i. electric charge dr. bill pezzaglia updated 2012aug05

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

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

• Famous theorems of similar triangles

• Amber rubbed with fur attracts straw

• “Amber” in greek:

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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

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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)

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2.a.1 Triboelectrification chart Dry human skinAsbestosLeatherRabbit'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

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2.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 ?)

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2.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)

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

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 Nm2/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

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B3. Units of Charge

• SI Unit of charge is “Coulomb” “C”

• Coulomb Constant: k=8.988x109 Nm2/C2

• Permittivity of free space: o= 8.85x10-12 C2/Nm2

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212

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4 R

qq

R

qqkF

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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