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. M

    Why can't we use the hydrostatic pressure gradient to do work?

    Is there any way to harness the hydrostatic pressure gradient to generate energy? The pressure at the surface of an ocean is atmospheric pressure.As we descend down the ocean, the pressure increases .After a point, the pressure will be very high.Why can't we use this pressure difference to do work?
  2. QuarkDecay

    MHD Equilibrium: Prove P+B2/8π

    Homework Statement Prove that in an MHD equilibrium state in a pinch with flux and Bθ(r) and Bz(r), the equation for pressure is; d/dr(P + B2/8π) + Bθ2/4πr - ρ( u2θ/r)= 0 B is the total Magnetic field, uθ(r) the velocity of plasma, ρ= density, and solving in a (r,θ,z) system Homework...
  3. A

    Pressure and a Manual Valve's Effect on Pressure

    Think that there is 6 bar air (or water) feeding the system and there is a manuel valve before air comes to regulator. My question is that if I open the manuel valve half position do I change pressure (reduce it) or just reduce the flow ? For example, I have pneumatic valve whose working...
  4. B

    Model pressure loss vs time for air in a cylinder w/ hole

    Related to an engineering project I will be working on. The situation is that essentially I will have a closed cylinder (5.5 in x 26 in) with a small 15mm hole on the side (not on a circular face), which allows the air inside to be at equilibrium pressure/temperature with the outside. Then a...
  5. navneet9431

    Is P_atm=P_gas? Solving the Homework Problem

    Homework Statement See this question. Homework Equations Pressure=Height*Density*Gravity The Attempt at a Solution I think P_atm=P_gas. I think so because for the two liquids(oil and Hg) to be in equilibrium the net force on them should be zero. And,since they are not getting displaced...
  6. Behrouz

    What is pressure accourding to Bernoulli's theorem?

    Hello everyone, In Bernoulli's theorem, I understand Potential energy (because of height) and Kinetic energy (because of velocity), but I don't understand pressure [energy]; Is it something like the vibration of molecules and bumping them into each other (in simple words). Any help or simulation...
  7. G

    Using Ideal Gas Law to Calculate Vertical Pressure Gradient

    Homework Statement Consider a cylindrical parcel of air of area A and infinitesimal height dz. If this air parcel is to remain stationary, the difference between the total pressure forces exerted on its top and bottom faces must be equal to its weight. Use this information and the ideal gas...
  8. T

    Fluid static problem with wooden block being pulled down

    Homework Statement The figure below shows a metal object (2) hanging by a thin cord from a floating wooden block (1). The wood block has a SG of 0.3 and dimensions of 50mm x 50mm x 10mm. The metal part as a volume of 6600m^3. Find the mass of the metal part and the tension in the cord. Work in...
  9. Beth N

    Writing Force equation in terms of pressure and area

    Homework Statement Problem 16.39 [/B] The viscous force of a liquid in laminar flow through length L is given by ##F_v=4\pi\etaL v_m## where ##\eta## is liquid viscosity and ##v_m## is maximum velocity of the fluid. Find an expression for ##v_m## in terms of p1, p2 (pressure at each end) and...
  10. C

    Calculating Pressure from Air Displacement in Gasoline Tank

    I am struggling with what seemed to be a simple problem and any help would be greatly appreciated. I have to determine the amount of pressure created when air is displaced buy gasoline flowing into a tank. The gas enters the tank at 10gpm. The entrance has a diameter of 1.5" and the vent has a...
  11. M

    Automotive Flash Point of Petrol & Diesel: Pressure & Self Ignition

    Flash point temperature of petrol is lesser than that of diesel at the same pressure. Does this mean, if I take petrol and diesel as liquids in separate containers and heat them, rapid vapourisation (rapid enough to sustain a combustion) of petrol occurs at a lower temperature(fire point) than...
  12. Zahid Iftikhar

    Bernoulli Equation and gauge pressure

    Homework Statement What gauge pressure is required in the city mains for a stream from a fire hose connected to the city mains to reach a vertical building of height 15m? Homework Equations Bernoulli Equation The Attempt at a Solution I have tried this sum. My confusion is regarding choice of...
  13. H

    Got a bowl stuck upside down in a pressure cooker

    I got a small bowl stuck in a pressure cooker. can someone help... I have attached a picture idk if its clear or not but anywayhttps://www.physicsforums.com/attachments/229166
  14. A

    How to calculate change in pressure from LN2 phase change

    I am trying to calculate the increase in pressure caused by liquid nitrogen when it changes from liquid to vapor within a closed, constant volume at atmospheric pressure. How can this be done? Do I need to include the heat of vaporization?
  15. Jonathan Scott

    A Explore Potential Energy, Pressure & Newtonian Gravity

    I've now found a different way to look at potential energy and its location in the Newtonian approximation to relativistic gravity which may perhaps shed some light on how the corresponding terms work in GR. This is related to a previous thread of mine on "Pressure and Newtonian potential...
  16. M

    Which equation to use for hydrostatic pressure?

    Homework Statement h = 10 ft RHO = 10 ppg (or pounds/gallon) Homework Equations P = 0.052 * RHO * h P = RHO * G * h I am very confused right now as why each equation gives a different answer! What are the correct units for the 2nd equation and how do I convert from second equation to the...
  17. K

    What is the difference between the centre of pressure and the centroid?

    [Moderator's note: This thread was created by moving posts from another thread, hence no homework template] The name "centre of pressure (COP)" indicates representative pressure will act through this point for a vertical plane inside liquid. If so, then total thrust= vertical area of the plane...
  18. chemisthypnos

    Does Increased Pressure Affect Buoyancy?

    Imagine that there are two metal spheres both with the same volumes and I am trying to get them to float up into the air. I fill the first sphere with 5 ATM of helium and I fill the second sphere with 6 ATM of helium. Will either sphere have a higher buoyancy force acting on it than the other...
  19. TheExibo

    How can heat change be measured under constant pressure?

    So enthalpy is the heat content of a system at constant pressure. Enthalpy change is equal to the heat absorbed or evolved by the system at constant pressure. If my understanding is correct, a system whose temperature goes up will return back to that starting temperature if pressure is kept...
  20. A

    Pressure upstream and downstream of a filter

    Dears I have one question When a filter gets clogged, the upstream pressure will go up. But what happens to the downstream pressure ? does it go down or stays the same in case of positive displacement pump vs centrifugal pumpThanks
  21. M

    Pressure drop necessary to have desired gas flow rate

    Hello, I'm a chemist and I'm currently working on my PhD on materials science. I'm trying to determine the pressure difference necessary to achieve the desired gas flow inside a chamber. The scheme of the chamber is the following: I have a big chamber which is evacuated to 0.2 bar, and another...
  22. Roy S Ramirez

    Adjusting the pressure drop in an injection plate

    Hello everybody, I'm designing a gas pressure feed system for a hydrogen peroxide - ethanol rocket engine, and after analyzing the devised injection plate, I noticed it is very sensitive to the propellants' pressure drops and discharge coefficients. Once the plate is manufactured, the only...
  23. Jehannum

    Cause of water pressure in pipework

    Am I correct in saying that water pressure (in typical domestic heating system pipework) is achieved by deformation of the pipes rather than compression of the water? I believe water to be virtually incompressible, so the 3 bar (or whatever) pressure must come from the elastic stretching of the...
  24. Roy S Ramirez

    Injector pressure drop in a liquid rocket engine

    Hello everybody, I hope you are doing fine. I'm currently designing an injector for a hydrogen peroxide - ethanol engine, and the following formula is confusing me: Q = Cd * A * sqrt( 2 * dP / rho); where Cd is the discharge coefficient, A the total area, dP the pressure drop, and rho the...
  25. E

    How Does Converting a Cylinder to a Sphere Affect Pressure Gradient?

    Have cylinder made from semipermeable material .There is positive pressure inside cylinder and negative pressure outside cylinder .How gradient of pressure will be changed if we convert from cylinder t o sphere? Thank you
  26. SchroedingersLion

    Pressure in canonical ensemble

    Greetings, I am having a hard time in understanding intuitively how pressure does not automatically stay constant in a canonical ensemble (=NVT ensemble). Pressure in a closed system is the average force of particles hitting against the wall of said system. The obvious way to manipulate...
  27. S

    Pressure test containment wall thickness

    Hi If I know the velocity, mass and diameter of a missile from a failed pressure test how can I calculate the minimum thickness of a containment sheet steel wall say 1 meter away?
  28. Roy S Ramirez

    Relationship between volumetric flow rate and pressure

    Hello everybody, This is my first post. I've been working on a liquid rocket engine for a while, but I have never gotten to work on its feed system (fluid mechanics), until recently. We are using a very simple pressure fed system, and its main goal is to deliver the propellants at a specific...
  29. A

    Discharge of a Pressure Vessel Containing Liquid Nitrous Oxide

    Homework Statement I want to design a closed tank with an orifice hole (that can be opened and closed remotely) at the bottom of the tank and model how the mass flow rate of liquid nitrous oxide changes with respect to time, as the liquid nitrous oxide leaves the tank due to a pressure...
  30. Bennett Haselton

    Use internal pressure to compute work done by an expanding gas?

    I would argue that if you are computing the work done by an expanding gas in a frictionless piston, in an irreversible expansion where the inner pressure is greater at all times than the (constant) outer pressure, that you should integrate the internal pressure over volume change, not multiply...
  31. S

    Chemical equilibrium vs pressure

    Homework Statement Consider the reaction : A⇔2B (⇔ stands for reversible). 1 mol of A and 2 mol of B are taken in a closed container at equilibrium. Suppose the pressure is increased, how much of B (by moles) will decompose to restore equilibrium ? Homework EquationsThe Attempt at a Solution...
  32. S

    Chemistry Relation between mole, volume and pressure fraction

    Homework Statement If mole fraction, pressure fraction and volume fraction are denoted by Xmol , Xp, XV respectively, of a gaseous component, then what is the relation between them? Homework Equations mole fraction = mole of component / total moles pressure fraction = pressure of component /...
  33. Baris Kalfa

    Change of of the pressure of water inside a container

    I am having difficulty solving an exercise prepared by our professor for hydrodynamics. I am unsure if Laplace pressure is the correct way to calculate the pressure inside. It comes to my mind that I should've used the equation for hydrostatic pressure instead (##P=P_0+\rho gh##). However, the...
  34. S

    Motion of body in water under 7 bar pressure

    Is entering force will be same when a body enter in two box one filled with water and other filled with air. Both boxes have same pressure 7 bar and velocity of body at entering point is 9 m/s
  35. Islam Hassan

    Pressure on the Walls of a Rocket Motor Bell Nozzle

    Is the pressure due to the hot gas exhaust stream on the inside walls of a rocket motor bell nozzle positive or negative? Does the rocket exhaust exert a outward pressure on the nozzle walls due to the sheer volume of the exhaust gases or is the pressure a negative one due to the exhaust stream...
  36. Ed Lenarduzzi

    Compressed air, heat and pressure?

    Yes compressing air increase its temperature but does that necessarily increase pressure? In an pellet gun a spring driven piston compresses the air which increases the temperature but wouldn't any increasing pressure simply retard the piston's advance? Because the chamber volume is not fixed...
  37. SpiffyPhysics

    Confused by temperature's effects on air pressure

    Hi, I'm an instructor at an online school, and my text says “ Air pressure is the amount of pressure the atmosphere places on the surface of Earth. Air pressure is usually measured as the weight of the column of air above a square meter (N/m2). As altitude increases, air density decreases and...
  38. V

    Flow Rate of Liquid O2 Cylinder w/ 24 Bar Pressure

    Hi. Just looking for an average answer to a practical scenario. At work we use a liquid O2 cylinder which includes an outer jacket that houses a regulated gas pressure of 24 bar. I need to calculate the flow rate in l/min when the supply tap (15mm diameter) is open fully? This is essentially...
  39. dRic2

    Dalton's law of partial pressure

    Does anyone know a rigorous proof for Dalton's law ? I think I saw it once, but I can not find it again anywhere. Thanks Ric
  40. M

    Low Air Pressure Training (exercise)

    Looking to do a experiment on the effects of air resistance training on the body. doing basic exercises such as jogging push ups. Maybe a couple of curls ;).got facility design herewondering what your thoughts and opinions on this would be.
  41. berkeman

    Breath Control and Blood Pressure

    When I free dive (skin dive without a scuba air tank), I've noticed that by the time I get out of the water after about an hour (abalone diving or just recreational free diving), I feel like Jello and my blood pressure (BP) feels very low. I feel extremely relaxed and warm and happy. And after...
  42. dromeascr

    What is the pressure surrounding a liquid at its boiling point?

    Hello everyone, before I start I just want to mention that I am not an expert in physics whatsoever, so please be as specific as you can get if you wish to provide an answer. (The question itself might be considered stupid to be honest) I read the definition of the boiling point recently and...
  43. C

    Pressure on Piston: Conservation of Energy Explained

    Homework Statement Suppose there is a tank filled with water and a piston of area S exerts a force F on the water. Suppose I divide the water boundary touching the piston to - N small equal " square " molecules. Then , the force on the upper face of each molecule is F/N . Also, the area of...
  44. JTC

    What causes pressure in form drag?

    Consider a ball flying through the air. When there is turbulence, and the flow separates, say on a SMOOTH ball, then in the rear, there is circulation in the wide wake. There is pressure on the front, but no pressure on the rear due to the fact that the fluid is "busy" circulating around. So...
  45. C

    Microfluidics in pressures lower than one atmosphere

    Hi everyone I am trying to build an experiment with microfluidics. I have a reservoir containing a bellow. This bellow is a compensation mechanism and makes sure that the liquid (in this case water) can be pushed out of the reservoir. Before storing the fluid in the reservoir, the reservoir is...
  46. T

    Estimation and validation of water pressure at nozzle

    ρ I am trying to estimate and validate the pressure of water exiting a nozzle. For an unknown reason, the validation is consistently twice as high as the estimation. Here the approach: Estimation: I am using the dynamic pressure equation for the estimation: q = 1/2 * ρ * u2 where, q = dynamic...
  47. oobgular

    Pressure in the corners of a box

    I have a fairly simple physics question, which I probably should know, but somehow I have never encountered it before. Suppose you have a pressurized cube, filled to a high uniform pressure P. Will the force from the pressure be larger at the edges and corners of the box? At the corner...
  48. FranciscoSili

    Fermion Pressure at Room Temperature

    Homework Statement I have to find the mean Energy $<E>$ and pressure of a system of N fermions with spin 1/2. The energy per particle is \begin{equation} \varepsilon = \frac{p^2}{2m}. \endu{equation} Homework Equations The relevant equations are the degeneracy of the system: \begin{equation}...
  49. C

    Heat released during combustion at constant pressure vs volume

    Greetings! I've been brushing up on some thermodynamics recently and came across a perplexing sentence in my notes and text from undergrad. It says that for a combustion reaction, such as the combustion of heptane: C7H16 (l) + 11O2 (g) ---> 7CO2 (g) + 8H2O (l) That this process carried out at...
  50. Avimanyu Ray

    Pressure not matching at same height in a U-tube

    Can we please look at the following attached file image? It shows a utube filled with a little water and the rest is oil. Thus, the height of oil in one arm will be more than the height of water in the other. When we compare the pressure at the height of the surface of water with that...
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