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In "Is electricity instantaneous?" there were several answers that differentiate between 'electron flow' and 'current flow': electrons move slowly through a conductor, but the resulting current moves near the speed of light.


Given: 1 Coulomb = "6.3 billion electrons passing one point in a circuit in one second" [noted that this is not reflected in the SI definition which is 'the constant current needed to produce a (given) force in parallel conductors...']. Most sources I have read go with the "electrons/sec" definition, or "charge/sec" definition - including the tag here for electric-current. As a chemist I equate "charge" with "electrons" (not 'current flow').

Question: is a Coulomb based on a 'count' of slow electrons, or on the fast electric field generated by the moving electrons? [acknowledging that as a derived unit the Coulomb is really based on the Ampere - but this seems to be kind of circular reasoning at some level]

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One Coulomb is defined as the charge transported by a current of 1 Ampere during 1 s. The Coulomb has nothing to do with the electric field generated by electrons.

As one electron has the negative charge of $1.602·10^{-19}C$, this means that 1 Coulomb has $6.24·10^{18}$ absolute values of electron, i.e. elementary charges. The Coulomb has thus a $6.24$ times a billion billion elementary charges

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...electrons move slowly through a conductor, but the resulting current moves near the speed of light.

Not quite. The distinction is not between "electron flow" and "current flow", but between the bulk flow of charge and the propagation of a signal through the charge.

Electrons move (relatively) slowly. What moves fast is a signal. A common way of thinking about this is a bicycle with a chain driving the wheel. The chain itself doesn't move that fast, but the rear wheel will move almost as soon as you push the pedal. The signal to move travels through the chain much faster than the links themselves move.

A Coulomb is a specific amount of charge. In this model, it is similar to a specific number of chain links. Neither is directly related to speed that a signal will move through a wire or a chain.

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There is a subtle difference between the concept of some physical thing, such as "electric charge" or "electric current" and the way we quantify it. The SI definition is not so much concerned with the concept itself, but rather with a rigorous method to quantify it.

So, conceptually, a current is the flow of electric charge and it literally means that a certain amount of charge flows past a point in a certain amount of time. However, if we want to make sure that we measure this current as accurately as possible, then we use the description given by the SI definition. These two descriptions are link via the understanding of the physical connection between then as given by the laws of physics. When a certain amount of electric charge flows past a point in a certain amount of time it will produce a certain force between the conductors within the specified situation as prescribed by the SI system.

A current (or an electric charge) is not an electric field, but again the two are linked by the laws of physics as described in Maxwell's equations. This link can often be used to say something about one, by measuring the other. So, if we measure a specific electric field, then, given a particular situation for the experiment, we can say how much charge there is, thanks to Maxwell's equations that link the two.

Hope this helps.

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