If you have a circuit of a battery, a Resistance and a light connected with a wire by example: in the middle the wire is cutted in two pieces. if you connect each piece of wire with a metal object and put those twee metal objects together so the current can flow to the resistance and the light. How is it possible that the electrons can flow from the wire to one of the metal objects into the other metal object and again in the wire? I do understand how electrons move in metals and wires, but i don't know how the transaction of electrons between those components goes. Is it really that simple that electrons just can flow from one object into another of is the (kinetic) energy passed through when an electron clashes with the edge and that energie is passed through to the other object?

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    $\begingroup$ The details are, unfortunately, somewhat complicated, but in the most simplified version you can imagine that a metal is a bunch of positive metal ions in a lattice that are surrounded by a cloud (sea) of electrons. This electron cloud reaches all the way to the surface of the metal and when two pieces of metal touch, the electrons from one cloud can move into the other without a significant barrier. The average motion of these electrons is very slow, it's called the "drift velocity" and they move at fractions of a mm/s. $\endgroup$ – CuriousOne Feb 25 '16 at 20:28

Metals consist of small crystals; within each crystal exists the "free electrons" which are shared by all of the atoms in the crystal lattice. The number of free electrons per atom depends upon the details of the atoms, but is most often 1 or 2. The free electrons are visualized as "the electron sea" in the Drude model, devised ~1900, and is semi-classical. Introductory condensed matter texts often start with this model. The situation is slightly more complicated with alloys, but the same ideas hold.

In the electron sea the electrons are electrically shielded from each other by (a) the net positive charges of the ion cores and (b) the uncorrelated motions, essentially random, of the free electrons, which are described using the kinetic theory of gasses.

The crystal boundaries serve to impede the free flow of these electrons from one small crystal to the next, and also serve as scattering sites which continually randomize the motions. The velocities of the free electrons are quite large. When an external electric field is applied, it appears as a net "drift velocity" in the electron sea. This is the current in that piece of metal.

When you bring two clean pieces of metal together all of the above is still true, but there is an additional restriction: each crystal has an effective "crystal voltage" on its interior, and for crystals of the same type it should be the same. But when different metals are joined, the difference in the crystal voltage causes a voltage drop when going in one direction, and an increase in the other. This voltage difference is known as the Seebeck effect, discovered in 1821. Since the internal voltages change slightly with temperature, it is possible to measure temperature change electrically; this is the physical basis for the thermocouple.

So adding additional metal increases the total resistance of the circuit, depending upon the resistivity of the additional metal, its dimensions, and other properties.

The current is the net flow of electrons; each individual electron barely moves, but the effects are passed down the line. With alternating currents for every move forward, there is a corresponding move backward -- hence no net motion from the electron drift at all.


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