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Pressure is normal force per area, or work done per volume, or mechanical energy per volume (mechanical energy density).

Static pressure, [itex]P[/itex], in a fluid (a liquid or gas or plasma), is measured across a surface which moves with the flow. It is the same in all directions at any point (unless viscosity is significant at that point). It is usually simply called "pressure".

Dynamic pressure in a fluid is the macroscopic kinetic energy density, [itex]\frac{1}{2}\,\rho\,v^2[/itex].
Total pressure in a fluid is pressure (static pressure) plus dynamic pressure, [itex]P\ +\ \frac{1}{2}\,\rho\,v^2[/itex]. It is the pressure measured across a stationary surface.

At any point in a mixture of gases, the pressure is equal to the sum of the partial pressures of the individual gases.

The SI unit of pressure is the pascal (Pa), equal to one joule per cubic metre (J/m), or newton per square metre (N/m), or kilogram per metre per second squared (kg/m.s).

Force = pressure times area:
[tex]\boldsymbol{F}\,=\,\int_SP\,\hat{\boldsymbol{n}}\,dA\ \ \ \ \ \ (F = PA\ \ \text{for constant pressure on a flat surface})[/tex]
where [itex]\hat{\boldsymbol{n}}[/itex] is the unit vector normal (perpendicular) to the surface S

Pressure in a stationary liquid of density [itex]\rho[/itex] at depth [itex]d[/itex] below a surface exposed to atmospheric pressure [itex]P_a[/itex]:
[tex]P\ =\ P_a\,+\,\rho g d[/tex]
Bernoulli's equation along any streamline of a steady incompressible non-viscous flow:
[tex]P\ +\ \frac{1}{2}\,\rho\,v^2\ +\ \rho\,g\,h\ =\ constant[/tex]
Bernoulli's equation along any streamline of a steady non-viscous flow:
[tex]P\ +\ \frac{1}{2}\,\rho\,v^2\ +\ \rho\,g\,h\ +\ \rho\,\epsilon\ =\ constant[/tex]
[tex]\frac{1}{2}\,\rho\,v^2\ +\ \rho\,g\,h\ +\ \text{enthalpy per unit mass}\ =\ constant[/tex]

Daniel Bernoulli (1700-1782)

Recent forum threads on pressure
> Classical Mechanics
>> Newtonian Dynamics

See Also
Bernoulli's equation
pr. measurement @ wiki


Extended explanation
If a pipe narrows, the fluid must flow faster, because of conservation of mass.

Since the energy is greater, the (static) pressure must be less, ultimately because of conservation of energy.

Dynamic pressure and Bernoulli's equation:

In fluid flow, we use measurements per volume or per mass. Density [itex]\rho[/itex] is mass per volume; energy density is energy per volume; and so on. So any ordinary dynamic equation should be convertible into a fluid dynamic equation by dividing everything by volume .

In particular, since work done per displaced volume is pressure, and since in steady non-viscous flow, energy minus work done per displaced volume is constant along any streamline, the ordinary equation for conservation of energy in a gravitational field,
[tex]\frac{1}{2} mv^2 + mgh + U = W + \mathrm{constant}[/tex]
becomes Bernoulli's equation for steady non-viscous flow:
[tex]P + \frac{1}{2}\rho v^2 + \rho gh + \rho\epsilon = \mathrm{constant\ along\ any\ streamline}[/tex]
In this equation, all four terms have dimensions of pressure. The first term is ordinary pressure (sometimes called static pressure); the second is kinetic energy density, usually called dynamic pressure; the third is gravitational potential energy density; and the fourth is internal energy density.

Atmospheric pressure:

For calculations involving a fluid, such as water, which is much denser than air, atmospheric pressure can be ignored, since it appears on both sides of the equation and can be taken to be constant, even at different heights. This is because the difference in pressure at different heights is [itex]\Delta P = \rho_{\mathrm{fluid}}g\Delta h + \rho_{\mathrm{air}}g\Delta h[/itex], so if the density of air is negligible compared with the density of the fluid, the difference in atmospheric pressure can be taken to be zero. This applies, for example, when calculating forces on the wall of a container and when calculating the speed of water exiting a hole.

Absolute pressure and gauge pressure:

Absolute pressure is another name for pressure, sometimes used to distinguish it from gauge pressure.

Gauge pressure is pressure minus atmospheric pressure. For example, the devices usually used for measuring tyre pressure measure gauge pressure.

Force on a surface:

Force = pressure times area, so for example:

The net force Fnet on a flat vertical wall of a container of water — that is, the force resulting from water pressure inside minus atmospheric pressure outside — is the integral of the net force on each horizontal strip of width W and height dD at a depth of D below the surface:
[tex]F_\mathrm{net} = \int PW\,dD = \rho g \int WD\,dD[/tex]

Speed of water exiting a hole:

If a hole is made in the side or bottom of a container of water at depth [itex]D[/itex] below the stationary top surface of the water, then the exit speed [itex]v[/itex] may be calculated by applying Bernoulli's equation along a streamline from the top surface (where the pressure is atmospheric pressure) to a point just outside the hole (where the pressure is also atmospheric pressure):
[tex]\frac{1}{2}\rho v^2 - \rho gD = 0[/tex]


Thomas Johnson @ 02:39 AM Mar8-14
I always wondered isn`t dark matter similar h*c/λ = (6.626*10^-34)*(3.00*10^8)/(557*10^-9) = 3.57*10^-19 J The energy rate (power) is then (2.9*10^17)*(3.57*10^-19) W = 0.104 W (4 pi radius2)∞

So it stands to reason that dark matter and dark energy is really what equals the 96% that is background radiation the pressure of which is binding the universe.

@ 12:41 AM Mar3-11

apep @ 05:36 PM Feb7-11
DPT = Displacement of pressure theory

The idea that movement through the field of medium becomes the pressure dynamics that create form, i.e. vortexes.

@ 02:37 AM Jan25-11
Thanks , that was very useful :)

tyisha56 @ 09:18 PM Jan6-11
to me pressure is like an emotion really.

@ 02:43 PM Nov5-10
I have had a look at the article. Is quite interesting. Good job!

tiny-tim @ 05:57 PM Feb19-10
Clarified dynamic pressure and total pressure by rearranging, and adding formula. No change in meaning.

CFDFEAGURU @ 11:43 AM Jul8-09
The form of the Bernoulli equation above should also state that it is for an incompressible fluid. When the fluid cannot be considered incompressible the Bernoulli equation has to be integrated along the streamline.

tiny-tim @ 08:42 AM Mar1-09
Added absolute pressure and gauge pressure to ext expl.

tiny-tim @ 04:56 PM Jan25-09
Thankyou Redbelly
Added atmospheric pressure, force on a surface, and speed of water exiting a hole.

Redbelly98 @ 07:47 AM Jan24-09
Corrected equation
F = P A
(was F = ρ A)

wiggler115 @ 07:54 PM Jan23-09
one question what is the constant for little p(roe)
~EDIT(tiny-tim) ρ(rho) is the density: is that what you mean?

Redbelly98 @ 06:52 PM Dec23-08
Edit and saved Definition/Summary section (no changes) to get rid of LaTex white background.

tiny-tim @ 05:18 PM Nov20-08
1. Suggestion implemented thankyou, skr777
2. U and ε incorporate compressible flow, and there is a link to Bernoulli's equation
3. Elaboration leads to stagnation

skr777 @ 03:12 PM Nov20-08
A good start, but I feel a few clarifications are required.

1. In the introduction/summary, it should be specified that pressure is the *mechanical* energy density.
2. Most of the discussion is valid only for incompressible flow. I don't recommend sweeping changes, but perhaps a statement to the effect, and a link to a separate page on gas dynamics.
3. I feel a more elaborate explanation of the difference between static and total/stagnation pressure is in order.