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 = 1.54 N/cm2

325 cm2

• 500 N = 76.92 N/cm2

6.5 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…