## Updating Circuit Theory: Answering the Catt Question

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By Ian Darney

Some readers of Electronics World may remember articles by Ivor Catt in which he posed the Question â€˜When the front edge of a voltage step is propagating along a transmission line, where does the negative charge on the bottom conductor come from?â€™

This brought to mind one of the lessons taught to final year students of Electrical Engineering at Glasgow University. It was explained that electrons could not possibly move at the speed of light. Nor did they need to, since there is an abundance of atoms in any copper conductor. If each atom has a free electron, then these entities need only move at a snailâ€™s pace to carry the rated current. The question as to how power is transported from Glasgow to London in less time that it takes to blink was left unanswered.

Part of the answer can be found at Wikipedia. â€˜The photon is a type of elementary particle. It is the quantum of the electromagnetic field including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, and they always move at the speed of light in vacuum.â€™

Since photons apply force, they can cause electrons to be ejected from copper atoms. This causes current flow in the conductor, rather like tipping over a pack of cards. Developing this idea allows a mechanism to be postulated; one which provides an answer.

After posing his question to several eminent scientists, Ivor Catt poured scorn on each and every reply. From his point of view, the fact that charges are delivered to the bottom conductor brings the whole science of Electromagnetic Theory into question. Even so, he has made no attempt to provide an alternative explanation.

The Catt Anomaly

Traditionally. when a TEM step (i.e. logic transition from low to high) travels through a vacuum from left to right, guided by two conductors (the signal line and the 0v line), there are four factors which make up the wave;

– electric current in the conductors
– magnetic field, or flux, surrounding the conductors
– electric charge on the surface of the conductors
– electric field, or flux, in the vacuum terminating on the charge.

The key to grasping the anomaly is to concentrate on the electric charge on the bottom conductor. During the next 1 nanosecond, the step advances one foot to the right. During this time, extra negative charge appears on the surface of the bottom conductor in the next one foot length, to terminate the lines (tubes) of electric flux which now exist between the top (signal) conductor and the bottom conductor.

Where does this new charge come from? Not from the upper conductor, because by definition, displacement current is not the flow of real charge. Not from somewhere to the left, because such charge would have to travel at the speed of light in a vacuum. (This last sentence is what those “disciplined in the art” cannot grasp, although paradoxically it is obvious to the untutored mind.) A central feature of conventional theory is that the drift velocity of electric current is slower than the speed of light.

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