I thank you and all who responded to me. I won't let you down! I'll get it eventually. 

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of Howard Winter
Sent: Wednesday, March 23, 2005 12:11
To: Microcontroller discussion list - Public.
Subject: Re: [EE] Learning Electronics: Question about AC/DC

On Tue, 22 Mar 2005 23:37:24 +0100, Lindy Mayfield wrote:

>...<
> If I think this way with AC, then I get:
> 
> >++++<---->++++<---->++++<----  One water Pipe
> <---->++++<---->++++>---->++++  The other pipe

The thing you're missing is that this isn't a *circuit*!  Ignoring static electricity, for current to flow 
there must be a circuit, and the current is the same all the way round the circuit (if it splits into more 
than one path then the sum of the currents is the same).

So in a DC circuit you have the current (which is the flow of electrons) doing this:

 --->---->---
 ^          V
 |          |
 ^          V
 --<----<----

(Please excuse the dreadful ASCII art!)

So in this case the current is always flowing clockwise through all the components.  If you want to think of 
it in terms of water, it's a ring of pipes with the water always going one way round, and if there are no 
leaks the rate of flow must be the same all the way round.

Now with AC the flow *alternates* between the flow above and this:

 ---<----<---
 V          ^
 |          |
 V          ^
 -->---->----

So the same electrons (or water in the plumbing analogue!) oscillate backwards and forwards at whatever 
frequency the Alternating Current is running.  As has been said, in cases like mains electricity this 
alternation is a sine wave, so it doesn't switch suddenly from one direction to the other, but slows down, 
stops, then speeds up going the other way, then slows down again, stops, etc.
 
> The water goes at 60Hz first one direction then reverses and goes in the other direction in on pipe, while 
in the other pipe it does the same but in opposite directions to the first pipe.  

Because the pipes are joined at both ends - if they aren't then no current (water or electrons) will flow.

>When I imagine this the water doesn't do anything.  They seem to cancel each other out.  This can't be 
correct.

The movement back and forth does the work - imagine having a propeller inside the pipe - it would be turned 
back-and-forth by the current, and this is one way to extract the energy from the pipe (connect it to some 
sort of ratchet if you want movement in only one direction).  Or have a small restriction in the pipe 
(resistance) and the friction of the water passing through would create heat, whether the flow was constant or 
alternating.
 
> With DC things seem to make sense in a way.  

They both do when you get that it's the flow that's important, not the actual movement of a particular 
molecule of water, or electron, from one part of the pipe to another over time - oscillating around one spot 
is fine!

Cheers,


Howard Winter
St.Albans, England



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