Pressure

• F=ma Earth 9.8m/s2 Moon 1.6 m/s2

• Weight: newtons, pounds

• SI unit of force is the Newton (N)

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Pressure

Pressure is a measure of force per unit area. Units for pressure include: atmospheres (atm), pascals (Pa), kilopascals (kPa), mm Mercury (mm Hg), and torr.

Pressure unit

A pressure unit that is equivalent to the pressure at sea-level.

1 atmosphere (atm) = 101.325 kPa = 101,325 Pa = 760 torr = 760 mm Hg.

Kilopascal (kPa)

• The kilopascal is a commonly used unit of pressure that is equal to 1000 Pa.

• The kilopascal is more commonly used than the pascal because the pascal is such a small unit of pressure.

• 101.325 kPa = 101,325 Pa = 1 atmosphere (atm) = 760 torr = 760 mm Hg.

The molecules in a gas are in constant motion. The purple balls represent gaseous atoms that collide with each other and the walls of the container. Gas molecules are in constant motion. "Pressure" is a measure of the collisions of the atoms with the container

Molecular motion in a gas

CollisionsPressure is proportional to the collision frequency of molecules against the walls of the container.                                       

                

Pressure determines the direction of mass flow                                                                                                                                                         

Dalton's Law of Partial Pressures:

The total pressure exerted by a mixture of gases equals the sum of the partial pressures of the individual gases.

                                  P (total) = P1 + P2 + P3 + ...

The water column is on the inside: PATM = PGAS + PWV + PWC

There is no water column: PATM = PGAS + PWV

The water column is on the  outside: PATM + PWC   = PGAS + PWV