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I. Electric Charge

Dr. Bill Pezzaglia

Updated 2012Aug05

I. Electric Charge

A. History of Electricity

B. Coulomb’s Law

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A. History of Electricity

1) The Electric Effect

2) Charging Methods

3) Measuring Charge

3 1a. Thales of Miletos (624-454 BC)

• Famous theorems of similar triangles

• Amber rubbed with fur attracts straw

• “Amber” in greek: ηλεκτρονηλεκτρονηλεκτρονηλεκτρον

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Here is a narrow tomb Great Thales lies; yet his renown for wisdom reached the skies

“elektron”

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

5 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.

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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)

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+ -

+ +

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)

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2. Methods of Charging

Three basic methods

a) Triboelectric (friction)

b) Conduction

c) Induction (Influence)

92.a.1 Triboelectrification chartDry human skin

AsbestosLeatherRabbit's furGlassMicaHuman hairNylonWoolLeadCat's furSilkAluminumPaper (Small positive charge)Cotton (No charge)Steel (No charge)Wood (Small negative charge)AmberSealing waxRubber balloonResinsHard rubberNickel, CopperSulfurBrass, SilverGold, PlatinumSynthetic rubberPolyesterStyrene (Styrofoam)Saran wrapPolyethylene (like Scotch tape)Vinyl (PVC)SiliconTeflon

10+

-

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

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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

112.a.3 Van Marum Machine (1784)

The biggest tribo-electrostatic generator ever built, could produce voltage with any polarity.

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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

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++ ++

2.c.1 Charge by Induction

Aka chargeby “influence”

(First done by 1759 - Francis Ulrich Theodore Aepinus ?)

152.c.2 Charge by Induction

Another way of doing it that is exploited by electrostatic generators

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2.c.3 Electrophorus (1775)

Uses method of induction to create charge

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Invented by Alessandro Volta (1745-1827)(also invents the battery in 1800 )

2.c.4 Wimshurst Machine (1880)

Two disks rotate in opposite directions, mutually inducing charge

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Invented by James Wimshurst (1832 – 1903)

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3. Measurement of Charging

Without really knowing what IS charge, how was it measured?

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(a) 1753 John Canton(1718-1772) 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?)

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3c Electroscope

•1786 Gold Leaf Electroscope invented by Abraham Bennet(1750 - 1799)

•1887 Braun Electroscope is less sensitive, but more accurate

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B. Coulomb’s Law

1) The Inverse Square Law

2) Coulomb’s Law

3) Units of Charge

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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

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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.

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Force due to gravity =

G is the “gravitational constant”, measured 100 years later by Cavendish: G=6.67x10-11 N•m2/kg2

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R

MGMF =

Newton 1643-1727

B2: Coulomb’s Law• Charles-Augustin de Coulomb 1736-1806

• 1785 using a “torsion balance”measures the inverse square law between charges.

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“q” is measure of chargek= “coulomb constant”

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R

qqkF =

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

27B3. Units of Charge

• SI Unit of charge is “Coulomb” “C”

• Coulomb Constant: k=8.988x109 N•m2/C2

• Permittivity of free space: εo= 8.85x10-12 C2/N•m2

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212

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4 R

qq

R

qqkF

πε==

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.

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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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II. Electric Field

Dr. Bill Pezzaglia

Updated 2012aug05

II. Electric Field

A. Faraday Lines of Force

B. Electric Field

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A. Lines of Force

1) Action at a Distance

2) Faraday’s Lines of Force

3) Principle of “Locality”

3 1. “Action at a Distance”

• 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

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"...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." -Newton

Sir Isaac Newton(1643-1727)

How does moon “know”the earth is there to falltowards it?

2a. Sir Humphry Davy 1778 - 1829

•1807 Electrolysis, used to separate salts. Founds science of electrochemistry.

•His greatest discovery was Michael Faraday.

•1813-15 takes Faraday with him on grand tour visiting Ampere and Volta.

62b. Michael Faraday 1791 - 1867

•1821 First proposes ideas of “Lines of Force”• Example: iron filings over a magnetic show field lines

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2c. Electric Lines of Force

•Electric charges create “electric field lines”•Field lines start on + charges, end on –•A plus charge will tend to move along these lines

82d. Other Properties

•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

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3. Principle of Locality

• Argues that the field lines have independent reality

•Force fields exist as distortions in the “aether” of space

•Alternative to “action at a distance”, charges Locally interact with force lines

•Ideas rejected by others. He can’t put them into mathematical form.

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I cannot conceive curved lines of force without the conditions of a physical existence in that intermediate space. (Michael Faraday)

B. Electric Field

1) Definition of Field

2) Sources of Field

3) Electrodynamics

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1a. James Maxwell (1831-1879)

•1855 essay On Faraday's Lines of Force , suggests lines are like an imaginaryincompressible fluid (obeying hydrodynamic equations)

•1861 paper On Physical Lines of Force , proposes “real” physical model of vortices for magnetic field

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• Definition: force per unit test charge(i.e. don’t want test charge to affect field)

• Units of Newton/Coul (or Volts/meter)

• So force on chare is: F=qE

1b. Definition of Field 13

=

→ q

FLimEq

rr

0+

EF

q

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1c. Analogy to Gravity

• Gravitational Force Field:force per unit test mass

• i.e. its an “acceleration of gravity” field

• Mass is the “charge” of gravity: F = mg

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=

→ m

FLimgm

rr

0

2. Sources of E Field

(a)Point Charge Source (monopoles

(b)Dipoles

(c) Field of Dipole (incomplete)

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2.a Monopole Sources

• A positive charge is a “source” of electric field. Field radiates outward from a point source

• A negative charge is a “sink” of electric field. Field radiates inward

• Field strength: E=kQ/r2

15 2.b Dipole Sources• An “electric dipole ” is a “stick”

of length “L” with + charge on one end and equal – charge on other.

• Dipole moment : p=QL• The vector “p” points along axis

from – to + charge

• Units (SI) is C•m• Standard in Chemistry is the

Debye: 1D=3.33564x10-30 C•m

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3. Electrodynamics

a) Force on monopole

b) Torques on Dipoles

c) Faraday’s Cage

17B.3.a: Force on a Charge (monopole)

• Force on positive charge is in direction of field

• Force on negative charge is opposite direction of field

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EqFrr

=

+E

Fq

-E

Fq

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B.3.b Torque on Dipole

• An electric dipole will want to twist and line up with the electric field

• Torque on a dipole in an electric field is:

• Recall dipole momentp=qL

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Eprrr ×=τ

Faraday Cage (1936)

An external electrical field causes the charges to rearrange which cancels the field inside.

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References•http://maxwell.byu.edu/~spencerr/phys442/node4.html

•http://en.wikipedia.org/wiki/Timeline_of_Fundamental_Physics_Discoveries

•http://www.oneillselectronicmuseum.com/index.html

•http://www.sparkmuseum.com/GLASS.HTM•http://neon.chem.uidaho.edu/~honors/crookes.html

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