D e s c r i p t i o n   o f   c h a r g i n g   p h e n o m e n a

      In electrically neutral substances, the values of positive and negative charges are equal and balance each other. Such substances are electrically neutral and do not react with their surroundings. Nevertheless, the charges can be transferred from one substance to another or move within one substance. They can not simply appear or disappear without any reason, can neither be created nor destroyed. According to the charge conversation principle, in an enclosed volume of space the amount of electric charge is always conserved. There are various mechanisms of creating generating electrostatic charges, but all of them consist in dividing the neutral charges that have already existed in a given volume of space.
The unit of electrical charge is Coulomb (C), and the elementary charge (of an electron) equals 1,60217655(35) *10-19 C.

A.  Contact electrification
      According to the Helmholtz's hypothesis, when two substances come into contact, charges are transferred from one to the other. Subsequently, the so-called electric double layers are created. Each of them consists of two layers of charges of opposite polarity that are placed near the surface. As a result, after separating the substances, in one of them there can be excess of electrons (negative charge), and in the other shortage of electrons (positive charge).

A1.  Potential difference when connecting two metals
      When two metals are brought together at a distance of about several diameters of an atom, the so-called quantum tunneling (Schottky effect) takes place. The electrones from the metal of lower work function tunnel through a barrier to the metal of higher work function, until levels of Fermi energy are equal. When different metals contact, potential difference between them is created. Volta has arranged the metals in such order that each metal, when contacted with one of the metals on its right side acquires positive charge.

            

A2.  Contacting the metal with semiconductor and insulator
      In semiconductors and insulators the conduction band is separated from valence band by a band gap. Only a powerful electric field can supply the electrons of the dielectric with the energy needed to tunnel to the conduction band from the valence band. When contacting a metal, thanks to quantum tunelling, electrons are able to move to the metal. Depending on material type and amount of dopants on the surface of the insulator, there may appear donor as well as acceptor additional energy levels. So the electrons can also be transferred from the metal to surface levels of insulator. Considering the instability of all the factors that can influence transference of charges, it is difficult to foresee which mechanism will be dominant. The sign of the accumulated charge is changeable.

A3.  Contact of non-conductive materials
      According to Lenard's hypothesis it is presumed that solid substances (just like polar liquids) can possess, resulting from interaction of intermolecular forces, outer surface layers of negative charge. In the case of more powerful polarization (substance with higher dielectric constant) negative charge of external layer is attracted with less strength to the positive charge that is present under that layer, than in a substance of lower dielectric constant. Hence the Coehn rule, which says that a material of higher dielectric constant loses its negative charges more easily (charges are transferred from it to the substance of lower dielectric constant). A substance of higher dielectric constant acquires positive charge in contact with a substance of lower dielectric constant (which in turn acquires negative charge).
           triboelectric series
      Both the strength and polarity of electric charge are variable depending on many factors arising from not very well recognized mechanisms. Presented illustrative order of chosen materials is not the only one possible.

      Creation of charges by contact of dielectrics can also be related to the presence of polar molecules (dipoles) that can create a double electric layer. External electric field provokes a polarization of dielectric molecules, by which the dipoles place themselves in a specific order, which in turn creates electric charge on the dielectric's surface. Such field can be created by the contact potential difference when two substances come into contact.

B.  Electrification through friction (triboelectrification)
      Electrification by friction is a special example of contact electrification. This type of electrification is able to create charges of great strength thanks to large quantity of contact points and rise of temperature. Energy emitted by friction can be so powerful that even ions from warmer parts are able to be transferred to other surface with lower temperature. Triboelectrification occurs also when contacting two materials of the same type. Charges that are created are not evenly distributed on rubbing surfaces. One material can be in different positions in triboelectric series, depending on how smooth its surface is. Triboelectrification occurs also when granules and powders are hit against solid surfaces and by friction of air.

C.  Electrification through induction
      If a conductive substance isolated from ground is placed into the electric field, forces in that field will split up charges inside conductor and gather them on the surface. After the external theexternal field disappears, charges in conductors become neutralized and practically disappear. However, if one were to section the conductor and put it out of the field, electric charges would remain on both parts. If the charges are split by the field in dielectric, the process of disappearing of the charges after removing the field may last for long hours.

D.  Electrification through partial (corona) discharge
      If sufficiently high voltage is applied to the electrodes of charging device, partial discharge will be created near the surface of electrodes. Gas around the electrodes will acquire electric charge adequate to polarity of the electrode. Forces of the field make ions that were created change their position. If they encounter a non-conductive material on their way, electrostatic charges will accumulate on its surface.

E.  Electrification of liquids
      Creation of charges by flow, decanting or splashing of liquids is enabled by phenomena that take place in the surface layer of liquid that contacts solid substance or gas. Thanks to the imbalance of Coulomb forces at the interfaces, polar charge is created (especially in the case of polar liquids). By decanting or colliding against an obstacle the charges are split. If the liquid is conductive (resistivity ρ<104 Ωm), there is a rapid flow of charges and potential equalization. If resistivity of liquid is higher, charges can accumulate on obstacles or pipes outlets or on the walls of pipelines and containers.

F.  Electrification through freezing
      Liquids, whose molecules possess electric dipole moment, during freezing enter in the process of organization of dipoles. This process is similar to the phenomenon of splitting the charges. Ions from the solution are absorbed by growing crystals and put in the appropriate order. As a result, crystals possess electric charge.

G.  Electrification through piezoelectric effect
      In some materials under the influence of mechanical tension electrical charges appear on their surface. Deformations cause displacement of centers of gravity of positive and negative charge. This results in electric polarization of material.

                        Sources:
                          "Elektryczność statyczna w przemyśle" J.Simorda,
                                   (translations: "Static electricity in industry"
                          "Elektryczność statyczna w pytaniach i odpowiedziach" J.Strojny
                                                (translations: "Static electricity in questions and answers"

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