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When you first learned about electric currents, you may have asked how the electrons in a solid material move from the negative to the positive terminal. In principle, they could move ballistically or ‘fly’ through the solid, without being affected by the atoms or other charges of the material.

But, this actually never happens under normal conditions because the electrons interact with the vibrating atoms or with impurities and collide with them. These collisions in a solid typically occur within an extremely short time, usually about 100 femto seconds (tenth of a trillionth of a second). So the electron motion along the material, rather than being like running down an empty street, is more like trying to walk through a very dense crowd.

Typically, electrons move only with a speed of 1 meter per hour, they are slower
than snails.

The generally accepted definition of electrical current is “the flow of electrons.” But this definition cannot explain how current (a signal) flows at the speed of light when electrons do not flow at the speed of light.

The “fluid model” of current flow is sort of like water flowing through a pipe. While the analogy of water flowing through a pipe is useful for illustrating some types of current concepts (such as resistance, capacitance and transmission line reflections) it is a very poor analogy in other ways. In particular, the fluid model (correctly) and the “flow of electrons” model (incorrectly) are criticized as being unable to deal with another aspect of current propagates at the speed of light but  electrons do not flow at the speed of light.

Signal Flow at the Speed of Light
Here is where communication breaks down. When we talk about current being the flow of electrons and current flowing at the speed of light, we are not talking about the same flows! Individual electrons actually flow very slowly through a conductor, much, much slower than at the speed of light. But, electrons (as a group) shift very quickly through a conductor, almost, well…. at the speed of light! We have to focus on the right thing.

Electron flow causes a shift in electrons along a conductor.

Electrons - Falling Ball

Figure above illustrates this point. We are all familiar with this desktop toy. The falling ball at the front hits the first ball and transfers its energy to it, which then transfers energy to the second ball, and so on. The last ball in the string pops out (in the absence of friction) as far as the first ball fell. We could think of this string of balls as being extremely long. Even so, in the absence of friction, the last ball would pop out almost instantaneously, almost at the speed of light, even though the first ball is not moving nearly that fast.

This is a rough analogy of what happens with electrons. And the distinction is very important. Current flow does not mean “one electron in – same electron out.” This happens very slowly as that individual electron travels through the conductor. Instead, it means “one electron in-one electron out.” This can happen almost instantaneously, as above figure suggests. So the criticism of the “flow of electron” definition of current, that it can’t handle the idea that signals flow at the speed of light, is incorrect. It’s a matter of understanding what we mean by “flow.”

Reference:
Technical Publication on
“What’s This Thing Called “Current”? Electrons, Displacement, Light or What?
By Douglas Brooks, President UltraCAD Design, Inc., December 2005
http://www.mentor.com

By Er. G Srinivasa Reddy
Divisional Engineer/Operation/Siddipeta

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