`When two bodies are kept in contact, electromagnetic forces act between the charged particles at the surfaces of the bodies'--chapter-6 (The Friction),article 6.1,page no.85, Concepts of Physics by Dr.H.C.Verma.

I m not geting that why the bodies on contact with each other gets charged at the surfaces.

Hi Nkumar,

They don't get charge at the surface, they already have charges on their surface.

As you probably know, all matter is composed of atoms that consist of protons (positivily charged) and electrons (negative charges). When you look at just the nucleus of an atom, all you see is positive charge. When you look at the atom at a whole, the charge of the electrons compensates for the proton charge, so as a whole, the entire atom is neutrally charged. However, this does not mean that there is no electromagnetic interaction between atoms: if you look at a large distance from an atom (eg. 1 mm = 10^-3m, an atom is typically 10^-10 m in size) then you see no electric field (the electric fields of the electrons and protons compensate eachother at large distances). Close to the atom however, they don't completely compensate and there is a small remaining electric field. This is refered to as the dipole electric field.

So atoms that are close to eachother can feel eachother's presence through electromagnetic interaction. In your example: electrons from one surface also interact with the protons of the other surface when they are in contact with eachother, and this electron-proton interaction is one of the causes of friction.

However, I would not attribute the cause of friction to electromagnetic interaction. Personally I'd say that friction is caused by imperfections in the surface of both bodies: these imperfections counteract eachother, and make it more difficult to move the two objects ( = friction) against eachother.

Bye!

Crisp

Originally posted by Crisp

So atoms that are close to eachother can feel eachother's presence through electromagnetic interaction. In your example: electrons from one surface also interact with the protons of the other surface when they are in contact with eachother, and this electron-proton interaction is one of the causes of friction.



hello crisp

As electron are in outer orbits and protrons are in inner neulceus
there should be some force of repulsion between electrons of one atom and other of the contacting bodies ,which should be larger then electron-proton interaction as because of inverse square law : Forces of interaction between 2 entities are inversly proportional to square of distance between them
bye
kumar

kumar,

'Friction' is not an altogether straighforward. If you have two rough surfaces in sliding contact, the friction is large. If you smooth them a bit, the friction usually decreases but if you polish them really smooth, the friction can increase again.

It's to do with the intimacy of the contact and the true area of contact between the high spots (called 'asperities') that occur even on apparently smooth surfaces. The tips of the asperities of both surfaces come into intimate contact and bond to create, in effect, small spot-welds. If you apply a shear stress, these welds will eventually break and new ones will form as the surfaces slide. That's why the coefficient of static friction is greater than that of dynamic friction; in the latter, there is no time for local creep to allow the welds to develop greatly. If the surface is really smooth, then you get less pronounced asperities but a lot more of them. To reduce the friction then you need to use a lubricant - even just air will reduce friction.

Cheers,

ron.