta U = Q_{in}+W{in}$. Frictional forces do work. If the phase doesn't change and $\Delta U$ is positive, the temperature increases. – Bill N Oct 09 '20 at 18:17