Pressure February 27, 2009

Last time… We considered molecules bumping into walls and the change in momentum meant an impulse (and a force) on the wall. We defined PRESSURE P as the force per unit area or:

Building Models Our model for the structure of matter must allow us to make predictions that can be tested by experiment. ▫If the predictions are borne out, they strengthen our belief in the model. ▫If they are not borne out, we must modify our model or abandon it and invent a new one. ▫FOR A SOLID, WE ALREADY HAVE SEEN THE VERIFICATION OF THE EXISTANCE OF ATOMS. ▫There is similar evidence of atoms in the gaseous phase!

Consider a box of air Air Box MAGNIFIED 4

Magnify Some More ● ● MOLECULES ● 5

Are Molecules Something Like Tennis Balls??? F HOW BIG IS THE FORCE ON THE WALL?? F 6

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The Force on the Wall INCREASES when …. The number of molecules in the box that can strike the wall increases. The AREA of the wall increases. The speed of the molecules increases. ▫This happens when the temperature of the gas increases. The “heavier” the molecules 8

The math Force = (constant) x (Area) x (number of molecules) x (speed) F=k n A T Define PRESSURE = Force per unit area = P P~nT 9

Units: Newtons/square meter ▫Pounds per square foot. ▫Bars.. Millibars … (later) 10

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An Example The pressure on a 5 square foot area wall is 10 pounds per square inch (psi). What is the TOTAL FORCE on the wall? P= F/A so F=PA F=(10 lbs/in 2 )(144in 2 /ft 2 )(5 ft 2 )= 7200 lbs 13

Air Pressure Normal Atmospheric pressure ▫14.7 lbs/in 2 ▫Also called a BAR  A 2005 storm had a pressure at its center listed as bars or 920 millibars.  Milli = one one thousandth ▫Question – What is one atmosphere in pounds per square FOOT? ▫1 sq. ft. = 12 in x 12 in = 144 in 2 14

To Ponder Imagine a pane of glass in your house that is one foot square. The force on it is 2016 pounds. 15

Balance! 16

Molecules Move Around and collide with each other! WALL 17

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Note- Aside from momentum, molecules (and other objects) have another important property. This is called ENERGY. This topic will be covered next week. Please read the chapter. The Monday after the break we will work on problems. The Examination is on Tuesday. Similar format.