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I. Electric Charge

I. Electric Charge. Dr. Bill Pezzaglia. Updated 2012Aug05. 2. I. Electric Charge. History of Electricity Coulomb’s Law. 3. A. History of Electricity. The Electric Effect Charging Methods Measuring Charge. 4. 1a. Thales of Miletos (624-454 BC). Famous theorems of similar triangles

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I. Electric Charge

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  1. I. Electric Charge Dr. Bill Pezzaglia Updated 2012Aug05

  2. 2 I. Electric Charge • History of Electricity • Coulomb’s Law

  3. 3 A. History of Electricity • The Electric Effect • Charging Methods • Measuring Charge

  4. 4 1a. Thales of Miletos (624-454 BC) • Famous theorems of similar triangles • Amber rubbed with fur attracts straw • “Amber” in greek:  “elektron” Here is a narrow tomb Great Thales lies; yet his renown for wisdom reached the skies

  5. 5 1.b. William Gilbert (1544-1603) • “Father of Science” (i.e. use experiments instead of citing ancient authority) • 1600 Book “De Magnete” • Originates term “electricity” • Distinguishes between electric and magnetic force • Influences Kepler & Galileo • Glass rubbed with Silk attracts objects • Invented “Versorium” (needle) used to measure electric force

  6. 6 1.c. Stephen Gray (1696-1736)[student of Newton!] • 1729 does experiment showing electric effects can travel over great distance through a thread or wire. • Classifies Materials as: • Conductors: which can remove charge from a body • Insulators: that do not.

  7. 7 + + + - 1.d. Charles Dufay (1689-1739) • 1733 Proposes “two fluid” theory of electricity • Vitreous (glass, fur) (+) • Resinous (amber, silk) (-) • Summarizes Electric Laws • Like fluids repel • opposite attract • All bodies except metals can be charged by friction • All bodies can be charged by “influence” (induction)

  8. 8 1.e. Benjamin Franklin (1706-1790) • 1752 Kite Experiment proves lightening is electric • Proposes single fluid but two state model of charge • + is an excess of charge • - is deficit in charge • Charge is conserved (objects are naturally neutral)

  9. 9 2. Methods of Charging Three basic methods • Triboelectric (friction) • Conduction • Induction (Influence)

  10. + 10 2.a.1 Triboelectrification chart Franklin and others contributed to determining the relative charge obtained by rubbing objects together. For example, amber on fur will give negative to amber, and plus to fur Dry human skin Asbestos Leather Rabbit's fur Glass Mica Human hair Nylon Wool Lead Cat's fur Silk Aluminum Paper (Small positive charge) Cotton (No charge) Steel (No charge) Wood (Small negative charge) Amber Sealing wax Rubber balloon Resins Hard rubber Nickel, Copper Sulfur Brass, Silver Gold, Platinum Synthetic rubber Polyester Styrene (Styrofoam) Saran wrap Polyethylene (like Scotch tape) Vinyl (PVC) Silicon Teflon 0 -

  11. 11 2.a.2 Otto von Guericke 1602 - 1686 • 1650 Invents Vacuum Pump (famous Magdeburg spheres that horses could not pull apart) • 1660 Invents static electricity generator, a large sulfur ball mounted on a pole inside a glass globe. The sulfur ball was rotated by a hand crank. The rotating ball rubbed against a pad generating static electricity sparks

  12. 12 2.a.3 Van Marum Machine (1784) The biggest tribo- electrostatic generator ever built, could produce voltage with any polarity.

  13. 13 2.a.4 Van der Graaf Generator (1929)

  14. 14 ++ ++ ++ ++ 2.b. Charge by Conduction If an uncharged conductor touches a charged one, the charge will be shared. When separated, they will both now have charge

  15. 15 2.c.1 Charge by Induction Aka chargeby “influence” (First done by 1759  -  Francis Ulrich Theodore Aepinus ?)

  16. 16 2.c.2 Charge by Induction Another way of doing it that is exploited by electrostatic generators

  17. 17 2.c.3 Electrophorus (1775) Invented by Alessandro Volta (1745-1827)(also invents the battery in 1800 ) Uses method of induction to create charge

  18. 18 2.c.4 Wimshurst Machine (1880) Invented by James Wimshurst (1832 – 1903) Two disks rotate in opposite directions, mutually inducing charge

  19. 19 3. Measurement of Charging Without really knowing what IS charge, how was it measured? (a) 1753 John Canton (1718-1772) Suggests deflection angle of Pith Balls is a measure of charge.

  20. 20 3b Henley’s Electrometer • 1770 First quantitative device. Deflection angle measures charge (its not however linear. Why?)

  21. 21 3c Electroscope • 1786 Gold Leaf Electroscope invented by Abraham Bennet (1750 - 1799) • 1887 Braun Electroscope is less sensitive, but more accurate

  22. 22 B. Coulomb’s Law • The Inverse Square Law • Coulomb’s Law • Units of Charge

  23. 23 B1a. Inverse Square Law

  24. 24 B1b. Inverse Square Law • Apparent Luminosity drops off inversely proportional to squared distance. • Sun at Jupiter (5x further away than earth) would appear 1/25 as bright. • Gravity works same • Electricity works same

  25. 25 B1c. Review: Gravity obeys inverse square law • 1666 probably derived first 3 laws • Law of Gravity probably done around the same time • 1687 He didn’t publish his work for some 20 years until Halley twisted his arm (Halley paid for it!) • Law of Gravity has inverse square law built into it. Newton 1643-1727 Force due to gravity = G is the “gravitational constant”, measured 100 years later by Cavendish: G=6.67x10-11 Nm2/kg2

  26. 26 B2: Coulomb’s Law • Charles-Augustin de Coulomb 1736-1806 • 1785 using a “torsion balance” measures the inverse square law between charges. “q” is measure of charge k= “coulomb constant”

  27. 27 This dial allows you to adjust and measure the torque in the fibre and thus the force restraining the charge This scale allows you to read the separation of the charges Coulomb's Torsion Balance

  28. 28 B3. Units of Charge • SI Unit of charge is “Coulomb” “C” • Coulomb Constant: k=8.988x109 Nm2/C2 • Permittivity of free space: o= 8.85x10-12 C2/Nm2

  29. 29 B3b. Fundamental Charge • Smallest charge in nature is: e=1.67x10-19 coulombs • This is the charge on the proton, and negative this is the charge on the electron. • The universe appears to be electrically neutral. We don’t know why its almost all matter, and hardly any antimatter.

  30. 30 References • Build simple electrophorus http://www.exploratorium.edu/snacks/charge_carry.html • Multimedia animations http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/index.html • In particular, static electricity animations at • http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/index.html#estatics • Make an electroscope http://www.mos.org/sln/toe/simpleelectroscope.html • More on electroscopes: http://www.sparkmuseum.com/ELECTROSCOPE.HTM

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