Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!bloom-beacon!primerd!zaphod!doug
From: doug@zaphod.prime.com
Newsgroups: sci.electronics
Subject: Re: Voltage and Amps Question (Huh?)
Message-ID: <44900003@zaphod>
Date: 29 Dec 88 16:25:00 GMT
References: <3415@ucdavis.ucdavis.edu>
Lines: 30
Nf-ID: #R:ucdavis.ucdavis.edu:-341500:zaphod:44900003:000:1338
Nf-From: zaphod.prime.com!doug    Dec 29 11:25:00 1988


Ohms Law:  V = IZ  

                     2     2
Power Law: E = IV = I R = E / R

First,  the current is indeed determined by Ohm's law.  The problem is that a 
car starter does not have a constant impedance.  When the starter is turning 
over quickly (low load, as in a warm day) the current draw is only 10->20
amps.  On a cold day the starter has to work much harder and will often draw
100 or more amps.  Note that all this is directly related to the work
(force x distance) the starter must do.  So on a warm day the starter is
using between 120 and 240 watts energy and on a cold day upward of 1.2 Kw.

In general for power reasons it is better to have high voltage and low current
in a line.  This is because the primary power loss is due to resistive voltage
drop in the line and is directly proportional to the current.  You can beat
this by lowering the current (therefore raising the voltage) or lowering the
resistance (e.g. using superconductors).  That's why power companies transmit
power cross country at hundreds of thousands of volts and do local power at
several thousand volts.  

Douglas Rand
 
  Internet:  doug@zaphod.prime.com
  Usenet:    primerd!zaphod!doug 
  Phone:     (508) - 879 - 2960
  Mail:      Prime Computer, 500 Old Conn Path, MS10C-17, Framingham, Ma 01701

->  The above opinions are mine alone.