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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:

• Our model for the structure of matter must allow us to Building Models 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!

4 Consider a box of air Air Box MAGNIFIED

5 Magnify Some More ● ● ● MOLECULES ●

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

7 http: //www. youtube. com/watch? v=ni. Wz. FSds 8 SY http: //www. youtube. com/watch? v=WIg. V 4 T 71 i. TU&feature= related

8 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

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

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

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13 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)(144 in 2/ft 2)(5 ft 2)= 7200 lbs

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 0. 920 bars or 920 millibars. Milli = one thousandth ▫ Question – What is one atmosphere in pounds per square FOOT? ▫ 1 sq. ft. = 12 in x 12 in = 144 in 2 14

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

16 Balance!

17 Molecules Move Around and collide with each other! WALL

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19 Map

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.