Pressure & Vacuum

1. Pressure & Vacuum Historical Development Rhine Feb 2011

2. Aristotle Natural state of materials Hot or cold air (gravitas & levitas) Nature abhors a vacuum His ideas widely held for next 2000 years!! Archimedes Buoyancy & fluid displacement Eureka moment Historical Timeline – Ancient Greece

3. Historical Timeline - Renaissance

4. Necessity of the mother of invention (or scientific discovery) Galileo (1564 – 1642) • Miners used suction pumps to pump water out of mining shafts • Pumps could not work at depths > ~ 30 ft • Galileo discussed problem issue “Dialogue of The Two New Sciences” (1638) • He thought water column “broke” at certain height • Could not support weight? • Interstitial vacuum holding matter together?

5. Water collapses in pipe - Gasparto Berti (1600 – 1643) • 40 foot lead pipe capped at top • Put in trough of water • When capped end elevated, water “collapsed” to 32 ft column • What was left in top?? • One of Galileo’s “Roman triumverate”, Rafaello Magiotti witnessed event

6. Evangelista Torricelli(1608 – 1647) • Expanded on Berti’s experimentation • Used mercury in glass tube – column collapsed to ~ 760 mm • Like Berti, he claimed vacuum must be in void at top. Why?

7. Letter to Michelangelo Ricci in 1644Evangelista Torricelli (1608 – 1647) • Discussed principle of his experiment with mercury • “We live at the bottom of a sea of air” • A column of air 50 miles high has weigh (gravity) and exerts pressure on mercury in dish • Pressure on dish can only support column of equal weight/area (pressure)

8. Wind and VaccumEvangelista Torricelli (1608 – 1647) • Proposed that void at top of column is a vacuum • Torricelli is credited with creating first sustained vacuum and for invention of barometer • Torricelli noted that winds must be created by geographical differences in pressure: wind = fluid (air) moves from high to low • Earlier in career he also experimentally showed that speed of fluid flow from hole in side of container is proportional to the square root of the depth (pressure) – special case of Daniel Bernoulli’s fluid flow law published in 1738

9. Before elevators, had to use stairs!Blaise Pascal (1623 – 1662) • Convincing demonstration of mercury barometer • Moved barometer from ground level to top of tall tower – noted drop in mercury level • Moved barometer from ground level to top of nearby mountain – even larger drop • Provided more compelling arguments for vacuum above column

10. What happens when you pump all of the air out of a barrel? Otto von Guericke (1602-1686) • Really, he did this! • Used suction pump to “pull” air out - barrels imploded! • Invented first efficient vacuum pumps

11. How strong is a vacuum? Otto von Guericke (1602-1686) • Invented Magdeburg Spheres • Pumped out air in two hemispheres, “gluing” them together • Higher external air pressure vs. lower internal • Hitch both ends to teams of horses • Amazed people by separating sphere with simple twist of a pressure valve • Like Torricelli, von Guericke alsonoted that wind created by differ-ences in air pressure(obervations fromseparating spheres)