What is the cause of triboelectric effect?

When a glass rod is rubbed with a silk cloth, glass rod loses electrons and silk cloth gains electrons. So, glass rod becomes positively charged and silk cloth becomes negatively charged. But why does the glass rod lose electrons and the silk cloth gain electrons? Why doesn't the silk cloth gain electrons and the glass rod lose electrons? Why doesn't the glass rod become negatively charged and the silk cloth become positively charged? And what is the cause of electron transfer? What factors decide as to which material will gain electrons and which material will lose electrons? What is the cause of triboelectric effect and what exactly is the triboelectric series? Please can someone explain? I am so confused.

• The triboelectric series is simplest so I'll just get that out of the way: It's just an ordered table of materials that tell you which when one will end positive and which will end up negative when you rub them together. It exists so you don't have to know the more technical stuff, just the results. Much like the galvanic series for metals (tells you which metal will take electrons and which will donate electrons when put into contact with each other.) The mechanism for why the series is the way it is is way over my head. Hopefully someone will answer you. Jun 2 '21 at 14:49
• But why do some materials get positively charged and some materials get negatively charged? What are the factors that decide their polarity? Jun 2 '21 at 15:22

2 Answers

Solid lattices are composed of molecules and atoms which are in a quantum mechanical bound state , the nuclei at the center, the electrons in various bands , described usually in the band theory of solids. . The electrons in the conduction band are shared with the whole lattice, they are very lightly bound so with the energy given by rubbing could change to which lattice they could go, leaving a hole in the other material. The particular attraction of electrons to a given lattice needs further detail in the modeling.:

The electrons in atoms are moving in orbitals about the nucleus, and when shared with a molecule the orbitals have specific shapes, example

Suitably aligned f atomic orbitals overlap to form phi molecular orbital (a phi bond)

Where in the lattice space there is a very small probability for an electron to be, the positive charge of the nucleus dominates , and thus chemical bondings can occur, joining into a lattice, like the play LEGO blocks.

For specific materials the specific bonding shape facilitates taking up electrons, which will leave positive holes in the appropriate other material, as with silk and glass.

I will describe a simple model to give the general idea.

Suppose we have two positive charges, both $$Q$$, and two negative charges, both $$-Q$$. Suppose that the first positive charge is sitting inside a solid sphere $$A$$ of radius $$a$$ and the second positive charge is sitting inside a solid sphere $$B$$ of radius $$b$$, with $$b > a$$. Suppose that each sphere has the negative charge $$-Q$$ spread out over its surface.

In this situation if we take either sphere on its own then we have a stable, bound system, and each negative charge will stay attached to its sphere. Outside each sphere the total electric field is zero.

Now let's bring these spheres close to one another. If they get close enough then the negative charges on the spheres will begin to push against each other, and this will distort the distributions. Now there will be a net electric field between the spheres, and some of the charge on sphere $$B$$ will move to sphere $$A$$, because there it gets closer to the positive charge (because the radius $$a < b$$). The configuration with the lowest energy overall is one where there is a bit more negative charge over at sphere $$A$$.

Now imagine pulling the two spheres apart again. There is no particular reason why the charge would go back to equal on the two spheres. Rather, one sphere will end up negative, the other positive.

This simple model gives an idea of what can happen when atoms or molecules of different types are brought alongside each other and then pulled apart. The positive charge represents an atomic nucleus; the sphere represents the electron clouds which (owing to Pauli exclusion principle) prevent the last electron from reaching a tightly bound state close the nucleus; the negative charge represents this last electron (or few electrons) which are not tightly bound.

Once you have thus charged two things, one positive, the other negative, each will subsequently either attract or repel further electrons which happen to be lying around, and thus gradually its total charge will go back to neutral.

final remark

Now I will 'come clean' and admit that I have not checked whether the scientific consensus is that the process I have described is indeed the right one to quote in this context. This answer described a simple process (correctly), and then conjectured that this is the essence of what is happening when things are charged by rubbing.