Al''s water analogy..
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> >If I punch a hole the size of my fist in the tank, more
> >water will flow per second, but it won't be under so much pressure
>
>Initially the pressure will be the same for both holes, as they would
>presumably be at the same depth from the surface of the water, the pressure
>will reduce faster for the larger hole.
Unless, of course, the tank is set up as a magically simple mariotte
siphon, then the pressure at the bottom is constant (down to a
point)! It is a fluidic "voltage regulator".
http://www.uswcl.ars.ag.gov/exper/mariotte.htm
>Consider that your water pressure literally comes
>from a big tank of water now (that's why water towers are -- well -- towers
>unless you live somewhere where they build them on a mountain or other
>elevated area). When you use your garden hose, water comes out and that's
>purely from the pressure of your city's water tower. If you occlude part of
>the hose orifice, the pressure goes up dramatically.
Unless you are standing right under the tank, there is considerable
"source resistance" in the pipe that leads from the water tower to
your house. It is analogous to a voltage divider. If your hose is
set for a small orifice, low flow, it is backed up by the full
available pressure. There is not much pressure drop in the piping up
to your house. Large orifice, high flow, the pressure drops across
that source resistance.
To put it another way: The closed faucet may be backed up by 85psi,
and that is as high as it gets, effectively the pressure in the city
system. As you try to get more volume flow, the pressure at your
faucet drops. When the faucet is full open, the flow rate is equal
to that 85psi divided by the total resistance of the plumbing from
the city's tank up to your faucet. And when all your neighbors are
irrigating their cabbages, your pressure drops too, because it is all
one big network of resistances, flows and pressures.
---
At school one time we were thinking about what would be a good
fluidic analogy for an inductor. Imagine that water in a pipe has to
flow through a closed paddlewheel arrangement, and the paddlewheel
turns a heavy flywheel. Then when you put pressure across it, the
flow has to build up slowly as the flywheel gains momentum. And the
flywheel can keep the flow going. If you try to stop the flow
suddenly, you get a big kick in pressure as the flywheel tries to
conserve momentum.
>Sometimes water analogies can turn to rubbish, but like you said, if it gets
>some work done....
...it's fun, it's tangible....
-- Tracy
> >water will flow per second, but it won't be under so much pressure
>
>Initially the pressure will be the same for both holes, as they would
>presumably be at the same depth from the surface of the water, the pressure
>will reduce faster for the larger hole.
Unless, of course, the tank is set up as a magically simple mariotte
siphon, then the pressure at the bottom is constant (down to a
point)! It is a fluidic "voltage regulator".
http://www.uswcl.ars.ag.gov/exper/mariotte.htm
>Consider that your water pressure literally comes
>from a big tank of water now (that's why water towers are -- well -- towers
>unless you live somewhere where they build them on a mountain or other
>elevated area). When you use your garden hose, water comes out and that's
>purely from the pressure of your city's water tower. If you occlude part of
>the hose orifice, the pressure goes up dramatically.
Unless you are standing right under the tank, there is considerable
"source resistance" in the pipe that leads from the water tower to
your house. It is analogous to a voltage divider. If your hose is
set for a small orifice, low flow, it is backed up by the full
available pressure. There is not much pressure drop in the piping up
to your house. Large orifice, high flow, the pressure drops across
that source resistance.
To put it another way: The closed faucet may be backed up by 85psi,
and that is as high as it gets, effectively the pressure in the city
system. As you try to get more volume flow, the pressure at your
faucet drops. When the faucet is full open, the flow rate is equal
to that 85psi divided by the total resistance of the plumbing from
the city's tank up to your faucet. And when all your neighbors are
irrigating their cabbages, your pressure drops too, because it is all
one big network of resistances, flows and pressures.
---
At school one time we were thinking about what would be a good
fluidic analogy for an inductor. Imagine that water in a pipe has to
flow through a closed paddlewheel arrangement, and the paddlewheel
turns a heavy flywheel. Then when you put pressure across it, the
flow has to build up slowly as the flywheel gains momentum. And the
flywheel can keep the flow going. If you try to stop the flow
suddenly, you get a big kick in pressure as the flywheel tries to
conserve momentum.
>Sometimes water analogies can turn to rubbish, but like you said, if it gets
>some work done....
...it's fun, it's tangible....
-- Tracy