Download Pressure

PRESSURE
WHAT IS PRESURE?
Pressure – Force applied per unit area.
Formula: Pressure = Force (N)
Area (m2)
EXAMPLE 1
If a ballerina has a force pushing on the floor of 500
Newtons and she is standing on both feet that have
an area in contact with 325 cm2 of the floor, what is
the pressure she is exerting on the floor? If she then
goes on her toes with an area of 6.5 cm2, what is her
pressure on the floor?
EXAMPLE 1 ANSWER
• Remember, Pressure = Force (N)
Area (m2)
• 500 N
=
325 cm2
1.54 N/cm2
• 500 N
=
6.5 cm2
76.92 N/cm2
• Based on this example, we can conclude that
pressure ____ as area ______ (inverse relationship)
TWO KINDS OF PRESSURE
• a. Atmospheric pressure
(measured by a barometer - Torricelli)
• b. Pressure of a contained gas
(tire gauge, psi)
• Where have you heard the term psi?
CONVERTING PRESSURE AND UNITS
1. SI unit of pressure is the pascal (Pa). 1 Pa = 1 N/m2
2. The units used for pressure are:
Kilopascal (kPa), Atmospheres (atm), Millimeters Hg (mmHg)
3. At sea level, the average air pressure is 760 mm Hg when the
temp. is 0 C.
4. Conversion factors:
1 atm = 760 mm Hg = 101.3 kPa (from the reference chart)
• Convert:
a. 785 mm Hg =
_______ atm
b. 115.9 kPa =
_______ atm
c. 2 atm=
_______ mmHg
PRESSURE: KINETIC
MOLECULAR THEORY
WHAT IS THE KINETIC
MOLECULAR THEORY?
• The Kinetic Molecular Theory (KMT) – Explains
the properties of gases in terms of the
energy, size, and motion of their particles.
KINETIC MOLECULAR THEORY’S
POSTULATES (CONCEPTS)
1. Gas particles do not attract or repel each other.
(Free to move without interference from the other particles)
2. Gas particles are much smaller than the distance between
them. (This allows gas particles to be compressed)
3. Gas particles are in constant, random motion. (They move
in a straight line until they collide with something)
4. No kinetic energy is lost when gas particles collide with
each other or with the walls of the container. (Collisions are
completely elastic)
5. All gases have the same average K.E. at a given
temperature. (Temp. increase = Energy increase and vice
versa)
• Actual gases do __NOT___ obey all of
the assumptions made in the Kinetic
Theory, but it is the best approximation
we have for gas behavior.
PROPERTIES & USES OF
GASES
PROPERTIES (CHARACTERISTICS)
OF GASES
• Particles are far apart
• Easily compressed
• Particles are constantly moving
• Expand to fill their container
• No definite (indefinite) shape - Take the
shape of the container
Illustrate each property in your notebook
USES OF GASES
• Air bags- protect passengers in car
accidents.
• Astronomy studies gas to determine if a new
star has formed.
• Air tank - Scubadiving
• Balloons, Respiration, etc…