Electric Charge. I. Electric Charge. Thales of Miletos ( BC) William Gilbert ( ) A. History of Electricity. B.

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1 Electric Charge I. Electric Charge 2 A. History of Electricity Dr. Bill Pezzaglia B. Coulomb s Law C. Capacitors Osher Lecture 1 Oct 15, 2007 A. History of Electricity 3 Thales of Miletos ( BC) 4 1) The Electric Effect Famous theorems of similar triangles Amber rubbed with fur attracts straw 2) Charging Methods 3) Measuring Charge Amber in greek: ηλεκτρον elektron Here is a narrow tomb Great Thales lies; yet his renown for wisdom reached the skies 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 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

2 Charles Dufay ( ) 7 Benjamin Franklin ( ) 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) 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) 2. Methods of Charging Three basic methods a) Triboelectric (friction) b) Conduction c) Induction (Influence) 9 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 10 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 Van Marum Machine (1784) The biggest triboelectrostatic generator ever built, could produce voltage with any polarity. 12 2

3 Van der Graaf Generator (1929) Schematic view of a classical Van De Graaf generator. 1) hollow metallic sphere (with positive charges) 2) electrode connected to the sphere, a brush ensures contact between the electrode and the belt 3) upper roller (for example in plexiglass) 4) side of the belt with positive charges 5) opposite side of the belt with negative charges 6) lower roller (metal) 7) lower electrode (ground) 8) spherical device with negative charges, used to discharge the main sphere 9) spark produced by the difference of potentials 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 c.1 Charge by Induction 15 2.c.2 Charge by Induction 16 Aka charge by influence (First done by Francis Ulrich Theodore Aepinus?) Another way of doing it that is exploited by electrostatic generators 2.c.3 Electrophorus (1775) 17 2.c.4 Wimshurst Machine (1880) 18 Invented by Alessandro Volta ( ) (also invents the battery in 1800 ) Invented by James Wimshurst ( ) Two disks rotate in opposite directions, mutually inducing charge Uses method of induction to create charge 3

4 3. Measurement of Charging 19 3b Henley s Electrometer 20 Without really knowing what IS charge, how was it measured? 1753 John Canton ( ) Suggests deflection angle of Pith Balls is a measure of charge First quantitative device. Deflection angle measures charge (its not however linear. Why?) 3c Electroscope 21 Storing Charge: Leyden Jar Gold Leaf Electroscope invented by Abraham Bennet ( ) 1887 Braun Electroscope is less sensitive, but more accurate The Leyden jar is a device for storing electric charge invented in 1745 by Pieter van Musschenbroek ( ). It was the first capacitor. Leyden jars were used to conduct many early experiments in electricity. Battery of Leyden Jars 23 B. Coulomb s Law 24 Daniel Gralath was the first to combine several jars in parallel into a "battery" to increase the total possible stored charge. He demonstrated its effects on a chain of 20 persons 1) The Inverse Square Law 2) Coulomb s Law 3) Units of Charge 4

5 B1. The Inverse Square Law 23 3a. Inverse Square Law 26 (a) Alkindus (al-kindi ), Based upon optics of Euclid, knew that light rays are scattered in a cone with the light source as apex, hence PROBABLY knew that the intensity of light drops off in proportion to the increase in the surface area (i.e. square of the distance) 3b. Inverse Square Law 27 Apparent Luminosity drops off inversely proportional to squared distance. Sun at Jupiter (5x further away than earth) would appear 1/25 as bright. Kepler knew this Gravity and Coulomb s law behave similarly, so is there a connection? B1b. Johannes Kepler ( ) Laws of Planetary Motion 1605 first two laws 1609 third law In his writings, it is clear that the inverse square law for intensity of light (e.g. from the sun, and planets) was well known at the time. He argues that planetary force does NOT follow the same law as light 28 Sir Isaac Newton ( ) Force due to gravity = 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. GM1 M F = 2 R G is the gravitational constant, measured 100 years later by Cavendish 2 29 B2. Coulomb s Law It is found that electric force obeys a law completely analogous to the law of gravity. Except: Gravity attracts, while like charges repel Plus & Minus charge, while only only Plus mass. 30 5

6 B2.a Joseph Priestley ( ) Friend of Franklin He shows there is no electric force inside a charged hollow conductor. He argues this is analogous to Newton showing there is no gravitational force inside a hollow mass shell By Analogy, argues electric force obeys inverse square law. 31 B2.b Henry Cavendish ( ) 1797 using a torsion balance measures the density of the earth (which leads to a value for the gravitational constant G ). Torsion Balance was invented by John Michell, but he died before the experiment could be done, and so the equipment was obtained by Cavendish little known fact that Cavendish determined that electric forces obey the inverse square law (cited by Maxwell), using charges on concentric spheres 32 B2.c Charles-Augustin de Coulomb ( ) 1785 using a torsion balance measures the inverse square law between charges. F = qq /r 2 1 dyne of force at 1 cm distance if charges are 1 statcoulomb (aka esu) 33 Coulomb's Torsion Balance This scale allows you to read the separation of the charges This dial allows you to adjust and measure the torque in the fibre and thus the force restraining the charge 34 B3. Units of Charge 34 B3a. Electrostatic Units Gauss shows all mechanical units can be written in terms of base units of mass, length and time Wilhelm Weber shows that all electromagnetic units can be defined by including one more base unit (for charge or current) esu or electrostatic unit of statcoulomb is unit of charge such that two 1 esu separated by 1 cm exert force of 1 dyne (cgs system of units!). Coulomb Law is simple: F = qq /r 2 Problem: can t relate it easily to magnetic units 6

7 B3b. The Coulomb Unit 36 B3c. Permittivity of Free Space Joule, Kelvin & Maxwell define unit of resistance, from which other electrical units can be defined (a column of 106 cm of mercury with 1mm x 1mm cross section at 0 C has resistance of 1 ohm). This was a cgs system units of Coulomb (and Amp) defined in mks system. Coulomb is amount of charge deposited by 1 amp in 1 second Amp is amount of current that delivers 1 Watt of energy passing through 1 ohm of resistance. In SI (mks) units, Coulombs law is, F = k q 1 q 2 /r 2 k=8.988x10 9 Or: force between 1 coulomb charges 1 meter apart is about 9 billion newtons. Constant k is analogous to the Cavendish constant G in Newton s gravity law. For reasons that make sense later we write k=1/4πε o Permittivity of free space: ε o = 8.85x10-12 B3d. Fundamental Charge 38 Geissler Tubes 18 Charge is quantized Smallest charge in nature is: e=1.67x10-19 coulombs 1838 Faraday notes strange arc of light in rarefied gas tube (dark region near cathode) 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. 1857(1864) Heinrich Geissler, shows different gasses fluoresce with different colors. Cathode Rays 19 William Crookes ( ) Plucker with better vacuum notes fluorescent green glow on glass wall, that can be deflected by magnetic field. Suggests they originate from the cathode Johann Wilhelm Hittorf (student of Plucker) observes that if there is an obstacle, a shadow is cast, proving the rays come from cathode and travel in straight lines Crooke did further work (i.e. most tubes are called Crooke Tubes ). In particular he showed that at low vacuum the tubes arc along curves, but at high vacuum the rays travel in straight lines. He postulates they are small charged particles. 7

8 Canal Rays 21 Heinrich Rudolf Hertz Goldstein coins the term cathode rays 1892 was unable to deflect cathode rays by an electric field. Discovers canal rays that go in the other direction (i.e. from anode). Proposes that cathode rays are a wave (pulses in the aether) rather than charged particle J.J. Thomson ( ) 1897 shows cathode rays can be deflected by electric field. 23 Electric Field Measures charge to mass ratio of cathode ray particle (i.e. the electron!) 1906 Nobel Prize Dr. Bill Pezzaglia "Could anything at first sight seem more impractical than a body which is so small that its mass is an insignificant fraction of the mass of an atom of hydrogen? --which itself is so small that a crowd of these atoms equal in number to the population of the whole world would be too small to have been detected by any means then known to science." Updated 23Jan2007 A. Lines of Force 1) Action at a Distance 2) Faraday s Lines of Force 3) Principle of Locality 3 1. Sir Isaac Newton ( ) Proposes gravity must act instantaneously, regardless of distance (else angular momentum not conserved). actio in distans (action at a distance), no mechanism proposed to transmit gravity "...that one body may act upon another at a distance through a vacuum without the mediation of anything else, by and through which their action and force may be conveyed from one to another, is to me so great an absurdity that, I believe no man, who has in philosophic matters a competent faculty of thinking, could ever fall into it." 4 8

9 Problems with Action at a Distance How does moon know that the earth is there pulling on it? 5 2a. Sir Humphry Davy How is gravity transmitted? Why does it follow inverse square law? Does it violate causality if instantaneous? 1807 Electrolysis, used to separate salts. Founds science of electrochemistry. Coulomb s law has same issues Newton himself writes: "...that one body may act upon another at a distance through a vacuum without the mediation of anything else, by and through which their action and force may be conveyed from one to another, is to me so great an absurdity that, I believe no man, who has in philosophic matters a competent faculty of thinking, could ever fall into it." 1675 Newton proposes an ether to transmit forces between bodies His greatest discovery was Michael Faraday takes Faraday with him on grand tour visiting Ampere and Volta. 2b. Michael Faraday c. Electric Lines of Force First proposes ideas of Lines of Force Example: iron filings over a magnetic show field lines Electric charges create electric field lines Field lines start on + charges, end on A plus charge will tend to move along these lines 2d. Other Properties 9 3. Principle of Locality 10 Field Lines can t cross (else physics would not be deterministic, ambiguity which way to go) Density of lines is proportional to the strength of the force I cannot conceive curved lines of force without the conditions of a physical existence in that intermediate space. (Michael Faraday) Argues that the field lines have independent reality Force fields exist as distortions in the aether of space Alternative to action at a distance, particles Locally interact with force lines Ideas rejected by others. He can t put them into mathematical form. 9

10 1a. James Maxwell ( ) 12 1b. Definition of Field essay On Faraday's Lines of Force, suggests lines are like an imaginary incompressible fluid (obeying hydrodynamic equations) 1861 paper On Physical Lines of Force, proposes real physical model of vortices for magnetic field Formal definition: force per unit test charge E = F/q Units of Newton/Coul (or Volts/meter) Better Definition: F E = lim0 q q (i.e. don t want test charge to affect field) 1c. Analogy to Gravity 14 References 47 Formal definition: force per unit test mass g = F/m i.e. its an acceleration of gravity field F = mg F E = lim0 q q (i.e. mass is the charge of gravity) Build simple electrophorus Multimedia animations In particular, static electricity animations at Make an electroscope More on electroscopes: Leyden Jars: References 47 Things to Do Get better Vad der Graff accelerator diagram/explanation More graphics on inverse square law Find tesla museum stuff RENUMBER SLIDES! 10