What is Pressure?
Pressure (P) is defined as the amount of force
(F) acting per unit area (A). The mathematical equation for
pressure can be written as:
P= F/A
= mg/A
where
P is pressure F is the normal force (g is acceleration) and
A is the area of the surface. Although the normal force is
a
vector quantity, pressure is a scalar
quantity (vector page).
The SI
unit for pressure is the pascal (Pa), equal to one newton
per square metre (N/m^{2} or 1kg/(ms^{2}).
Other
units of pressure, such as pounds per square inch and bar,
are also in common use. The CGS unit of pressure is the barye
(ba), equal to 1 dyn·cm^{2} or 0.1 Pa.
A good
example of how a force on small area can result in a very
high pressure is seen in women's shoes with high spiked heels.
Did you ever get stepped on by a woman wearing high heel shoes?
It would be less painful if she wore a flat shoe because the
soles are larger and the pressure is less.
As an
example an average shoe distributes the weight of the person
over about 20 square inches. Thus, a 150pound person applies
150/20 = 7.5 pounds per square inch on the floor. Since a
spikeheel is only 0.25 square inches, the 150pound person
would be applying 150/0.25 = 600 pounds per square inch on
the floor at the heel.
Fluid Pressure
All matter is made up of tiny particles called
atoms. The forces that exist in fluids are caused by the mass
and velocity of these atoms making up a fluid.
The pressure
exerted by a static fluid depends only upon the depth of the
fluid, the density of the fluid, and the acceleration of gravity.
(see hydrostatic pressure
page for more details)
Atomospheric Pressure
Atmospheric
pressure is the force per unit area exerted on a surface by
the weight of air above that surface in the atmosphere. In
most circumstances atmospheric pressure is closely approximated
by the hydrostatic pressure caused by the weight of air above
the measurement point. The standard atmosphere (symbol: atm)
is a unit of pressure equal to 101325 Pa. Equivalent to 760
mmHg (torr), 29.92 inHg, 14.696 psi. The atmospheric pressure
at the sea level is equal to 760 mm of height of the column
of mercury.
What is Vapor Pressure
Vapor
pressure (or equilibrium vapor pressure) is the pressure (at
a given temperature) that is exerted by a gas in equilibrium
with either a solid or liquid that is in a closed container.
The equilibrium vapor pressure is an indication of a liquid's
evaporation rate. Vapor pressures increase with temperature.
The vapor pressure is an indication of a liquid's evaporation
rate.
The units for vapor pressure :There
are several units used for vapor pressure: Pascals (Pa), tor
(mm Hg), atmospheres (atm) and bar (bar).
The torr (symbol: Torr) is a unit of pressure,
now defined as exactly 1⁄760 of a standard atmosphere.
Thus one torr is exactly 101325⁄760 pascals (~133.3
Pa). Historically, one torr was intended to be the same as
one "millimetre of mercury".
The torr (symbol: Torr) is a nonSI unit of
pressure with the ratio of 760 to 1 standard atmosphere, chosen
to be roughly equal to the fluid pressure exerted by a millimeter
of mercury, i.e. a pressure of 1 Torr is approximately equal
to 1 mmHg.
1 Pa = N/m2 1 bar = 10^5 Pa 1 atm =
1.01325 bar760 Torr = 1 atm 1 torr
What is Boiling?
Boiling
occurs when a liquid is heated to its boiling point. At this
temperature the vapor pressure of the liquid is equal
to the pressure exerted on the liquid by the surrounding environmental
pressure (e.g., air pressure).
