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/m2 or 1kg/(m-s2). 

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·cm2 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 150-pound person applies 150/20 = 7.5 pounds per square inch on the floor. Since a spike-heel is only 0.25 square inches, the 150-pound 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 non-SI 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 bar--760 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).

Test your Understanding:

1. The SI unit for pressure is the:
a) Newton
b) Pascal
c) Pounds
d) Torr

2. What is the pressure exerted by a 2kg block sitting on the floor equal to, if the surface area in contact with the floor is 1.5 square meters ?
a) P= (2kg) x (9.8 m/sec2)/ (2.25 sq. m)
b) P= (2 kg)(2,25 sq. m)
c) P= (2 kg)/ 1.5 sq. m
d) P= (2 kg)x(9.8 m/sec2) /1.5 sq.m

3. When the vapor pressure of a liquid is equal to the pressure exerted on a liquid by the surround environment, the liquid will:
a) start to evaporate
b) start to boil
c) start to freeze
d) begin to increase in temperature

4. Which of the following does the pressure exerted by a static fluid depend upon?
a) depth of the fluid
b) density of the fluid
c) acceleration of gravity
d) all of the above

5. The standard atmosphere at sea level is
a) 3.0 N/m2

b) 760 mmHg
c) not measurable