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I fill coolers in my house with a pipe connected to a tap, as soon as it is full i place a thumb on the open section of the pipe effectively shutting off the outflow of water. Keeping my thumb in place requires force against the pressure exerted by the water, But i have noticed when i go to the tap and turn it off the pressure inside the pipe seems to increase and it starts flowing against the pressure of my thumb which was sufficient enough before i turned off the tap, i cannot seem to find any good reason for this. One thought might be that the water is more compressed after closing the tap but i cant visualise that happening.

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I think the structure of the tap is responsible.

Structure of a simple tap

When the tap is open the hindered water can flow backwards in more volume since the washer part of it is open but once it is closed the washer is closed so the volume decreases cause an increase in pressure since the compression of fluid increases so yes you are right.

Edit: since most of the tap contain rubber washer then when we close the tap the rubber washer seals the area of water incoming while sealing it sorts of push water on either of the sides which contributes more to pressure what do you think?

Please forgive me if I am wrong it is just an attempt to answer I am not as much of an expert as everyone else is here but I am just sharing what I understand.

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  • $\begingroup$ That seems reasonable at first glance but i am imagining the stopper to be like a thin iron barrier that divides the water inside into two parts, just like placing a iron sheet inside a beaker of water that doesnt seem to increase the pressure in the later case why is it increasing it in the first case $\endgroup$ – SOSXX Jun 24 '20 at 7:27
  • $\begingroup$ Please check the edit $\endgroup$ – Prateek Mourya Jun 26 '20 at 1:46

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