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Orifice and Hose Pressure
Name: Brian
Status: Student
Grade: Other
Location: NC
Country: United States
Date: May 2007
Question:
We have a little bet here if you put your finger over the
end of a water hose is the pressure the same, less, or greater than
when your finger is not over the end of the hose?
Replies:
The problem with betting on science is that you have to define the bet very
carefully... :)
The answer depends on where you are measuring the pressure... just before
your thumb, the pressure is higher (closer to the upstream pressure). Just
after the thumb, the pressure becomes the same (roughly atmospheric). This
assumes the hose is horizontal (neglecting static pressure head).
Hope you won the bet!
Burr Zimmerman
Hi Brian,
It depends on where you are measuring the pressure, but the
general answer is the pressure will rise when you block the
end of the hose.
Let us look at a typical situation consisting of the water
pipes in your home, connected to a tap, which then feeds a
length of hose, that is free to discharge water from its open
end. A typical water pressure in the pipes in your home is
about 70 PSI (pounds per square inch). As the water flows
through the tap, it encounters a restriction, so the result
is that after the tap, the water pressure is much lower as it
enters the hose. The remaining pressure pushes the water
along to the overcome the resistance of the hose. As the
water gets closer to the end of the hose, its pressure gets
less and less, because there is less and less remaining hose
that restricts the water's flow. Finally at the outlet of the
hose, the water pressure falls to zero. The water may flow
out strongly, but this is caused by its momentum, not
pressure. It has been flowing fast inside the hose, and as it
emerges, it naturally continues to flow just as quickly away
from the hose, as it flowed inside the hose.
Now, if you block the end of the hose, all water flow stops.
The restrictions to water flow (such as the resistance of the
hose, and the restriction of the tap) do not matter any more,
because there is no water flowing any more. Restrictions to
water flow can only operate if water is flowing; but you have
your finger over the end, so no water can flow. The pressure
in all parts of the hose now rises to the same pressure as is
in the house's pipes. Without your finger blocking the hose,
the pressure at the end as the water flows out, is zero. When
you block the end, you can certainly feel the pressure is no
longer zero! So clearly, the pressure at the end of the hose
rises very substantially when you block it.
So, Brian, who wins the bet?
Regards,
Bob Wilson.
Dear Brian,
If the end of the hose is open, the pressure will be just the pressure of
the air -- atmospheric pressure. If you put
your finger over the end of the hose so no water can escape, the pressure
will increase to the pressure in the pipe
when the faucet is closed. If the finger pressure is reduced so some water
escapes, the pressure at the end of the
hose will be intermediate.
Best, Dick Plano
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