Electrostatics

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Electrostatics

• Dr. Don Franceschetti
• Physics 1010
• April 12, 2007

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Electromagnetism in ancient times

• Atmospheric Electricity
• Electric Fish
• Torpedo (used by Roman physicians)
• Electrophorus
• Amber (elektron)
• Lodestone

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Thales of Miletus 624-527 B.C.E.

• Knew about static electricity.

• Knew about the lodestone.

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William Gilbert, M. D.
Wrote De Magnete and distinguished electrical
from magnetic phenomena.
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Robert Symmer 1759

• Beginnings of two fluid theory
• Wore silk stockings over worsted.
• On taking the two stockings off together and
  then drawing the one from inside the other, he
  found that both became inflated so as to
  reproduce the shape of the foot, and exhibited
  attractions and repulsions at a distance of as
  much as a foot and a half.

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Abbé Jean Nollet

• Conducted electricity demonstrations in the
  salons of Paris.
• Rival of Ben Franklin

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Dr. Ben Franklin

One fluid theory Lightning rod Debunked Mesmer
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Luigi Galvani M.D.
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Count Allesandro Volta
Voltaic Pile, first source of current electricity
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AldiniGalvanis nephew

• Went on the road to popularize electricity.
• Visited England with shocking literary results.

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Sir Humphrey Davy
Used electrolysis to discover new elements. Did
public demonstrations using cadavers
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Mary Shelly
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Getting serious now

• Electric charges can be positive or negative.
• Bulk matter is positively charged if it has fewer
  electrons than protons.
• Bulk matter is negatively charged if it has more
  electrons than protons.
• Electrons get transferred when dissimilar
  materials are rubbed against each other. This is
  static electricity.

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An electric relationship

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Attraction and Repulsion

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Charge is

• Conserved (a new conservation law)
• Quantized (comes in packages of proton size
  charges)
• Electrical force is so much stronger than
  gravitational force that it is real hard to build
  up a large concentration of charge.

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Conductors and Insulators

• Conductors (metals) Electrons are free to move
  over large distances.
• Insulators Electrons not free to move.
• Semiconductors are intermediate in character.
  Both electrons and holes contribute to
  conduction. Very sensitive to doping.
• Superconductors Materials that exhibit zero
  resistance below a certain temperature.

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Tricks for storing charge

• Use parallel metal plates (capacitor or Leyden
  jar) separated by a thin insulator.
• Excess charge arranges itself to screen out
  electric field inside conductors (van de Graff)

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A Capacitor

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The Coulomb

• 1 coulomb (C) is a lot of charge.
• Elementary charge is 1.6x10-19C.
• Charge on protons in 1 g. of hydrogen is 96,500 C.

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Coulombs Law

• F kq1q2/d2
• K9,000,000,000 Nm2/C2
• An inverse square law like the law of gravity but
  can be either attractive or repulsive and much
  stronger.

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Charging

• Can occur by contact.
• Or induction.
• Lightning charged area of cloud induces opposite
  charge in ground below.
• Sharp points allow discharge. Franklins
  lightning rod.

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Charging by induction

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Lightning protection

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Charge Polarization

• Rearranges charges on an atomic scale.
• Electronic and or dipolar reorientation
• Carried to extreme rips electrons out entirely to
  produce spark or lightning bolt

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Polarization
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Induced Polarization

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Water has a permanent dipole

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Because of the dipole in water

• Water is a liquid at room temp.
• Water expands when it freezes.
• Salt dissolves in water.
• Ponds do not freeze completely in winter!
• And life as we know it is possible.

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Electric Field

• EF/q, measured in N/C
• Points in direction of force on a positive
  charge.
• Field lines start on positive charges and end on
  negative ones.
• Microwave oven is source of alternating electric
  field, drags molecules around, creates heat.

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Electric Shielding

• No electric field inside a conductor.
• Recall van de Graaff

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van de Graaff

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Electric Potential

• Moving electrically charged objects subject to
  electrical forces changes their potential energy.
• Electric potential
• Electric potential energy/charge
• volt joule/coulomb

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Review questions

• 22.1 In terms of attraction and repulsion, how do
  negative particles affect negative particles? How
  do negatives attract positives?
• 22.2 Why does the gravitational force between the
  Earth and moon predominate over electrical forces.

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Review Questions

• 22.6 What is a positive ion? A negative ion?
• 22.7 What do we mean by saying charge is
  conserved?
• 22.8 What is meant by saying charge is
  quantized.
• What particle has exactly one quantum of charge?

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Review questions

• 22.20 Give an example of something charged by
  induction.
• 22.21 What occurs when we ground an object?
• 22.22 What is the purpose of a lightning rod?