MadSci Network: Engineering
Query:

Re: speed of electricity

Date: Mon Aug 31 00:15:36 1998
Posted By: William Beaty, Electrical Engineer / Physics explainer / K-6 science textbook content provider
Area of science: Engineering
ID: 901847196.Eg
Message:

The word "electricity" has no single definition. I think that this is your problem. For example, in an AC system the charges wiggle back and forth, yet the energy moves rapidly forward. Books typically say "electricity moves back and forth in an AC circuit". They also say "electric companies send ELECTRICTY from the generator to the consumers." Taken together, these statements are meaningless. An analogy: when sound waves move through air, the AIR vibrates but does not flow, while the ACOUSTIC ENERGY flows forward very rapidly. Now if we had just a single word for both air and sound, nobody could discuss them or understand them. Everyone would think that air WAS sound! When we use the term "electricity", we mix together the charges, current, and energy into a conceptual mess. A solution: don't say "electricity".

Conductors contain charges, and during an electric current the conductors' charges flow. This flow is fairly slow. And in an AC system, charges do not flow forward at all, instead they sit in place and vibrate.

When a generator or battery sends electrical energy to a light bulb, the energy travels as electromagnetic fields which are guided by the connecting wires. If the wires are bare, then this energy travels at the same speed as the speed of light in atmosphere. If the wires are covered with plastic, then this energy travels more slowly, depending on the geometry of the plastic and the permittivity of the material. If you send electric energy along a 75-ohm coaxial cable, then there is an equation for calculating the propagation velocity (I don't have a reference here which contains it though.)

For instructions on how to calculate the speed of electrons flowing in metal wires, see SPEED OF ELECTRICITY on my Misconceptions web page.

About your "power surge". I don't think that had anything to do with the electric company. It resembles a common description of St. Elmo's Fire, which is a corona discharge created by the strong e-fields below a thunderstorm. This sort of discharge does sometimes travel along metal wires. But it also travels along barbed wire surrounding farmer's fields. I would predict that your glowing ball would still have moved along that power line even if the generators were turned off. For a bit more about this, see:

BALL LIGHTNING

SCIENTIFIC AMERICAN "ASK EXPERTS"


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