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- Basic principles of electricity -

Electricity is made by a flow of electrons (e-) moving through conducing matter, like the metals (silver, copper, gold, platinum,...).
The other chemical elements (non-metals) don't allow a flow of e- through themselves, or with more difficulty (sulfur, boron, gases, all solid oxides and salts, organic molecules) because their e- are more strongly bound to each atom.

Also ionic solutions in water (formed by dissolved salts that release their ions with opposite charges) are conducing, but the charges moving through the solution are not e-, but ions, like H+, OH- , Cl-, K+, Na+, Ca++, Cu++ and so on and their mobility in the solution depends on their dimensions and charge, if the electric field in the solution remains the same.

e- are negatively charged particles with a very little mass, 9.11 * 10^(-31) Kg (nearly 2000 times less that of neutrons (N) and protons (p+) forming the nucleus) and a charge of 1,602 * 10^(-19) Coulombs, equal but opposite to that of protons.
The e- move around the nucleus of atoms in equal number to that of p+, to balance exactly their positive charge, in the neutral atoms.
Every e- moves in an energy orbital that can contain maximum 2 e-.

In metallic atoms, the most external e- are more weakly tied to the nucleus and circulate in a chance way from an atom to the other across the whole volume of the metallic object, strongly binding one another all atoms of the metal.
This, because there's no energy gap among the level of a metallic atom and those of the surrounding ones, so that every e- moves into an unique energy level across the whole metal piece.
The contrary happens for not metallic elements (oxides, salts and organic compounds) with e- more tightly bound to atoms, needing much more energy to leave them; they are not conductors and used in electric devices as insulating matters or semi-conductors .

If no external forces are applied to the e-, they move in a totally chance way across the whole metal but, if we insert it into the electric field generated between two metallic pieces, the one with a lack of e- (positive electrode) and the other with an excess (negative electrode), the e- are immediately attracted by the opposite charge of the positive one, creating an electric current.

Conventionally, the sense of the electric current is opposite to that of the e- movement, hence, directed to the negative electrode.

Every current flow must always move within a closed circuit of conductors

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