150110-2032 EST
Time limit.
Edited version.
150110-1614 EST
aarena:
When you provide a reference to something specific that I should look at, as you did in your first post, then provide sufficient information that I can easily find the point where the information is located. Reference to a many page document is much too broad. I think I found your reference location at paragraph 1.18 .
Components that are very good conductors have a low resistance between two measurement points. You need to understand that resistance and conductance are relative measurements when trying to compare different materials and applications. Some materials with relatively low resistance for a given mechanical shape are: silver, copper, aluminum, gold, iron, and stainless steel.
In the "Handbook of Chemistry and Physics". 40th Edition 1958-1959 starting at page 2587 is a table of Resistivity for various materials. This probably can be found in the Index in any edition under "Resistivity of metals".
Code:
All the following are * 10^-6 ohm-cm
Silver 99.98% at 0 C = 1.468
Copper hard drawn at 20 C = 1.77
Gold pure drawn at 20 C = 2.44
Iron 99.98% at 20 C = 10
Mercury liquid at 20 C = 98.5
Mercury solid at -183.5 C = 6.97
Nichrome at 20 C = 100
Steel vanadium at 20 C = 121
Graphite at 0 C = 800
See
http://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity note that the units are different so the exponent changes.
What Mike was saying is that material for heater heating elements is of a higher resistivity than the material in wiring conductors because they serve two different purposes. For a heater you want high resistivity material to minimize size and cost, and in distribution wiring you want low resistivity material to reduce mass, cost, and wasted energy.
Suppose you have a perfect source voltage of 100 V that does not vary with a change in load current. Assume copper wire resistance is constant with temperature, and that #14 copper wire has a resistance of 2.5 ohms per 1000 ft. Connect 1000 ft to the 100 V source and the current is 40 A, and the power dissipated is 4000 W. Change the material to nichrome and the resistance is about 100/1.77 = 56.5 times greater, or 2.5 * 56.5 = 141 ohms. Now the current is only 0.71 A, and the power dissipated is 71 W. To make the nichrome #14 wire produce 1500 W we need to shorten it by 1500/71 = 21.1 times. This becomes a length of 1000/21.1 = 47.4 ft. Because of the physical characteristics of nichrome wire we can reduce its diameter to increase the resistance per foot and make a practical 1500 W heater with only a few feet of wire.
On the other hand we don't want to waste power in the distribution of energy to the heater so low resistance copper wire is used for the conductors from the voltage source to the heater. Silver would be better, but it is much more scarce and in turn higher in cost. For the 1500 W heater you might use #12 instead of #14 for distribution copper wiring.
In the 1870s Edison understood that to build a practical power distribution system that the loads needed to be in parallel and the distribution of energy needed to be at high voltage and have a low source impedance. Many theoretical people at the time wanted to operate in a maximum power transfer mode.
However, today in a solar system you do want to operate in a maximum power mode. This is a different criteria. As an excerise determine what value of source resistance will produce maximum power transfer to a fixed resistance load.
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