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Updating Circuit Theory: Flow of Partial Currents


By Ian Darney

The analysis of the drift velocity of electrons along a conductor conjures up a picture of water sluggishly flowing along a ditch in the countryside. This is misleading. A more appropriate visualisation of the flow of charges along a conductor is that of an avalanche hurtling down a mountainside.

A reprise of a lesson copied from a blackboard sixty years ago derives a figure for the drift velocity when a copper conductor is carrying its rated current. This was described as a snail’s pace. The simulation of the flow of partial currents along a transmission line provides a dramatically different scenario.

If an electron is ejected from a copper atom, that atom becomes positively charged. If an electron is attached, the charge on the atom goes negative. If photons happen to illuminate a small patch on the surface of a copper conductor, then this will initiate a chain reaction. Positive charges will depart along the wire in one direction with negative charges flowing in the other direction.

If a step voltage is applied at the near end of a twin conductor cable, then a wavefront will propagate along that cable, with positive charges flowing along one conductor and negative charges keeping pace along the other conductor.

With a short-circuited termination at the far end the chain reactions continue unabated. A negative wavefront arriving from the return conductor reappears as a positive wavefront departing along the send conductor. A positive wavefront arriving from the send conductor reappears as a negative wavefront departing along the return conductor. 

The total current at any point in a conductor is the sum of the partial currents.

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