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PRESSURE

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**
Pressure** is the ratio of

force to the area over which that force is distributed. In other words, pressure is force per unit area applied in a direction perpendicular to the surface of an object

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**FORMULA
**

• Pressure is the effect of a force applied to a
surface. Pressure is the amount of force acting
per unit area. The symbol of pressure is *p
*

* P=F/A
*where:
• *p* is the pressure, *F* is the normal force, *A* is

the area of the surface area on contact

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**SCALAR QUANTITY **

Pressure is a scalar quantity. It relates the vector

surface element (a vector normal to the surface) with the normal force acting on it. The pressure is the scalar proportionality constant that relates the two normal vectors.

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UNITS The SI unit for pressure is the Pascal (Pa),

equal to one Newton per square meter (N/m2 or kg·m−1·s−2). This special name for the unit was added in 1971;before that, pressure in SI was expressed simply as N/m2.

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Examples

• As an example of varying pressures, a finger can be pressed against a wall without making any lasting impression; however, the same finger pushing a thumbtack can easily damage the wall. Although the force applied to the surface is the same, the thumbtack applies more pressure because the point concentrates that force into a smaller area.

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• Pressure is transmitted to solid boundaries or
across arbitrary sections of fluid *normal to*
these boundaries or sections at every point.
Unlike stress, pressure is defined as a scalar
quantity.

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TYPES OF PRESSURE

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Pressure in Fluids • Fluids include both

gases and liquids

• Pressure of a fluid is caused by the atoms/molecules of that fluid pressing against the container

• Equally exerted in all directions

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Pressure in Fluids

• As depth increases, pressure increases

• Pressure is affected ONLY by depth and type of fluid, not size or shape of container

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Explosion or deflagration pressures

• Explosion or deflagration pressures are the result of the ignition of explosive gases, mists, dust/air suspensions, in unconfined and confined spaces.

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Surface pressure

• There is a two-dimensional analog of pressure – the lateral force per unit length applied on a line perpendicular to the force.

• Surface pressure is denoted by π and shares many
similar properties with three-dimensional
pressure. Properties of surface chemicals can be
investigated by measuring pressure/area
isotherms, as the two-dimensional analog of
Boyle's law, *πA* = *k*, at constant temperature.

**π=f/l
**

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Pressure of an ideal gas

• In an ideal gas, molecules have no volume and do not interact. Pressure varies linearly with temperature, volume, and quantity according to the ideal gas law

**P=nRT/V
**

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where:

• *P* is the absolute pressure of the gas
• *n* is the amount of substance
• *T* is the absolute temperature
• *V* is the volume *R* is the ideal gas constant.
• Real gases exhibit a more complex

dependence on the variables of state.

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Vapor pressure • Vapor pressure is the pressure of a vapor in

thermodynamic equilibrium with its condensed phases in a closed system.

• All liquids and solids have a tendency to evaporate into a gaseous form, and all gases have a tendency to condense back to their liquid or solid form.

• The atmospheric pressure boiling point of a liquid (also known as the normal boiling point) is the temperature at which the vapor pressure equals the ambient atmospheric pressure.

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• With any incremental increase in that temperature, the vapor

pressure becomes sufficient to overcome atmospheric pressure and lift the liquid to form vapor bubbles inside the bulk of the substance.

• Bubble formation deeper in the liquid requires a higher

pressure, and therefore higher temperature, because the fluid pressure increases above the atmospheric pressure as the depth increases.

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Liquid pressure

• When a person swims under the water, water pressure is felt acting on the person's eardrums. The deeper that person swims, the greater the pressure. The pressure felt is due to the weight of the water above the person..

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• As someone swims deeper, there is more water above you and therefore greater pressure. The pressure a liquid exerts depends on its depth

• Liquid pressure also depends on the density of the liquid. If someone was submerged in a liquid more dense than water, the pressure would be correspondingly greater.

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**P=ρgh
**• where:
• *P* is liquid pressure
• *g* is gravity at the surface of overlaying

material
• ρ is density of liquid *h* is height of liquid

column or depth within a substance

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Kinematic pressure
** P=p/ρo
** is the kinematic pressure, where “**p”** is the

pressure and “**ρo”**constant mass density. The
SI unit of *P* is m2/s2. Kinematic pressure is
used in the same manner as kinematic
viscosity in order to compute Navier-Stokes
equation without explicitly showing the
density .

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**THE END
**

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