All About Pressure

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All About Pressure
Put in your foldable! ?
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What makes you feel pressured?
FRIENDS ?
PARENTS ?
SCHOOL ?
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How do you handle pressure?

• How does pressure feel?
• When is it greatest?
• What can you do about it?

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Pressure Defined

• Force over an area
• Force measured in pounds or Newtons
• Area measured in square cm, in2, mile2
• Pounds per square inch (psi) or Newtons/m2
  pascals (Pa)

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Barometer

• Measures atmospheric pressure

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Demo upside down test tube

• The atmosphere can support water in an upside
  down test tube 33.9 ft tall!
• That would take a huge instrument just to measure
  pressure!

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• Mercury is heavier than water.
• The atmosphere can only support 760 mm Mercury
  (30 inches)

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Weather

• High pressure days are usually sunshiny

• Sudden drop in pressure accompanies rain and
  stormy days.

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Units of Pressure

• Atmospheres (atm)
• Torr
• mm Hg (Barometer)
• 1 atm 760 torr 760 mm Hg
• (by definition, so these conversions are exact,
  according to your textbook)
• 1 atm 101.325 kPa

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Consider this 5x5x10cm box.

• Box1 kg

5 cm
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What P does the box exert?

• Mass 1 kg
• Area 5 cm x 10 cm
• Pressure 1 kg/50 cm2

• Mass 1 kg
• Area 5 cm x 5 cm
• Pressure 1 kg/25 cm2

Which exerts the greater pressure?
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Compare Pressures of These

• High heels v. Tennis Shoes
• What about snow shoes?

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Compare Pressures of These

• Walking on thin ice
• vs.
• lying down on thin ice

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Compare Pressures of These

• Tank treads vs. car tires vs. snow chains

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Gas Pressure

• Pressure exerted by a gas is caused by the
  collisions of tiny gas particles with the walls
  of their container.
• Standard Pressure 1.00 atm

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Characteristics of a Gas(Kinetic Molecular
Theory)
Real vs. Ideal

• Tiny particles (point volumes only)
• Constant random motion, straight line paths
• Gas particles bump into one another transferring
  KE from one to another
• Perfectly elastic collisions

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Characteristics of a Gas(Kinetic Molecular
Theory)

• KE relates to temperature
• Gas particles have more internal energy than
  solids or liquids because the particles are not
  packed tightly together they have space to move

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Characteristics of a Gas(Kinetic Molecular
Theory)

• Gas particles neither attract nor repel each
  other
• Real gases generally obey gas laws except under
  high pressure or low temperature.

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Boyles Law and Maybe Charles Law lab
Monday-Tuesday

• Relating pressure and volume
• Relating temperature and volume
• Do both pre-labs.