Electrostatics

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Electrostatics

• Physics

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Electric Charge

• Term Charge has new meaning from when it was
  first defined.
• Charge Carriers are
• Electrons, protons, ions
• Charge is quantized
• Indivisible discreet amounts of charge carried by
  electrons / protons.
• Units
• Coulomb 1 Coulomb 6.25 x 1018 electrons
• Charge of electron is 1.6 x 10-19 C
• Law of Conservation of Charge
• Charge cannot be destroyed nor created, only
  transferred from one object to another.

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Transfer of Charge

• Conductors Atoms / Molecules with several
  loosely bound electrons in outer energy levels
• Exs Gold, Silver, Copper, Aluminum,
• Insulators Atoms / Molecules whose electrons
  are more tightly bound to the nucleus.
• Exs Hydrocarbons (plastic, acrylic, rubber,
  ..), Air, Wood
• Semiconductors Atoms / Molecules that have one
  loosely bound electron for every few atoms
• Transistors Act as switches by applying a
  small electric field more or less electrons
  enter the material making it either a conductor
  (on) or insulator (off).

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3 ways objects get Charged

• Charging by Contact / Friction
• electrons are scrapped off one material and
  adhere to another
• Charging by Induction
• A charged object is used to separate the charges
  in a nearby conductor. Then the opposite side of
  the conductor is grounded
• Charge Polarization
• A charged object is held near another object and
  polarizes the atoms / molecules in the other
  object.

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Electric Force Coulombs Law

• Fe k (q1 q2 ) / r2
• k 9 x 109 N m2/C2
• Ex
• Magnitude of electric force compared to gravity.
• Calculate the electric force between an electron
  and proton in a Hydrogen atom, r 5 x 10-12 m.
  Electron mass 9.11 x 10-31 kg, Proton mass
  1.67 x 10-27 kg.
• Fe
• Fg

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Tinsel Levitation

• Determine the gravitational force pulling down on
  a strand of tinsel.
• By using the induction method, charge a pie tin
  positively and DROP a loop of tinsel onto the pie
  tin and observe what happens.
• Why does the tinsel attract to the pie tin and
  then repel?
• Approximate the height above the pie tine that
  the tinsel levitates.
• What is the net force on the tinsel?
• What is the Electric Force supporting the tinsel?
• By assuming the charge on the tinsel and the
  charge on the pie tine to be the same, what the
  amount of charge on each?

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Electric Field

• Definition an area of space in which if a
  charged object were placed in, it would
  experience a force.
• Magnitude
• E Fe / qtest
• (Honors) E k q1 / r2
• Direction the direction of the field is taken
  to be the direction of the force felt by a
  positive test charge.
• Shielding Inside of a conductor, the electric
  field is always zero!
• Exs

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Anatomy of a Lightning Bolt

• Rising Air, Water Droplets, and / or Ice
  crystals gain or loose electrons leaving the
  bottom of the cloud typically (-).
• The bottom of the cloud induces a charge on the
  surface of the ground below.
• Leaders branch out from the top of the cloud,
  the base of the cloud and from the ground.
• When the leaders connect, the air ionizes and
  becomes a good conductor.
• The Ionized air then allows for an avalanche of
  electrons to flow from (-) to ().
• Tall, pointed objects on the ground prevent
  charge from building up
• If a lightning is produced it typically strikes
  tall, pointed objects (trees, poles,
  buildings,..)
• To be safe, stay away from tall objects, crouch
  down into a ball

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Electric Potential Energy

• PEe q E d
• Similar to gravitational PE (mgh)
• Assumes Electric field is constant through the
  distance, d.
• (Honors) If distance d is considerable as
  compared to the size of the object that is
  establishing the field, then use other PE
  equation!
• (Honors) PEe k (q1 q2) / r

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Electric Potential

• Electric Potential Electric Potential Energy
  per Coulomb of charge
• V PEe / q
• Honors V k q / r
• NOTE PE and V are scalars not vectors!
• Ex 120 Volts means that there is 120 Joules of
  energy for every Coulomb of charge present.
• Exs

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Capacitance

• Capacitors store PEe by separating positive and
  negative charge by a small distance.
• The smaller the separation distance the more
  energy they can store per charge.
• The larger the surface area the more charge they
  are able to separate.
• Capacitance is used as a description of how much
  charge a capacitor can separate per Volt
  applied to it.
• C Q / V (Honors)
• Exs
• Net Charge of a capacitor is always ZERO They
  store SEPARATED charge (equal amounts of and -)

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Batteries

• Act as an electric pump They do not store
  charge!!!
• Because of the different affinity for electrons
  between different metals, charge will move from
  one metal to another.
• This movement is of electrons and ions so there
  needs to be a medium in between the metals that
  allows for the movement on ions (electrolytes,
  acids, etc..)