I. Electric Charge Dr. Bill Pezzaglia Updated 2012Aug05

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

A. History of Electricity 1)The Electric Effect 2)Charging Methods 3)Measuring Charge 3

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

1.b. William Gilbert ( ) “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 5

1.c. Stephen Gray ( ) [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. 6

1.d. Charles Dufay ( ) 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)

1.e. Benjamin Franklin ( ) 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) 8

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

2.a.1 Triboelectrification chart 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 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 0

2.a.2 Otto von Guericke 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 11

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

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

++ 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 14 ++

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

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

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

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

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

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

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

B. Coulomb’s Law 1)The Inverse Square Law 2)Coulomb’s Law 3)Units of Charge 22

B1a. Inverse Square Law 23

B1b. Inverse Square Law 24 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

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. 25 Force due to gravity = G is the “gravitational constant”, measured 100 years later by Cavendish: G=6.67x N  m 2 /kg 2 Newton

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

Coulomb's Torsion Balance 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 27

B3. Units of Charge SI Unit of charge is “Coulomb” “C” Coulomb Constant: k=8.988x10 9 N  m 2 /C 2 Permittivity of free space:  o = 8.85x C 2 /N  m 2 28

B3b. Fundamental Charge Smallest charge in nature is: e=1.67x 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. 29

References Build simple electrophorus Multimedia animations In particular, static electricity animations at Make an electroscope More on electroscopes: 30