©2019 L A Waygood
In the post I wrote on Misconceptions about Electric Current, I explained that an electric current is a flow of charges; in metal conductors, these charges are free electrons. These charges are in a constant state of chaotic movement. But, when under the influence of an electric field, while the chaotic movement continues, there is a tendency for these electrons to move through the conductor in a direction determined by the direction of the electric field. This movement is v-e-r-y slow —usually measured in millimetres per hour. So slow, in fact, that an individual electron is unlikely to travel the length of a flashlight’s lamp filament within the lifetime of that flashlight’s battery!
In the case of alternating current, the net movement of individual charges continually reverses direction, which means that there is no progress of electrons around the circuit at all!
Many students believe that energy is transferred from its source to a load by individual electrons, as though they were acting as a ‘conveyor belt‘: delivering energy to the load, then returning to the source, depleted of energy, only to collect more energy in order to deliver it, once again, to the load!
This is a complete misconception because, as explained above, in the case of direct current, the transfer of charge carriers from source to load is extremely low, whereas the transfer of energy is practically instantaneous. And, in the case of alternating currents, well, the charge carriers don’t move around the circuit at all; they merely vibrate backwards and forwards!
So, if the ‘conveyor belt’ model of energy transfer is wrong, how does energy get transferred from the source to the load?
Well, while we know how energy isn’t transferred from the supply to the load, scientists are not altogether certain how energy is transferred from the supply to the load! A number of theories have been advanced, and perhaps the most well-known of these theories is one that describes the interaction of a circuit’s magnetic and electric fields to create what is known as the ‘Poynting field’, named in honour of the British physicist, John Poynting (1852–1914), which causes energy to ‘flow’ in a direction perpendicular to both these fields, and causing that energy to be transferred through empty space around —not through— the conductors.
The following, extremely-simplified, diagram attempts to convey the general idea:
In this illustration, a pair of conductors connects a battery to a load, and (conventional) current flows in a clockwise direction around the circuit, as shown. The conductors are surrounded along their entire length by a magnetic field, and an electric field stretches between the upper (positive) conductor and the lower (negative) conductor. The white arrows are perpendicular to both the magnetic and the electric fields, and represent the Poynting field which acts to transfer energy entirely through the empty space that surrounds the conductors between the supply and the load.