Are you feeling the PRESSURE?

Gravity exerts a pressure onto the ground

Pressure is inversely proportional to surface area

Pressure α 1 / surface area

Snowshoe Application

• Increase the surface area and thus decrease the pressure

• Result?• You can walk

on the snow without sinking!

What other areas of technology can you think of that compensates for

changes in pressure?

• Space travel• Flying on an airplane• Scuba diving• Climbing/living on mountains

Gas Pressure

• Like snowshoeing example, the pressure of a gas is also the force per unit area

• Force is created by moving molecules as they collide, mainly against the walls of the container

Units of Pressure• International units of pressure• SI unit is the pascal (Pa)

– 1 Pa = 1N/m2

– 1 kPa = 1000 Pa = 1000N/m2 (unit used for atmospheric pressure)

• At Sea Level – Average atmospheric pressure is 101.325 kPa

= 1 atm– (Standard ambient pressure 100 kPa)

Standard Laboratory ConditionsSTP : Standard Temperature and Pressure• Temperature = 0ºC• Pressure = 101.325 kPa (1atm)

SATP : Standard Ambient Temperature and Pressure• Temperature = 25ºC• Pressure = 100 kPa

Table 1:SI and Non-SI Units of Gas Pressure

Unit Name Unit Conversion

Pascal Pa 1Pa = 1N/m2

Atmosphere atm 1atm = 101.325 kPa

Millimeters of mercury (Hg)

mm Hg 760 mm Hg = 1 atm = 101.325 kPa

Torr torr 1 torr = 1 mm Hg

Conversion Practice!

Pressure

kPa atm mm Hg torr

(a) 1.25

(b) 98.8

(c) 235

(d) 2.60

(e) 635

0.0123 9.38 9.38

84.6 0.836 635

263 1980 1980

31.3 0.309 235

0.975 741 741

Learning Checkpoint

Assigned Questions:

p. 425 Practice UC # 1, 2, 3