Scott Huffman writes in a message to All
 SH> Miscellaneous questions: The following are some questions I 
 SH> have that nobody seems to be able to answer for whatever reason. 
 SH> I thought I'd ask yet another time as maybe I'll get a response. 
 SH> 
 SH> (1) Why do mains transmission lines ("power lines") transmit 
 SH>  alternating current more efficiently (with much less loss) 
 SH>  than direct current? [which is one of the two reasons why 
 SH>  we use A.C. for mains power..] 
First some definitions:  (you may know these, but they are for others to 
follow)
P=power in Watts
I=current in Amperes
E="EMF" or voltage expressed in Volts
R= resistance in Ohms
AC is transmitted at many thousands of volts, and relatively little current.  
Power is lost on the line by the relationship P=R*I^2 for a line with a total 
power of P=I*E.  Therefore, for any transmission line of any DC or low 
frequency resistance, the less current, the better.  Power loss increases as 
the *square* of the current flowing in the line.
  
AC power is used because you can use relatively low-cost transformers (with 
reasonable efficiency) to convert it from the many-thousands-of-volts 
transmission line levels to the 110V (or 220V) found in homes and business.  
If you sent DC at such high levels, you would have to typically convert the 
power to some form of AC, then reduce it through a transformer *or* use some 
sort of switching power supply arrangement to reduce the voltage to 
reasonable levels that won't throw sparks at you just by it's nearby 
resence.
  
The voltage on the transmission line is chosen in a balancing act between the 
leakage across insulators at the higher voltages versus the I squared times R 
loss that I mentioned above.
  
Make sense?
  
 SH>  I can only conjecture that maybe the capacitance present in 
 SH>  all conductors attenuates DC moreso than AC because the  alternatic 
 SH> cycles and phases of AC counteracts the rectification  caused 
 SH> by capacitance in the line.... or something to that affect.. 
Way off base.  There is *some* very minor capacitive loss in AC systems - DC 
transmission lines would not have this minor loss (because the voltage never 
changes).  Any capacitance loss is overwelmed by the I squared R loss and 
even by the leakage across the insulators.  The reason to use AC is the 
ability of simple, inexpensive transformers to change the voltage at will 
with little loss.  
 SH> (2) Why do high frequency RF signals attenuate more rapidly 
 SH> than low  frequency RF signals. Wheras, for every doubling 
 SH> of frequency  in MHz or kHz, the attenuation is 6dB greater 
 SH> as given by the  free space formula: FSL = 20 Log 4 Pi Distance 
 SH> / Wavelength. 
I think you will see that the formula gives equivalent field strengths (in 
volts/meter) at the same number of wavelengths away from the source.  In 
other words at 1000 wavelengths from the source, the field strength would be 
the same, given that the source emitters have the same power and the same 
antenna efficiency.  I'm not sure why the world works this way, just that it 
does.  
 
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