If I have two 20ml test tubes connected at the base via a thinner flexible U-shape tube and I pour a set amount of water (40ml) into one test tube whilst both are at level height, the water equilibrates so that it is at the same level in each tube. If I then introduce a stopper to meet the surface of the water in one tube and raise the other open tube to a higher level of lets say 2cm, how can I determine the pressure exerted onto the stopper by the water as the system once again tries to reach equilibrium? I'm not sure what equations I need in order to work out everything that is going on in the system. I'm trying to mimic a certain pressure against the stopper but first need to understand what the current pressure is and what factors I am working with in order to change the height of the other tube to gain the pressure I am after. Below is a simple schematic of what I mean, any help to get me started would be great - thank you!

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  • $\begingroup$ Are you familiar with the equation $\Delta p=\rho g h$ $\endgroup$ Commented Jan 6, 2021 at 12:36
  • $\begingroup$ Thank you - this makes sense, I thought it would be more complicated. $\endgroup$ Commented Jan 6, 2021 at 13:30

1 Answer 1


Pressure tubes

The pressure at the top of the left hand tube is simply given by Pascal:

$$P=P_0+\rho g h$$

where $P_0$ is the atmospheric pressure.

  • $\begingroup$ That's great, thank you so much for the schematic. I see now that it is a simpler situation than I thought. $\endgroup$ Commented Jan 6, 2021 at 13:31

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