Pressure = a force per unit area
 SI: Force = Newtons (N)
 SI: Area = m2
 SI Units of pressure = N/m2 = Pascals (Pa)
What happens to pressure as area increases?
It decreases.
Practice Problems:
A force of 9.0 N is applied to an area of 2.0 square meters; what is the pressure in Pascals?
4.5 N/m2 = 4.5 Pa
A book weighing 19 N is lying on a table face up. The surface area of the book is 522 cm2;
what pressure does the book exert on the surface of the table?
522 cm2/1000 = .0522 m2  364 Pa
We are often concerned with atmospheric pressure, the force per unit area that air
particles exert on their surroundings.
Normal atmospheric pressure is on average, at sea level:
 1 atmosphere (atm)
 101.3 kilopascals (kPa)
 760 torr
 760 millimeters of mercury (mm Hg)
These are called standard pressure. This is often referred to as STP, along with a
standard temperature of 0°C.
As altitude increases, atmospheric pressure decreases, and vice versa.
Sometimes it is necessary to convert from one unit to another. We can use the values for
standard atmospheric pressure as our conversion factors and do dimensional analysis.
Example #1 - Convert 0.875 atm to mmHg.
We know that 760mmHg = 1 atm  0.875atm x 760 mmHg = 665 mmHg
1
1 atm
Notice that the atm values - one in the numerator and one in the denominator - cancel,
leaving mmHg.
Example #2 - Convert 740.0 mmHg to kPa.
We know that 760mmHg = 101.3 kPa  740 mmHg x 101.3 kPa = 99 mmHg
1
760 mmHg
Try these on your own – convert 955 mmHg to each of the following:
a. atmospheres
1.26 atm
b. kilopascals
127 kPa
c. pascals
127000 Pa
127000 N/m2
d. Newton per square meter
e. torr
955 torr
Convert 62 kPa to each of the following:
a. atmospheres
0.61 atm
b. millimeters of mercury
c. pascals
62000 Pa
d. Newton per square meter
e. torr
465 mmHg
62000 N/m2
465 torr
Atmospheric pressure is measured with a barometer. A barometer uses the height of a
column of mercury to measure gas pressure in millimeters of mercury or torr (1 mmHg = 1
torr). The mercury is pushed up the tube from the dish until the pressure at the bottom of
the tube (due to the mass of the mercury) is balanced by the atmospheric pressure.
Gas pressure can be measured with a manometer.
A closed-end manometer is not open to the atmosphere, so the difference in the height of
the mercury in each tube equals the gas pressure.
The device used to measure blood pressure is called a sphygmomanometer. Sphygmos is
Greek for pulse.