Path: utzoo!utgpu!watmath!clyde!att!ucbvax!hplabs!hpfcdc!hpfcmr!ron
From: ron@hpfcmr.HP.COM (Ron Miller)
Newsgroups: sci.electronics
Subject: Re: Voltage and Amps Question (Huh?)
Message-ID: <1240002@hpfcmr.HP.COM>
Date: 29 Dec 88 21:29:40 GMT
References: <3415@ucdavis.ucdavis.edu>
Organization: HP Fort Collins, CO
Lines: 45

> 
> 	Could someone please explain why it is better to have a circuit with
> high amps and low volts rather than the other way around?  

It depends on what you are using it for.  Low voltage isn't as dangerous.
Low voltage requires big wires and is lossy. etc etc etc etc

For cars, notice that 12V is now standard. Used to be you could find 6V
automobile electrical systems. Go too low in voltage and it's a bad idea.
(Also makes for dim headlights in practice....)

>Does increased amps
> lead to increased power in the same proportion as increased voltages leads to
> increased power?  

Using Ohm's Law, yes. This characteristic is not useful by itself.


>Why are car batteries touted as having more cranking amps, 
> isn't your amperage held constant by:
> 
> 			V = I R  V = 12, R = Constant  Power =  V^2/R
>  
> 	What am I missing here? A brain perhaps?
> 	    Thanks in advance, -Mark

[What you're missing is the background to analyse the problem. (Practical
EE as opposed to mathematical EE)]

V is not constant, I is not constant because R is not constant. A starter motor
is only a long length of wire until it begins to turn. (It develops something
called 'counter EMF' which tends to oppose current flow.)  Battery output 
voltage tends to drop with increasing load current due to the internal 
resistance of the battery (I**2 R losses)

The only reason why "cranking amps" is important is that it takes considerable
power to budge a car engine. The more amperage available, the more power
is available to the starter motor to crank over the engine. (Cranking
amps may also give insight into cold temperature battery performance.)

A practical 'motors 'n rotors ' class would make sense of all this.
I didn't get mine in college. I got it in Naval Nuclear Power School in
order to deal with BIIIIG !!! motors 'n rotors. :-)

Ron Miller