Electronic – Why is there no net current in a wire without a voltage applied


Atoms of materials with loosely bound outermost electrons constantly exchange charges between each other over time, and these materials are called conductors. Now, the conducting process is different from the one often described in the electrical engineering textbooks.

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This implies that in order for current to flow in the circuit, an electron has to move from one lead all the way to the other, which is simply not true. Reality is something like this:

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The electron at the far left coming from the negative lead of a battery, for example, is then colliding at the nearest atom and because of its acceleration it's knocking out the electron which is revolving at this shell level. The knocked electron is heading to its closest atom and in turn it's doing the same, knocking out an electron which creates a chain reaction. So, basically, electrons move just a little bit, but the overall outcome is virtually instantaneous.

What I don't understand is if we take a regular conductive wire WITHOUT applied voltage on it, electrons still constantly bounce from atom to atom which means that literally there is "an electron flow" in the wire, but if we connect the wire to a LED diode nothing would happen. So, what I am really asking is how differs "an electron flow WITH applied voltage" from "an electron flow WITHOUT applied voltage" in a wire.

Best Answer

Statistically, there are as many electrons moving in one direction as there are in the 180º opposite so there is effectively no net current. What we know as "current" is the movement of more electrons in one direction than all the others (1D, 2D or 3D through a piece of metal). That's how you can have "tons of free electrons" but no net currents flowing or measurable.

The random agitation of those electrons has a name: thermal noise. This agitation is proportional to temperature so you get more of it as you heat things up. However, the average motion is always zero so you can never do any useful "work" or equivalently extract usable energy from the process.

This is in agreement with the laws of thermodynamics.