Al''s water analogy..
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Hi Al -
Can't resist sticking my $.02 in: The analogy is - An analogy.
And a pretty good one for the general relationship between DC
voltage & current. If you want something which models a DC
power source a bit more closely, use a Mariotte syphon instead
of a plain tank - A constant pressure source, up to a point.
As for flow as a function of hole size, it's fairly regular over
the range, say, 1" -3". However, quantify the function, and then
calculate flow through a .010" orifice drilled in the same tank -
Your answer is wildly wrong. Other factors have come into play.
Similarly with the analogy, you can only push it so far. This
does'nt detract from its value as a pedagogical device.
regards, Jack
p.s. - http://www.uswcl.ars.ag.gov/events/exper/mariotte.htm
Al Williams wrote:
>
> Well, I am notoriously bad at "physical stuff" but what I was thinking of
> would be a water hose. 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.
>
> Granted, I'm known to be "bad" at pretty much anything mechanical so I might
> well be wrong, but it seems to me that the small hole will have more
> pressure because it has less flow. Energy will be conserved so less flow
> will turn into more pressure.
>
> As for draining the tank (the other reply) I'm assuming a big enough tank
> just as you don't plan for a battery draining to empty.
>
> I'll be the first to say that these analogies are not perfect. If you really
> want to get into the details of hydrodynamics, the flow is probably not
> laminar which leads to other effects that make the analogy quite bad. Also,
> the resistance of a pipe is not really linear over all possible flow rates.
>
> Oh well, I don't want to get to far into water pipes because I don't know
> much about it, but that's my take.
>
> Al Williams
> >
> > Hey Al, Its my turn to be a smart-***.
> >
> > >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.
> >
> > Sometimes water analogies can turn to rubbish, but like you said,
> > if it gets
> > some work done....
> >
> > Chris
Can't resist sticking my $.02 in: The analogy is - An analogy.
And a pretty good one for the general relationship between DC
voltage & current. If you want something which models a DC
power source a bit more closely, use a Mariotte syphon instead
of a plain tank - A constant pressure source, up to a point.
As for flow as a function of hole size, it's fairly regular over
the range, say, 1" -3". However, quantify the function, and then
calculate flow through a .010" orifice drilled in the same tank -
Your answer is wildly wrong. Other factors have come into play.
Similarly with the analogy, you can only push it so far. This
does'nt detract from its value as a pedagogical device.
regards, Jack
p.s. - http://www.uswcl.ars.ag.gov/events/exper/mariotte.htm
Al Williams wrote:
>
> Well, I am notoriously bad at "physical stuff" but what I was thinking of
> would be a water hose. 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.
>
> Granted, I'm known to be "bad" at pretty much anything mechanical so I might
> well be wrong, but it seems to me that the small hole will have more
> pressure because it has less flow. Energy will be conserved so less flow
> will turn into more pressure.
>
> As for draining the tank (the other reply) I'm assuming a big enough tank
> just as you don't plan for a battery draining to empty.
>
> I'll be the first to say that these analogies are not perfect. If you really
> want to get into the details of hydrodynamics, the flow is probably not
> laminar which leads to other effects that make the analogy quite bad. Also,
> the resistance of a pipe is not really linear over all possible flow rates.
>
> Oh well, I don't want to get to far into water pipes because I don't know
> much about it, but that's my take.
>
> Al Williams
> >
> > Hey Al, Its my turn to be a smart-***.
> >
> > >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.
> >
> > Sometimes water analogies can turn to rubbish, but like you said,
> > if it gets
> > some work done....
> >
> > Chris