# Re: Electricity as water

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On Sun, 3 Jan 1999, cwells wrote:

>
> From: "Frank I. Reiter" <FIR at istar_ca>
> I have never figured out a simple way to represent watts in the
> "Electricity
> as water" metaphor.  The product of pressure and volume is probably
> proportional to  ability to drive a turbine, which would relate it to
> total
> energy (watts), but few listeners have real world experience of that to
> relate to.
> Frank.
>
> Frank try replacing "volume" with a rate of water flow (ie current) and
> it works very well.
> Current has the same meaning for both electrons or molecules of water.
> A quantity of something moved at a particular rate.
> In fact you could say that a pump requires a certain amount of power or
> watts to pump water through a given circuit at a given rate even if the
> pump was turned by hand.

The analogy between water and electricity is OK conceptually, but it falls
apart quantitatively.  Water flow is linear (like electrical current) only
at very low flow rates.  Under most conditions water flow is nonlinear.

The water quantity most comparable to electrical power is:

P = gamma*Q*H

Where gamma is the specific weight of water (pounds force per cubic foot),
Q is the discharge (cubic feet per second) and H is head - a measure of
pressure - across which the water is flowing (feet).  In those units P
comes out in foot pounds per second, which can be converted to horsepower,
watts, or whatever you care to use.

Practical application of this formula isn't possible unless you also have
a handle on the efficiency of the device in question and/or the frictional
resistance of the pipes, fittings and so on that are being used.  Those
resistance terms are where the big nonlinear bite happens.  Unlike
electrical resistance, resistance to the flow of water is not reasonably
constant.

Roger Miller

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