What is Pressure: Definition and 1000 Discussions

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. G

    Air pressure holds coaster on glass: What if only half full?

    Hi. If I fill a glass with water up to the top and close it with a coaster, I can turn it upside down without the coaster falling off and spilling everything. Usually, this is explained with the atmospheric air pressure being bigger than the pressure of the water inside the glass (hence, it...
  2. E

    Statistical pressure for a canonical ensemble

    So the pressure for a canonical ensemble is: P = kbT dZ/dV P = pressure P = -∑pi dEi/dV Z = ∑e-βEi pi is the probability of being in microstate i Ei is the energy of state i β = 1/kbT <E> = U = average energy U = -1/Z dZ/dβ = -d(Ln(Z))/dβ How can the pressure (given above) be derived in...
  3. P

    The effect of exit pressure and temperature on a turbine

    I still struggle with the interchangeability of pressure and temperature and how they affect the performance of various bits of turbomachinery. Microscopically they're very interconnected, in that the measure of pressure is the result of the confinement of the measure of temperature, however...
  4. E

    What does it mean for there to be uniform pressure?

    Homework Statement A closed hemispherical shell of radius R is filled with fluid at uniform pressure p. The net force of the fluid on the curved portion of the shell is given by:Homework Equations The Attempt at a Solution [/B] I'm not even posting the answers because I'm not looking to get...
  5. R

    Calculating water pressure and the required mass of water

    Basically what I'm trying to do is calculate the amount of water it would take to generate a given pressure at the water tap. Or to put it more simply I'm trying to figure out where the water line is damaged, well more for fun then actually being practical. The water system is simply a well on...
  6. T

    Effect of Fluid Expansion on Pressure vessel/chamber

    Hi, I have a vessel/chamber filled partially (25%) with silicon fluid (coefficient of thermal expansion is 0.00096 cm^3/cm^3/ °C) and the rest is filled with nitrogen gas at a certain pressure (P1) and temperature (T1). my question is, how to predict the final pressure (P2) If I increased the...
  7. T

    Hydrostatic equlibrium and Centre of pressure

    Homework Statement A uniform star has a mass of M and radius of R,use the hydrostatic equilibrium equation to find its centre pressure. (note: not the minimum centre pressure) Homework Equations $$\frac{dp}{dr}=-\frac{Gm\left(r\right)\rho \left(r\right)}{r^2}[1]$$ The Attempt at a Solution...
  8. N

    Pressure Drop Across a Change in Diameter

    Greetings, I have very little experience with fluid dynamics and I was wondering how I can calculate a pressure drop across an abrupt change in diameter of the piping used for water. Any help is appreciated! Thanks!
  9. A

    How much time to pressure balance two air chambers

    A smooth flexible tube 1mm in diameter and 1 foot in length holds air at normal / atmospheric pressure (14.7psi). the tube is open at one end. the other end is connected to a tank by an open valve. the tube is peristaltically compressed so all the air in the tube is forced into the tank. the...
  10. S

    Pressure in a tank with continuous flow

    I feel like this is easy to answer but I'm coming up with answers I don't trust. Basically, I have a tank of air where I want to keep the pressure at a certain value. Let's say 1 psig for the sake of argument. I have a 1/2" ID hose connecting to inlet flow to the tank and an outlet hose of 3/4"...
  11. Asmaa Mohammad

    Total peripheral resistance and the diastolic pressure

    Hello, I came across this line in my textbook: "Total peripheral resistance (TPR) increases diastolic pressure more than systolic pressure." I don't understand how the peripheral resistance would increase the diastolic pressure more than the systolic pressure? Could some one explain that...
  12. J

    Pressure and Volume Relationship in Closed Bottles

    Homework Statement I have two closed bottles one has a smaller volume that the other. (initially atmospheric pressure in both and then closed with a cap) If I heat both bottles up, which one will increase in pressure more the larger bottle or smaller one. Homework Equations PV=nRT ? The...
  13. Aleoa

    Pressure formula and infinitesimal

    I'm studing classical physics and I'm stuck with the simple pressure formula defined as: P=\frac{dF_{\perp }}{dS} Now, i know some calculus and the concept of infinitesimal in physics; however what i don't understand is : 1) according with the fact that in Calculus dF_{\perp } represent an...
  14. BillTre

    Immune Cells Involved with High Blood Pressure

    High blood pressure can affect health in many ways. No one treatment can control it in all cases. Treating immune cells may help control it. Science mag news article here.
  15. A

    Using an IR Thermal Gun to check for steam passing valves

    Greetings I am asking about the reliability of using a thermal gun to check if there is passing downstream the valve. The fluid is either steam or air. Wouldn’t the pressure and temperature drop sharply downstream the valve if it is closed but passing ? again I am talking about air or steam...
  16. M

    Pressure carrying capacity of a hose

    Hi There, Why is a pipe with a smaller diameter able to withstand a greater internal pressure than a pipe made from the exact same material and wall thickness but larger diameter? Thank you
  17. dRic2

    Pressure gradient in wakes around a cylinder

    Hi, Let's consider a cylinder of infinite length and fluid flowing "over" (I'm not sure of which words I should use, sorry) it like in the figure: Let's consider x>>D in order to neglect what's happening near the rear surface of the cylinder. Let's get rid of static pressure which doesn't...
  18. M

    Understanding Pressure Drop in Pipes: Causes and Solutions

    Hi There, I have constantly come across the fact that as fluid (hydraulic oil say) travels along a pipe you will see a continual drop in pressure the further you travel down the pipe. Am I right in saying that the pressure at any point is simply the result of the resistances (friction with pipe...
  19. MermaidWonders

    MHB Integral Applications - Hydrostatic Pressure + Force

    Question #1 - A lobster tank in a restaurant is 1 m long by 0.75 m wide by 60 cm deep. Taking the density of water to be 1000 kg/m$^3$, find the water forces (a) on each of the larger sides of the tank; (b) on each of the smaller sides of the tank.
  20. N

    Fluid Mechanics: Choosing a fuel line diameter for a required pressure drop

    Homework Statement [/B] Homework Equations - The pressure drop equation. [/B]The Attempt at a Solution
  21. Dane P

    The effect of pressure gradient across dissimilar fluids

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  22. physea

    Understanding Pressure Losses and Calculation Methods

    Hello, The Darcy losses are pressure losses in the dynamic, static or total pressure? Also, the pressure losses in various fittings (eg. bends) are they dynamic, static or total pressure? Also, how can we calculate the static pressure? Is it the hydrostatic pressure of the amount of water...
  23. F

    Euler's equation pressure difference

    Homework Statement I am after PC - PA However I must do so without breaking into components. My problem has different values L=3 H=4 SG=1.2 downward a = 1.5g horizontal a = 0.9g and my coordinate is conventional positive y up and positive x to the right cos##\theta## = 3/5 sin##\theta## =...
  24. E

    Why is internal energy not a function of pressure?

    I'm reading a book on thermal physics and the author says this: "In general, the internal energy will be a function of temperature and volume, so that we can write U =U(T,V) " How do we know this intuitively and how do we know that internal energy is not a function of pressure as well?
  25. F

    Hydrostatic Pressure and its Effects: A Visual Explanation

    Homework Statement I drew a picture[/B] Homework Equations I drew a picture The Attempt at a Solution I drew a picture
  26. P

    Solving Pressure Equations: P=pgh & Bernoulli's

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  27. H

    Thick walled Pressure vessel -Principle stresses

    Homework Statement Hi, I trying to calculate principle stresses for pressure vessel (thick walled) which is pressurized from inside. I calculated all 3 stresses radial, hoop and axial and looking for what formulae to use to get to principle stresses. thanks Homework EquationsThe Attempt at a...
  28. B

    Volume, pressure and final temperature

    Homework Statement a) A mass of 0.12 kg of air has an initial temperature of 500°C and pressure 0.8 MPa. If the air is expanded according to the law pV1.2 = c to a final volume of 90 litres, determine i) its initial volume, ii) its final pressure, iii) its final temperature...
  29. M

    Liquid volume loss during pressure test

    Hi, I'm doing a pressure test with a liquid medium. I have a fixed volume and I know the pressure drop, and temperature change - how do I calculate volume of liquid lost in a leak?
  30. J

    Hydrostatic pressure in a tilted tube

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  31. T

    Residual pressure in a fire sprinkler system

    I'm aware that the term means the remaining pressure at a point while there is flow. Say the distribution pipes downstream the sprinkler valve have a calculated total head loss of 3 bars during the designed flow. Does that mean that the residual pressure at the sprinkler valve needs to be at...
  32. Richie Smash

    Find volume of pesticide after pressure decreases

    Homework Statement Here is a pesticide spray vessel, of total volume of 10 litres, containing 8 litres of pesticide. Valve B is closed first,and the space above the air is filled with air at a pressure of 200KPa until the desired pressure is reached, and then valve A is then closed. The...
  33. J

    How is a plate with a hole in it sealed

    Say I have a simple 2 or 3mm thick sheet of plexiglass or aluminum, and I have a closed chamber box out of these sheets. Now, I want to build a pressure inside the chamber, but the prototype only has a simple O shaped "hole" on the box. I can choose the diameter of the hole, and I have a rubber...
  34. Kevin

    Pressure Gradient for Nitrous Flow

    Hello everyone, I am currently working on an undergraduate club team for the Intercollegiate Rocket Engineering Competition. I am attempting to do a calculation to determine the pressure needed in a vessel leading to another pressurized combustion chamber to achieve a desired mass flow rate...
  35. B

    Hydrostatic Pressure: Resultant Thrust & Overturning Moment

    Homework Statement [/B] Hello, am i on the right track with this problem. I am not sure if i have done the it correct. The part i am unsure on is b). Not sure if i have calculated it correct. Part of a sea defence consists of a section of a concrete wall 4 metres high, and 6 metres wide. (See...
  36. E

    A Change in phonon dispersion with pressure

    Hi, I'm looking into how phonon dispersion changes with pressure analytically and need to know how the atomic spacing in copper changes with pressure in order to model the crystal. I can't find any helpful papers online :( Any help would be appreciated thanks e
  37. ZiHao

    Volume of Balloon Unchanged When Pressure Doubled & Temp Changes

    A balloon is filled with a gas to a certain volume at a certain pressure at 22.0 degrees Celsius. If the pressure exerted on the balloon is doubled. explain what must the temperature ( in degrees Celcius) be so that the volume of the balloon doesn't change. If the temperature changes, the...
  38. A

    Determine the pressure gradient for suction

    Hello, Im designing a product which will pump components as vacuum cleaner does. I have 2 questions: Im trying to calculate the pressure different in order to pump those metal component with : M=0.2 kg , dimensions (mm) cylinder with diameter 5.5 mm length 600mm . the formula for pressure...
  39. DaveC426913

    Optimum shape for a pressurized structure in a pressure gradient

    I came across this pie-in-the-sea concept: (Obviously, the pictured structure would be extremely susceptible to complete catastrophic failure, having no apparent internal means of water-tight seals to prevent complete implosion. Which is why you'd more logically build a city in tube-and pod...
  40. Suyash Singh

    Force due to a chain falling on a table

    Homework Statement a uniform fine chain of length l is suspended with lower end just touching a horizontal table. Find the pressure on the table, when a length x has reached the table.. Homework Equations Pressure = force/area The Attempt at a Solution let mass density, m= mass/l...
  41. F

    What causes pressure and is it isotropic in a moving fluid?

    I'm studying fluid dynamics and we just had a lecture about the momentum equation. We started the lecture by talking about pressure in terms of molecules moving across a hypothetical surface element and carrying their momentum with them (in both directions). There are 2 things confusing me about...
  42. M

    How to measure the force generated by a tyre when it fails?

    Hello everyone, I am trying to work out how much force is generated when a tyre fails at a certain pressure. The tyre will be inside an OSHA section 1910.177 compliant tyre cage. What calculation would be used to solve the following?: The tyre size is 35 x 11.5-17 with a rated inflation of...
  43. leo92ra

    Simulation with COMSOL - uniform pressure

    Hi, anyone using COMSOL? I'm new to this program and I'm trying to get a plot of the pressure distribution on a cut-plane of a cube. I have applied a boundary load of 4 MPa to the upper surface of the cube and fixed constraint to the bottom surface of the cube. Now, as a check, when plotting...
  44. C

    A Elastically anisotropic sphere under pressure

    Does a sphere made of an elastically anisotropic material (eg. a material of cubic symmetry) subject to an hydrostatic pressure retains its spherical shape ?
  45. Mamat31

    Reducing pressure in a chilled water system

    Hi all, I am new on this forum, hope you are doing good. I have a question related to a project I am currently working on. The system is a 40+ floors mix used building, with a cooling system composed of a couple of centrifugal chillers, and a primary/secondary pumping configuration. There is...
  46. Jese Reyes

    Calculate Sound Pressure Level

    Homework Statement Spaker located at 90 feet from a auditorium chair. The sound pressure level at the chair is 70 dB. The first reflection from the roof takes 200ms to get to the chair. What distance does the first reflection travel to get to the chair if the room temperature is 303.15 K? What...
  47. PHstud

    No drag thin airfoil theory / Pressure drag

    Hello everyone. I have a question about the fact that there is no drag in the thin airfoil theory. I have read that it comes from inviscid and incompressible flow (potential flow) but what i can not understand is why there would be no drag from pressure differences ? The skin drag is of course...
  48. ZiHao

    Pressure in Gases: Boiling Water in a Can Explained

    A teacher heats some water in a drinks can until it is boiling and steam comes out of the can. She quickly turns the can over and puts it into some water. Explain in detail why the can collapses. My solution will be: By boiling the water, the water changes state from liquid to gas, which is...
  49. I

    Bernoulli's Equation, Find gauge pressure at 2nd point

    Homework Statement At a certain point in a pipeline, the velocity is 1 m/s and the gauge pressure is 3 x 105 N/m2. Find the gauge pressure at a second point in the line 20 m lower than the first if the cross-section at the second point is one half that at the first. The liquid in the pipe is...
  50. jaechung

    Why Do Blood Pressure Levels Vary Throughout the Cardiovascular System?

    Hi, I am studying about blood pressure. And I have some questions. The blood pressure in the left ventricle is about 105mmHg, and that in the right atrium is about 0 to 2 mmHg, Why do pressures decrease through our vasculature? And what kind of pressures are the pressures in the blood? Is it...
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