I can understand the pressure increase below,but why did pressure decrease above as more bricks were added?
Image source: https://www.youtube.com/watch?v=4REdJIW6-Kg
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2 Answers
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Because as the piston was moved down by the mass of the bricks the volume above the piston was increased.
With the valve closed then the fluid expanded to still fill that increased volume and the pressure decreases.
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$\begingroup$ Isn't oil incompressible, though? $\endgroup$ Commented Apr 3, 2023 at 16:58
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$\begingroup$ oil is considered "nearly" incompressible in most applications. Small changes in volume can increase/decrease pressures dramatically in a closed system. $\endgroup$– user13416Commented Apr 3, 2023 at 22:03
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$\begingroup$ "Isn't oil incompressible, though?" It might be incompressible but it is pressurisable (if that's a word). Think of water pressure increasing with depth in the ocean. It's not compressed but it is under increasing pressure with depth. $\endgroup$ Commented Apr 4, 2023 at 7:21
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$\begingroup$ @OsvaldoSilva check out engineeringtoolbox.com/bulk-modulus-elasticity-d_585.html then think about what might happen to a fluid subjected to reduced pressure... gas coming out of solution? $\endgroup$ Commented Apr 4, 2023 at 7:34
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$\begingroup$ The commentator in the video mentions that hydraulic oil is slightly compressible below the piston and slightly "Stretchable" (his term) above the piston. $\endgroup$– RichCommented Aug 18 at 4:48
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Picture a cylinder with a solid block at the bottom, a steel piston in the middle and a solid block on top. These three items completely fill the piston. Now use a strong magnet to exert a downwards force on the piston. The block below will be slightly compressed. This can be an exceedingly small amount; all materials are slightly compressible including fluids, (unless you are considering an idealized fluid). The block above the piston will also move down the tiniest amount, it doesn't matter how small, this will leave a space - a vacuum. So there will be vacuum pressure there, above the cylinder.
This is true for the hydraulic fluid in the cylinder. The piston need only move the smallest amount to cause a partial vacuum above it.
