StaticElectricity: Its All Around Us

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(Static)Electricity Its All Around Us
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Courtesy Dr. Joseph Alward, University of the
Pacific Physics Dept.
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Electricity

• In innumerable gadgets and high technology
• It holds atoms together
• Its in lightning and other sparks

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Electricity Comes From Charged Particles in Atoms

• Nucleus contains positive protons
• Negative electrons buzz
• around nucleus
• Nucleus also contains
• neutral neutrons
• Net charge of atoms is
• zero - neutral

Courtesy Science Museum, London
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Courtesy www.energy.ca.gov/education/story/
story-html/chapter07.html
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Electricity Basics

• Like charges repel
• Unlike charges attract

Courtesy psych.hanover.edu/Krantz/
neural/charge2anim.html
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Attraction and Repulsion
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Evidence for Two Kinds of Charge

• Acrylic rod charged by rubbing with wool attracts
  pith ball
• After touching, rod repels ball
• Rubber rod rubbed with silk attracts same ball

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Conservation of Charge

• In atoms positive and negative charges balance
• Removing electron produces positive ion
• Ions are charged atoms
• Imbalance can also come from adding an electron
  to make negative ion
• Electrons are neither created nor destroyed,
  merely transferred
• No one has ever witnessed electric charges, or
  -, destroyed or created

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Positive or Negative?
You rub cloth and add electrons. Are you
positive or negative? You rub something else
and lose electrons. Are you positive or
negative?
Negative - - - - - - - - -
Positive
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Coulombs Law
F k q1 q 2 /d2 q1 is amount of charge of one
particle q2 is amount of charge of other
particle d is distance between particles Unit of
charge is the coulomb, abbreviated C k 9 000
000 000 N-m2 /C2 9 x 109 N-m2 /C2 k Coulombs
constant ___________
Like gravity law) F Gm1m2/d2
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Repelling Students
Two students one meter apart each carry a charge
of one Coulomb. What is the force between them?
F k q1 q 2 /d2 9 x 109 N-m2 /C2 x 1C2 /1 m2
9 x 109 Newtons
What would happen to our school if this were
actually the case?
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How would they accelerate?

• a F/m 9 x 109 N/(6 x101 kg) 1.5 x 108 m/s2
• v at 1.5 x 108 m/s2 (1 sec) 1.5 x 108 m/s
• Speed of light 3 x 108 m/s

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Conductors and Insulators

• Conductors are materials in which electrons are
  free to move around, especially metals.
• Good conductors of
• electricity
• In insulators such as rubber, paper, glass and
  styro- foam electrons are tightly bound to atoms.
  
• Poor conductors of electricity

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Electrons Are Free To Move in Metals
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Semiconductors

• Good insulators when pure
• Become much better conductors when tiny amounts
  of impurities are added
• Ex. Germanium and silicon, used in transistors
  and microchips

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Superconductors

• Metals that become infinitely conducting at low
  temperatures
• Electric current can flow forever without energy
  input

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Methods of Charging

• Friction rubbing transfers charge
• Contact touching leads to charge transfer
• Induction bringing charged object near causes
  redistribution of charge
• Grounding charges repel or attract to or from
  an effectively infinite reservoir of charge such
  as the ground

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Charging by Friction
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Charging by Contact
Some electrons transfer from rod to ball
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Charging by Induction(conductors)
Courtesy of the Physics Classroom
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Charging by Induction
Such separation of charge is called charge
polarization
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Polarizing Atoms
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Explain This
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Explain This
A charged comb attracts little bits of paper
Courtesy Dr. Joseph Alward, University of the
Pacific Physics Dept.
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Explain This
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Applications of Electrostatic Charge
Negatively charged paint adheresto positively
charged metal
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Application
Fine mist of negatively charged goldparticles
adhere to positively chargedprotein on
fingerprint.(From Eugene Hecht's Physics, 2nd
Edition Brooks/Cole Publishing)
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Electrostatic Air Cleaner
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Xerox machine
You find out how it works
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Charge Coupled Device (CCD)