Date: Sat, 4 Apr 2009 23:17:32 -0400
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Learning Electricity Part One
In-Reply-To: <c4e7c5f90904041513r1665f89ga88b61869bbc985a@mail.gmail.com >
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At 06:13 PM 4/4/2009, neil N wrote:
>Looks like a helpful tutorial.
Here are some fundamentals of automotive electricity for the electricity-shy.
First of all, it's very useful to think of electricity as a
compressible fluid, like steam; and an electrical circuit as a
circuit of steam piping and components. The major difference is that
steam condenses into water, and electricity doesn't, so the analogy
breaks down there -- so compressed air is actually a better analogy,
but I think steam may be more familiar as a working fluid.
Both circuits have pressure -- in steam we call it psi; in
electricity, volts.
Both have volume -- in steam we might call it a pound; in electricity
it is a coulomb. A pound of steam is made up of an enormous number
of water molecules, and a coulomb of electricity is an enormous
number of electrons. About 602,300,000,000,000,000,000,000 in fact.
Both have a flow rate -- in steam it might be pounds per second; in
electricity it's coulombs per second, which we call amperes or amps.
Both have internal resistances to the flow of working fluid. Some
are intentional but others are intrinsic to the piping or wiring in
the circuit. In steam I have no idea what name if any that
resistance has -- it's more usually spoken of in terms of its
effect. In electricity the unit of resistance is called the ohm. In
both circuits resistance has no effect if there is no flow, but
causes a pressure drop in the circuit when there is flow. That is, a
gauge or meter measuring between upstream and downstream of the
resistance will show a pressure which increases proportionate to the
flow rate. In most automotive circuits the only intentional
resistance is in the operating device, or load; all the rest simply
waste power as heat.
In both circuits, the pressure drop caused by a resistance and the
amount of power developed by that resistance are in proportion to the
ratio of that resistance to the total resistance in the circuit. If
the pressure drops across all resistances in the circuit are added up
they will equal the pressure produced by the power source -- boiler,
battery etc.
Likewise in both circuits the flow rate is the same in any part of
the circuit. If there are parallel branches within a circuit, the
flow rate in each branch will be inversely proportional to the
resistance in that branch and the total flow will be identical with
the rest of the circuit.
Both do work of some sort -- in steam we might measure it in
horsepower-hours; in electricity it is watt-hours.
Both do that work at a certain rate; that rate is known as power --
in steam, horsepower; in electricity, watts (incidentally, 746 watts
equals one horsepower). In both cases the power is the product of
the pressure and the flow rate. In electrical terms, Watts = Volts x Amps.
--
David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
'89 Po' White Star "Scamp"
