Talk:Pascal's law

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I dont think that this (hydrostatic pressure is due to height of water column) is not what's commonly known as Pascal's Law. Pascal's Law (as I understand) is that force applied to an enclosed fluid is spread as an equal pressure throughout the fluid. ie. the concept behind pistons, syringes, car tyres,... Dougalc 23:57, 21 Oct 2004 (UTC)

• I agree. I googled Pascal's Law and all mentioned the principle you explained above. In fact, Pascal's law and Pascal's principle are supposed to be the same. Bubbachuck 16:03, 7 August 2005 (UTC)
  • The two principles may both be Pascal's Law; they are both foundational priciples for fluid statics and hydrostatics. The "hydrostatic paradox", discovered by Simon Stevin explains that the downward pressure of a liquid is independent of the shape of the vessel, and depends only on its height and base. Pascal discovered his principle by moving a barometer to different elevations in the atmosphere. Often, the hydrostatic paradox is said to be the precursor of Pascal's principle. So they may both be the same principle. The spreading of pressure may be the "higher" principle. Also some state that Pascal's law is Pressure = density * gravity * height. This equation would explain both principles. I am checking on it further. Steven McCrary 19:13, August 7, 2005 (UTC)

OK, here is what I found: Applied Fluid Mechanics, Third Edition, by Mott (New York: Macmillan, 1990) uses the term "Pascal's laws". Fluid Mechanics with Engineering Applications by Daugherty and Franzini (New York: McGraw-Hill, 1977) says, "all points in a connected body of constant density fluid at rest are under the same pressure if they are at the same depth below the liquid surface." So they are the same principle, but could be called principles, if necessary. Steven McCrary 14:55, August 8, 2005 (UTC)

I have added in the alternative formula, although i am not sure how to make the text in the same format as the original formula. --LeakeyJee 05:40, 30 May 2006 (UTC)

......

Clarification needed:

The wording of the intro is technically correct, but not very clear as worded: "...pressure exerted anywhere ... is transmitted equally in all directions throughout the fluid such that the pressure variations (initial differences) remain the same." This is saying that the variation with elevation, (the "initial differences") remains the same after some pressure changes.

It is much clearer to say that: "When there is a pressure change at one point ... there is an equal change at every other point."

Following the pattern of the original intro wording: "...a principle in fluid mechanics that states that a pressure change anywhere in a confined incompressible fluid is transmitted throughout the fluid such that the same pressure change occurs everywhere in the container. The "All direction" is redundant because of the word "throughout".

It's not spread as an _equal pressure_ throughout the volume because the pressure in a volume is not one value since it varies with depth. The *change* is what spreads equally. That is, if there is an increase at one point there will be an equal increase throughout. This allows for the variation in hydeostatic pressure with depth. This follows from the fundamental principle that at any point, pressure acts in all directions. This stems directly from Newton's Third Law.

E.G if pressure goes up 1 PSI at one point in a container, it will go up 1 PSI everywhere in the container. If a container of water is 14 PSI at the top and 15 PSI at the botom, then if the presure at the top goes to 15 PSI, the bottom will go to 16 PSI. The *increase* is the same throughout. The hydorstatic variation with eleveation must remain since it is a fundamental phenomenon.

NASA's GRC wording is much better: "Pascal's law states that when there is an increase in pressure at any point in a confined fluid, there is an equal increase at every other point in the container."These words are From: https://www.grc.nasa.gov/www/k-12/WindTunnel/Activities/Pascals_principle.html Regards, -- Steve -- (talk) 07:56, 13 July 2016 (UTC)

Pascal's law concept beats me

ok so volume of a liquid has to be conserved (given it is incompressible, non viscous blah blah) but why should the force per unit area be conserved and be the same in all directions? Basically if I just model everything as volume conservation and simply try and say that the liquid "en mass" will try to attain a level of minimal potential energy, I still end up with the same results. However to make the assumption that dF/dA is conserved seems off beat to me —The preceding unsigned comment was added by 220.227.207.194 (talk) 09:29, 5 March 2007 (UTC).

It is a static situation and nothing “ will try to attain a level of minimal potential energy”. Force is a vector describes interaction between two bodies (two parts of liquid). Pressure is a scalar describes properties of point on the surface. When you are considering p=dF/dS you must indicate surface S. This surface divide liquid on two parts. This parts are reacted. Problem, where is a starting point of vector force. I mean that centre of mass is a good point or point on the indicated surface. Gas or liquid is the set of material point in this model. 83.26.189.120 (talk) 08:21, 18 November 2008 (UTC)

I still do not agree, Pascal's law is extrapolated and can never be 1. either measured correctly 2. nor extrapolated mathematically. —Preceding unsigned comment added by 192.18.192.21 (talk) 09:26, 30 November 2009 (UTC) well here is a simple observation consider 2 sides of a U tube, one with area A1 and other A2 A1.d1 is the displacement of one side and A2.d2 is the displacement of the other A1.d1=A2d2 if fluid is incompressible hence work done F1.d1=F2.d2 hence d1/d2=F2/F1=A2/A1 Hence: F1/A1 = F2/A2 —Preceding unsigned comment added by Alokdube (talk • contribs) 11:06, 25 November 2009 (UTC)

However the above has no bearing to how the liquid has the same level or why the pressure should be same inside the liquid —Preceding unsigned comment added by Alokdube (talk • contribs) 11:08, 25 November 2009 (UTC)

Another example is as follows: Take a U tube with unequal width arms. The U tube is stable because of the base stand, if you suspend it on a pivot/via a string, while liquid make seek the same level, the tube will dip towards the heavier side. In case the tube is on the table, we easily assume pressure is equal, forces somehow balance out etc. — Preceding unsigned comment added by Alokdube (talk • contribs) 08:11, 28 March 2011 (UTC)

Major Style Revisions Needed

This article has worthwhile information in it, but it was extremely poorly written. I've given it a once over, but someone with more time and experience should deal with it. Of particular note, I removed two of the entries under applications:

• 'Pascal's burst barrel demonstration': a long and narrow vertical pipe is connected to the contents of a large, sealed barrel. Adding 2x to the pipe increases the pressure throughout the system. Adding a small amount of water to the pipe is enough to burst the barrel.
• Atmospheric pressure diminishes with height, a fact first verified on the Puy-de-Dôme and the Saint-Jacques Tower in Paris, on the instigation of Blaise Pascal himself.

The first is poorly written enough that I cannot tell what it's trying to say (and I am unfamiliar with the demonstration). It also appears to be anecdotal at best, and is hardly an "application". The second is misleading: air is not an incompressible fluid, and is therefore not strictly subject to Pascal's law. -Athaler (talk) 18:05, 19 February 2009 (UTC)

Terminology

In engineering, the term fluid encompasses both gases and liquids. Typically, gases are compressible fluids (increasing or decreasing pressure can change the volume occupied by a gas), while liquids are incompressible fluids (applying pressure does not change the volume occupied by the liquid). Though in lay language, a fluid is usually construed as a liquid, when a textbook or other source says Pascal's Law applies to fluids, it is covering both gases and liquids—not one or the other.Writeswift (talk) 21:07, 1 May 2014 (UTC)

Energy interpretation

A more correct interpretation, though, is that the pressure change is caused by the change of potential energy per unit volume of the liquid due to the existence of the gravitational field.

It's not at all obvious to a non-physicist how energy sneaks into this simple equation in such a way as to warrant depiction as "more correct". — MaxEnt 19:15, 2 May 2014 (UTC)

Please someone take notice of this difficulty: I personally do not think energy (potential energy) is even to be considered here - pressure is about force (per unit area) being "exerted" - which is not the "potential" of exertion - but rather naive "force" in itself.

It is highly sad no one is trying to either correct it or clarify as MaxEnt requested - his/her request is lying neglected since more than 3 years? MakingSenseOfSenses (talk) 17:52, 2 October 2017 (UTC)

=MaxEnt and MakingSenseOfSenses,

I know I'm coming in late here. I just happened to notice this from another change. Pressure is potential energy. It has the potential to do work if allowed to. A piston in a pressurized cylinder can, if allowed, push a block, or turn a crank and be converted to kinetic energy and/or friction-heat. Until it does some work, it still only has the potential to. I agree that sometimes the conservation of energy thing gets thrown out as an explanation, which may be true, but it is at too high a level for the particular part of the discussion. The basic concept is that any force is has the potential to do work. Hope this helps. -- Steve -- (talk) 14:54, 18 September 2018 (UTC)

Pascal's law in compressible fluids

Are we sure, that Pascal's law is correct only in incompressible fluids? I've found phrases in literature, that Pascal's law is consequense of law of conservation of energy and is correct for compressible liquids (gases). Dinamik (talk) 14:15, 21 May 2016 (UTC)

The problem I see is that for compressible liquids (such as gases), the density changes with pressure, and so the simple form presented on the page (:$\Delta P=\rho g(\Delta h)\,$) doesn't work. However, the derivative form should:$dP/dh=\rho g$. Klbrain (talk) 21:12, 31 August 2016 (UTC)

Well another problem is that Pacal law only works for liquid that is not moving. Otherwise we have additional inertia and viscosity terms. In compressible fluids pressure change causes movement Alex Bakharev (talk) 21:02, 9 July 2019 (UTC)

article needs attention

This article is a total mess. Under "Definition," it has two inequivalent statements of the law. The second one, stated as an equation, is valid only for fluids in static equilibrium, and is also stated in a mathematical form that makes it invalid except when the fluid is incompressible and the gravitational field is constant. There is a graph, referenced nowhere in the text, with this caption: "Pressure in water and air. Pascal's law applies only for fluids." The term "fluid" is usually defined in physics and engineering to include both liquids and gases, and in any case the graph doesn't demonstrate anything about Pascal's law. I'm adding the { { Expert-subject } } tag.--76.169.116.244 (talk) 17:48, 29 July 2016 (UTC)

Pascal's barrel

See the elements given in fr:Crève-tonneau de Pascal : the experiment never was performed by Pascal... Micheletb (talk) 04:32, 3 July 2020 (UTC)

"Explanation" Section

The explanation section does not provide an explanation, just an example. For instance, there's no explanation of why, when gravity acts only downward, that there should also be pressure on the sides of the container. A conceptual (not just mathematical) explanation is needed. DrMattB (talk) 02:36, 8 November 2021 (UTC)

Indeed. The sentence "The pressure that the left piston exerts against the water will be exactly equal to the pressure the water exerts against the right piston." just restates the law. For this reason, I'll rename the "Explanation" section to "Application" and merge it with the existing section. I'll also remove redundant entries from there. Kotlopou (talk) 19:35, 9 January 2022 (UTC)