I know that friction is cause by the electromagnetic force between electrons from 2 contacting surfaces but how come it only applies to physical contact with a solid surface?
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Long range forces: https://physics.stackexchange.com/questions/312820/long-range-forces#:~:text=The%20term%20long%20range%20force,any%20physical%20contact%20to%20exist. – Aug 07 '20 at 09:32
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It is because matter is made up of approximately equal numbers of positive and negative charges so the long range effect of these charges is negligible ie the bodies concerned are electrically neutral and do not interact with one another.
Friction is to do with electrostatic forces between particle which are nanometres apart and have formed bonds with one another.
For bodies to interact to any significant degree the electron shells on the surface of each body must "touch" ie friction is a contact force.
Farcher
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As a toy model, suppose we're a distance $d$ from the surface, where one layer of charges is a distance $\delta$ further away. The function $1/d^2-1/(d+\delta)^2$ approximates $1/d^2$ for $d\ll\delta$, but only $2\delta/d^3$ for $d\gg\delta$. – J.G. Aug 08 '20 at 09:51
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As you know, atoms are made of positively charged nuclei, surrounded by an equal number of negatively charged electrons. Say the charge of a particular atom is $Z$. There is a distance between the positive and negative charges - the "radius" of the atom - and so we know that perhaps $Z/2$ electrons are "closer" to the contact surface than the positive charge of the nucleus. We can grossly model this as half of the electrons are "away" from the contact surface, and half are "toward" the contact surface, for both surfaces. Further simplifying to model each half as a point charge, and the nucleus itself as a point charge.
The electrostatic force between the two surfaces can be derived easily. Since the force is inversely proportional to square distance, the $1/D^{2}$ repulsive term dominates when the surfaces are close together. When the surfaces are far apart compared to $d$, the two electric dipoles representing each atom cancel.
Of course, this extremely simplistic line of argument can be extended to include surface charges instead of point charges, and to improve some approximation with respect to charge distribution, but the gist is there.
This is not, of course, meant to suggest that friction happens due to some shear force between perfectly parallel planar charges. This was to address the question of how the long range electromagnetic field can cause friction to happen only over short distances.
jpf
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Friction arises generally from the deformation, or even breaking, of the surface of one body by the other—perhaps mutually. This deformation entails the stretching (or breaking) of intermolecular or interatomic bonds, which resists the potential or actual motion between the two bodies. This type of friction arises when the interface between the two bodies is not very flat, so that the relative movement of the bodies cannot be exactly planar.
Even when the contact between the bodies is planar, and at a set of outermost points of the bodies, there may be some friction arising from interatomic attraction between the respective neighbouring surface atoms of the two bodies. The extent to which this happens depends very much on the chemical constitution and surface preparation of the bodies, ranging from very low (compared to the general type of friction described above) to very high—the latter being the case, for example, when the presenting surfaces are certain metals that are very clean and free of termination by atoms of gases.
John Bentin
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