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Updated: 09/22/07
Current
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IT ALL BEGINS HERE /
Capacitance Resistance simply means the "opposition to the flow of electric current". As electric current is defined as the movement of free electrons, then resistance is the opposition to the motion of free electrons. Wherever free electrons are in motion, there is resistance to their movement. This is often called d-c (direct current) resistance. Later on I will include a page on alternating current as well. All metals offer electrical resistance. Resistance is important because it has a controlling effect on the amount of current which flows with an applied voltage. Materials that have so few free electrons (very high resistance) that current cannot flow through them are called "Insulators". Materials which have free electrons are lower in resistance and are called "Conductors"...current flows freely through them...and there are of course all materials in between the two extremes.
Radio circuit resistance range from a
fraction of 1 ohm to several million ohms. The metric prefix milli means
one-thousandth, kilo means thousand, meg means one million, and micro
means one-millionth....these are all used in connection with resistance. The symbol
of ohm is the Greek letter omega and looks like a Comparing the specific resistance of a metal gives us a way of determining the usefulness of the particular metal as a conductor. In order to determine specific resistance, the resistance in ohms must be determined by comparing a standard amount ( 1 cubic centimeter) of the substance being measured at a particular temperature (68 degrees F). The Resistance of a substance varies with its length. The longer the conductor, the more resistance it will have. The basic rule is "the resistance of a conductor is directly proportional to its length". The "thickness" or cross section of a wire made from a particular substance has a significant effect on its resistance. Think of the wire as a pipe conducting water...a bigger the cross section of the pipe, the more water that will flow in a given time...so to with a larger cross section wire. The greater the cross section, the lower the resistance. If you have two equal lengths of the same wire, with one having twice the cross section of the other, the larger cross section wire will offer one-half the resistance. The basic rule here is "the resistance of a conductor is inversely proportional to the cross-sectional area." (Do not confuse cross-sectional area with diameter..they are not the same Area = Pi (RxR) where R=radius
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The unit of resistance is the Ohm. The word is the name of
George Simon Ohm, a German scientist who advanced many theories in the study of
electricity.
. Several physical features of
conductors determine the amount of resistance they present to the flow of current.
They are (1) kind of material; (2) length; (3) cross-section; (4) temperature.
