Is the pressure inside a body of water different from the atmospheric pressure nearby? Is it less pressure or more?
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1$\begingroup$ Right at the surface of water it is atmospheric pressure, but any lower it is higher. This because you have the weight of the atmosphere + the weight of the water pushing on you $\endgroup$– xXx_69_SWAG_69_xXxCommented Jan 27, 2021 at 16:21
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For a non-moving body of water the pressure at the surface is the atmospheric pressure, and "inside" the water the pressure increases with depth due to the weight of the overlying water. You can look up the relationship in any basic physics text.
For water that is moving, it is more complicated; see a thermodynamics text.
Reponse to your comment. For a capillary tube with one end in a pan of water, the level of water inside the small tube is higher than the level of water in the pan due to surface tension effects inside the small tube. Water rises up into the tube till the total upward force due to surface tension is equal to the downward weight of water inside the tube. At the water/air surface at the top of the tube the pressure is the atmospheric pressure and at the water/air surface in the pan the pressure is also the atmospheric pressure.
In the water in the tube, at the interface the pressure is lower than the atmospheric pressure. See the following discussion of Jurin's law on Wikipedia: https://en.wikipedia.org/wiki/Jurin%27s_law
Search the web for capillary physics for more details.
answered Jan 27, 2021 at 16:25
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$\begingroup$ Thanks, that clears it up. Just one question. At the surface of water at the top of a capillary tube, water pressure is lower than the level below (classic capillary water scenario), which is higher. Does that mean air pressure up there is also lower? Or is it the same at the top and outside the capillary tube? $\endgroup$– user137288Commented Jan 27, 2021 at 16:40
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$\begingroup$ What I mean is this, does air pressure vs water pressure tend to be the same at water-air interfaces? $\endgroup$– user137288Commented Jan 27, 2021 at 16:44
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$\begingroup$ Excellent!! At the top of the capillary tube, then, there is indeed difference between air and water pressure, correct? Which there isn’t for the water in the pan outside, which is the same pressure as atmospheric. Correct? Kindly confirm this description? $\endgroup$– user137288Commented Jan 27, 2021 at 17:42
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$\begingroup$ I think that makes sense as an approximation because the water outside the tube doesn’t have any significant curvature, and the Laplace formula for excess pressure considers meniscus curvature for excess pressure. What do you think? $\endgroup$– user137288Commented Jan 27, 2021 at 17:58