Tuesday, June 10, 2008


May 28, 2008 Educational Radio Net, PSRG 2nd session

One of the topic suggestions offered during last weeks edition was impedance and antenna matching. This was (and is) a great topic but one of enormous scope and complexity. After some thought I felt it appropriate to more or less serialize the topic and spread it over several weeks so that we can deal with the subject in more detail.

Impedance is, certainly, one of the corner stones of electrical theory in general and radio systems in particular. To really grasp the significance of the symbol "Z" requires at least some understanding of the big three circuit elements... resistance, inductance, and capacitance. Additionally, some understanding of electrical current and voltage relationships is necessary to get the big picture. So, this week lets start the narrative by discussing the most elemental idea in electrical physics... that of "charge", both moving and stationary. In the weeks following we will address the circuit elements in turn and then merge them together into some coherent structure.

Much like any radio talk show I will "set up" the topic and then allow time at the end for questions or comments. Truth be known this subject is a mathematical adventure but, given that we do not have a "white" board for graphic illustration, I will attempt to convey simple ideas verbally.

So, lets get started with the concept of "charge".

In the formative years of electrical theory it was understood that something clearly moved when influenced by electrical forces. That hypothetical "something" was given the name "charge" and considered to be positive due to the direction it would move when in the presence of some motive force. Modern electrical theory has shown that charge is an electron and, in fact, the elemental electronic charge is negative. So, from a historical perspective, the early and assumed positive charge became known as "conventional" in contrast to the now better understood "electron" theory of charge composition.

The electron is very tiny and is normally attached to some atom... helium or hydrogen for example. When agitated by forces yet to be discussed the electron can break free of the "mother" atom and become a free electron in contrast to being a "bound" electron before agitation. One free electron bumping along a wire would be impossible to locate or measure without some fancy laboratory instruments however large numbers of moving electrons are readily detected by nanoamp, microamp, milliamp, and just plain ampmeters. There are some very important definitions which are associated with charge.

First... the coulomb. This is simply a fixed number of electrons. In fact 1 coulomb is an assemblage of 6.24 x 10^18 electrons.

Second... the ampere. This is defined as 1 coulomb of charge moving past a point in 1 second and is assigned the electrical symbol "I". In physics, something per time is known as a "rate". Good examples are miles/hour, feet/second, apples/minute, coulombs/second, etc.

So, there you have it... a coulomb, with symbol Q, is just a known quantity of charge... in a bag for example... and electrical current is just a known quantity of charge moving past a fixed point. A coulomb sitting around doing nothing is electrostatic charge whereas a coulomb marching down a wire is electrodynamic charge.

This concludes the first in a series of elementary ideas and basic definitions.

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