Pressure Definition and 999 Threads

Pressure (symbol: p or P) is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled gage pressure) is the pressure relative to the ambient pressure.
Various units are used to express pressure. Some of these derive from a unit of force divided by a unit of area; the SI unit of pressure, the pascal (Pa), for example, is one newton per square metre (N/m2); similarly, the pound-force per square inch (psi) is the traditional unit of pressure in the imperial and U.S. customary systems. Pressure may also be expressed in terms of standard atmospheric pressure; the atmosphere (atm) is equal to this pressure, and the torr is defined as 1⁄760 of this. Manometric units such as the centimetre of water, millimetre of mercury, and inch of mercury are used to express pressures in terms of the height of column of a particular fluid in a manometer.

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  1. EastWindBreaks

    Pressure of a piston cylinder device after heated

    Homework Statement I am confused on when will pressure of two states be the same for a piston cylinder device. Below are two problems where one's final pressure equals the initial pressure and one is not. Homework EquationsThe Attempt at a Solution Initially, I thought a piston-cylinder...
  2. Oscar Gonzalez

    How to calculate flow loss between 90 and 45 degree fittings

    Hi everyone. Let me give a little bit of background on the problem I need help understanding. I want to plumb an aquarium that has a sump and move the water from the sump to the main aquarium using a pump and 1" PVC pipe. The total system pipe length is going to be roughly 10 feet total with...
  3. R

    B Pressure Vessel Length Contraction and Pressure Comparison in a Spaceship

    I have 2 identical pressure vessels P1 and P2, exactly 1000 cc each. I pressurise them both to exactly ##1000 lbs/in ^2##. I place one on a spaceship and keep the other beside me. The ship blasts off. As the ship whizzes past me, as a result of length contraction, I see that the vessel on the...
  4. axer

    What happens to the pressure change inside a rising balloon?

    Hello, so i have a question regarding the pressure change inside a rising hydrogen balloon, I know that the pressure change of air outside is decreasing(as we go up = less pressure) And as the balloon is rising it's increasing its volume. but what happens do the pressure change inside the...
  5. S

    Split Hopkinson pressure bar data

    Currently coding a wave separation script for the split hopkinson pressure bar. Is there a place I can get raw data for the split hopkinson pressure bar to test my code?
  6. Rorshach

    Tympani in a concert hall: calculating Sound Pressure Level (SPL) and delay

    Homework Statement Hello. I have a problem, which is as follows: A seat in a concert hall is 84 ft from the tympani. The tympanist strikesa single note. The sound pressure level of the direct sound of the note at the seat is measured to be 55 dB. The first reflection from the nearest sidewall...
  7. F

    Pressure measurement using a Piezometer

    Hello everyone, I understand the hydrostatic fluid pressure in static fluids: static pressure varies linearly with depth and the infinitesimal fluid parcels located at the same depth in the fluid experience the same isotropic hydrostatic pressure. By Pascal principle, the atmospheric pressure...
  8. J

    Fluid Velocity and Pressure in a *Closed* System

    In a closed fluidic system (with a pump), fluid velocity is constant throughout the system but what puzzles me is the pressure drop due to viscous effect. In a horizontal pipe, pressure decreases gradually (assuming low friction) down the pipe due to viscous effect. In this concepts, it's hard...
  9. Nikhil Rajagopalan

    Is Pressure Constant in U-Tube Arms with Different Density Liquids?

    A liquid in a U-tube maintains the same level in both the arms. When another liquid which does not mix with the first one is poured in it, the air interface with either of the liquids in both the arms will be at different levels. Considering a point at the top surface of liquid A, the pressure...
  10. Franz Rojas Ayala

    Maximum pressure for a combustion chamber

    Greetings All! I am not sure if this is the correct place to post this, but I think that since rocket fuel is considered a chemical reaction, this sections seems appropriate. Right, to the topic. Me and my team from the spaceflight society are looking forward to building a hybrid rocket fuelled...
  11. W

    Partial Pressures and Vapour Pressure

    Homework Statement Two systems in diffusive equilibrium have equal chemical potentials. We can use this fact to solve the following problem. We begin with a closed system consisting of a liquid such as water in diffusive equilibrium with its vapour. At the start, only the liquid and its vapour...
  12. MacLaddy

    Pressure loss in an HVAC system

    I'm working on my first engineering project in the real-world and I'm hitting a bit of a snag. It's regarding pressure loss analysis in an HVAC system. We have a system that is capable of providing a maximum of 2.0-in.wg pressure at the air handler, but we are finding very little air movement at...
  13. T

    Central heating - available differential pressure

    I'm reading up on the use of the valve seen in the link below. On page page 5-6 there is the mentioning of available differential pressure. I struggle with understanding what that means exactly. Say I have a set of main supply/return pipes along a building, with some branches along the way...
  14. HastiM

    Change of radiation pressure of sunlight w.r.t. distance

    Hello, Let me first describe the situation I am thinking of: suppose we consider an object in the form of a square with some thickness (e.g. a mirror). Suppose further, that the gravitational force pulling the object towards the sun and the radiation pressure due to the sunlight are in balance...
  15. S

    I Exploring Fermi Pressure in Relativistic Conditions

    Hello! I am reading a derivation for Fermi pressure and the author assumes that the electrons in a box are cooled so much that they occupy all the states in the momentum space from p=0 up to a maximum value of p. Then after he obtains a formula for the pressure, he simplifies the formula...
  16. Raffaele

    Pressure Required for Fireman to Send Water 12m High | Physics Problem

    <Moderator's note: Moved from a technical forum and thus no template.> My daughter, who attends the italian equivalent of US K9, asked me some advice about a physics hydrostatic problem. > Which pressure (in atmospheres) a fireman must use to send water at 12 > meters height. I tried to use...
  17. EastWindBreaks

    Why must we use integral to find the resultant force?

    Homework Statement Hydrostatic force on a plane surface ex: Hydrostatic force on a gate: Homework Equations The Attempt at a Solution why can't we just use the formula in the red box above for problem 3.57? instead I have to use integral I am confused, how does this gate different from...
  18. A

    Determining entropy as function of pressure

    I read this discussion but I am interested in how the entropy is obtained as a function of pressure. Namely, how can you determine a following integral for an ideal gas: $$S(p) = -\int_{0}^{p} \frac{nR}{p}dp $$ when you need to start from 0 pressure?
  19. T

    Variation of (pore pressure) due to consolidation

    Homework Statement In the last picture , we can see that at time between 0 and infinity , the variation of pore pressure across depth is parabolic curve , why is it so ? Homework EquationsThe Attempt at a Solution I think it's incorrect . I think it should be linearly decrease with depth ...
  20. Z

    Pressure drag acting on a sphere

    Homework Statement I am trying to show that the pressure drag acting on a sphere is 2πμaU by integrating around the surface of the sphere where U is the speed of the fluid the sphere is in, a is its radius, and μ is the viscosity. Homework Equations The pressure at a given position r can be...
  21. S

    Energy density versus pressure?

    If i have Energy Density (U) -> U.Area= F but F.Area = pressure (p) but p must be U . I'm confused! In which cases we can say that energy density is pressure?
  22. T

    Pressure caused by beam of molecular oxygen

    Homework Statement A beam of molecular oxygen containing 1010 molecules/cm3 and average speed of 500 m/s strikes (elastic collision) a plate at an angle of 30º with the normal direction. Calculate the exerted pressure on the plate. Homework Equations P = Impluse x Flux The Attempt at a...
  23. Matt James

    Thermodynamics: Calculating Pressure Increase From Work

    Homework Statement Estimate the pressure increase required to impart 1 J of mechanical work in reversibly compressing 1 mol of silver at room temperature. What pressure rise would be required to impart 1 J of work to 1 mol of alumina at room temperature? For alumina take the molar volume to be...
  24. D

    Boyle's law pressure calculation

    Homework Statement An pillow is inflated with an pressure of let's say 20 hPa. Afterwards we place a weight of 10 Kg on the pillow, what is the pressure increase within the pillow? P athmosthere = 1013 hPa P cushion = 1013 + 20 = 1033 hPa m = 10 Kg g = 9.81 m/s^2 Amount of air in the pillow...
  25. D

    Calculating pump to cylinder pressure ratios

    This may not be the proper forum but i need to find a way to calculate how much effort it will take to move a cylinder using a manual helm pump. The hydraulic cylinder i intend on using is a 32mm bore with a 16mm diameter. Stroke of 178mm. Volume of 107 cc The helm pump (attaches to a steering...
  26. M

    Questions involving airflow and pressure around a car

    Homework Statement 1.Since the roadster seats only two people, it has only two side windows--one on each side. Each window is aligned squarely on the vehicle, so air can flow straight past the window as the vehicle heads forward. However, the air flowing around the roadster's windshield has to...
  27. Likith D

    Regarding the pressure at a point in a "rested" fluid

    Hey! I just came across this text ; from http://www.feynmanlectures.caltech.edu/II_40.html I tried to find "the law of hydrostatics" on the internet, to find no other mention of it. (would appreciate links to more details on it or any ideas regarding the topic) What is really puzzling is how...
  28. M

    How do I calculate pump size for low pressure Aeroponics?

    How can I calculate the water pump size I need for these sprayers? http://www.amazon.com/dp/B007D2RX1C/?tag=pfamazon01-20 Flow rates : 31.2 GPH. The current system I'm trying to figure out the pump for has a center rezzy and it needs to feed aprox 10, 10' pipes with diametre 1''. I have 108...
  29. EastWindBreaks

    Pressure reported from a water line, gauge or absolute?

    Homework Statement Homework EquationsThe Attempt at a Solution I thought the pressure in a water line is in gauge pressure, but the solution suggests that 1630 kPa is in absolute pressure, do we always assume absolute pressure unless otherwise specified?
  30. M

    Fluids Bernoulli and a pressure field

    I have a text that writes a pressure balance for a cylindrical shape of fluid, where the linearized Bernoulli gives the pressure field ##p = p_0+\rho\partial_t \phi : \vec{v} = -\nabla \phi## where ##\vec{v}## is the velocity vector. ##p_0## is the static pressure required to maintain the...
  31. A

    How Do Hydraulic Systems Apply Pascal's Law?

    Homework Statement The large cylinder in a hydraulic press has 3 times the surface area of the small cylinder. What force should be applied to the small cylinder to create a lifting force of 7200 Newtons In a hydraulic-brake system, a force of 25 N can be applied to a surface area of 5 cm^2...
  32. Jonathan Scott

    A Pressure and Newtonian potential energy

    Recent discussions on pressure as a source of gravity and the related Tolman paradox have reminded me that few people seem to be aware that even in Newtonian gravity the pressure is related to the potential energy, which I've mentioned a few times on these forums before. This is easy to show...
  33. I

    Deriving the vapor pressure latent heat dependence

    Homework Statement There is a picture attached showing the entire problem. Equation 2.78 is the Clausius Clapeyron equation. Homework Equations Clausius Clayperon equation. L = L'T (since there is a linear dependence on temperature) The Attempt at a Solution $$ \frac{de_s}{dT} =...
  34. R

    Dynamic pressure variances in sealed and pressurised systems

    Hi Last did physics about 50 years ago but have become interested in how pressuried fluids behave in sealed containers when they are rapidly agitted or otherwise brought into motion. Envisage a large circular container with a central power driven agitator. Two cylinders are attached . The whole...
  35. J

    Thermodynamics help in finding Temp, Volume and Pressure

    Hey Guys! I need some help for solving question 2 of this example as my HSC exam is in 5 days and I want to be able to solve this question if I receive it on the paper. Question: Graph: Answer for Q1 (Just the formula) : Work I have done towards this question: I have realized that from...
  36. J

    Pressure, Volume, and Temperature at a Point in a Thermodynamic Cycle

    Homework Statement Question: https://imgur.com/a/8rUw1 Graph: https://imgur.com/a/Z1oBO Homework Equations Answer for Q1 (Just the formula) : https://imgur.com/a/uesqN The Attempt at a Solution I have realized that from 1-2, there is an increase in pressure, decrease in volume and increase...
  37. S

    Can a Model Accurately Simulate Pressure Variations in Tillage Tools?

    Hi all, I have just measured the pressure on a point of a tillage tool and the resulting signals is reported in the following link. In literature, tillage tool are modeled with a constant pressure, but i reality the pressure changes over time and I would like to model the pressure...
  38. longz

    Pressure loss to double wall pipe (ventilation)

    Hi every body, I need help to calculate the pressure drop to double wall pipe with information as below, i understand need calculate pressure loss to the straight pipe, elbows and supports but I don't know exactly how. - Fluid : air - Double wall pipe (stainless steel) with outer pipe...
  39. W

    Pressure between a membrane and a solid body

    I would like to understand this a bit better, so I am looking for a reference to refer to and not necessarily an explanation in a reply. I have a non-porous membrane of a compressible material wrapped around a solid body. I then draw out the air between the membrane and the solid body. I want...
  40. Greg Bernhardt

    Negative pressure in house after winterizing

    I sealed my attic door and a set of 3 old windows downstairs with plastic. When everything else is closed up the two sets of plastic become extremely tight and you can see/feel the pressure pushing the plastic into the house. Almost to the point of ripping the tape off holding the plastic. Today...
  41. P

    B Why is there no proton degeneracy pressure?

    Hello. I usually heard about electron degeneracy pressure and neutron degeneracy pressure. But I´ve never heard about a proton degeneracy pressure. Why is this?
  42. MathematicalPhysicist

    Partition Function at a Fixed Pressure

    Homework Statement I don't quite follow the solution to this problem (problem 2.11 in Bergersen's and Plischke's textbook), here are the quoted problem and its solution: problem: solution: Homework EquationsThe Attempt at a Solution My problem is with the solution to (a), it seems they...
  43. K

    Why Does the Chain Rule in Thermodynamics Include a Negative Sign?

    I'm trying to understand why $$\left(\frac{\partial P}{\partial T}\right)_V = -\left(\frac{\partial P}{\partial V}\right)_T \left(\frac{\partial V}{\partial T}\right)_P$$ where does the minus sign come from?
  44. G

    Calculating Liquid Pressure: Water and Mercury

    < Mentor Note -- thread moved to HH from the technical physics forums, so no HH Template is shown > Notes: Water density = 1000 kg/m3 Mercury density = 13600 kg/m3 g = 10 N/kg Hi all, please help on this questions. The attempt from me a) (0.7m)(1000kg/m3)(g) = (h)(13600kg/m3)(g) h = 0.0514m h...
  45. J

    Internal energy of an ideal gas as a function of pressure?

    Assuming all gases in the combustion reaction of benzoic acid (C6H5COOH) behave ideally, what is the "exact" change in internal energy? The context in which this question is being asked is after a calorimetry experiment. For all the intents and purposes of calorimetry, the change in internal...
  46. A

    Troubleshooting Pressure Gauge Issues on Compressed Air Machines

    hi every body, I have a machine with compressed air. the problem is the pressure gauges is not working. and when I take the gauges into other compressed air line the pressure gauge work properly. why this happening ? I need your help
  47. M

    How to calculate pressure of a bulb in a system

    p1=Pa+(pgh) p2=p1+pgh Bulb A SG= 0.87 pressure = 200kn/m^2 Bulb B SG= 13.6
  48. F

    Atmospheric air pressure on the human body

    Hello, It is well known that the atmospheric air pressure is significantly strong but our body does not get crushed by it because and equal pressure pushes from the inside our body. That said, does it mean that air can exist and be diffused inside solid organs like the lungs, our heart, etc...
  49. Ron19932017

    I Pressure term in the energy stress tensor

    Hi all, I am reading Bernard Schutz's a first course in general relativity. In Chapter 4 it introduced the energy stress tensor in two ways: 1.) Dust grain 2.) Perfect fluid. The book defined the energy stress tensor for dust grain to be ## p⊗N ##, where ##p## is the 4 momentum for a single...
  50. J

    Calculating pressures from flow rate & pressure differential

    It seems there must be a way, but I cannot seem to wrap my head around it. Here's the scenario... I have a 24" line flowing with a wet gas (combustion flue gas) into a dryer. It comes in at 90 degrees F, at a rate of 180 SCF/M. It leaves the dryer at 73 degrees F at a rate of 124...
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