Chapter 19: Electric Charges, Forces, and Fields

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Chapter 19 Electric Charges, Forces, and Fields

• Brent Royuk
• Phys-112Concordia University

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Electric Charge

• Electrostatics vs. Electricity
• Historical Development
• Elektrum and Magnesia

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Electric Charge Basics

• Like repels, opposites attract
• Charge can move
• Two kinds of charge
• How do you know its only two?
• Two materials insulators and conductors
• Neutral means balanced charge
• So a water stream is always attracted
  polarization
• Charge by contact or induction
• Charging by induction which charge does it get?

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Electric Charge Basics

• Charge by contact or induction
• Charging by induction which charge does it get?

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Electric Charge

• Franklins single fluid model, 1747
• surplus (glass) or deficiency (rubber/amber)
• Unfortunate coin toss
• Actually, we know its electrons that move
  surplus , deficiency .
• And electron is negative, proton is positive.
  Bummer.
• Triboelectric charging, Table 19-1 p. 628
• Metric unit of charge the Coulomb (a lot of
  charge)
• 1 Coulomb 6.25 x 1018 electrons
• Usually more useful charge of electron
• e 1.6 x 10-19 C
• Notice inverse relationship
• Classical electromagnetism does not require the
  use of electrons
• Charge symbol, q, and q ne (charge is
  quantized)
• Van de Graaf Wimshurst

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Franklins Kite

• BEN FRANKLIN'S KITE WAS STRUCK BY LIGHTNING?
  Never happened!
• Many people believe that Ben Franklin's kite was
  hit by a lightning bolt, and this was how he
  proved that lightning was electrical. A number of
  books and even some encyclopedias say the same
  thing. They are wrong. When lightning strikes a
  kite, the spreading electric currents in the
  ground can kill anyone standing nearby, to say
  nothing of the person holding the string! So what
  did Franklin actually do? He showed that a kite
  would collect a tiny bit of electric charge out
  of the sky during a thunderstorm. Electric
  leakage through the air caused his kite and
  string to become electrified and so the hairs on
  the twine stood outwards. Twine is slightly
  conductive, so the imbalanced charge spread to
  all parts of the kite string. Franklin used the
  twine to electrify a metal key, and tiny sparks
  could then be drawn from the key. (He used a
  metal object because sparks cannot be directly
  drawn from the twine, it's not conductive
  enough.) This suggested that some stormclouds
  carry strong electrical net-charge. It IMPLIED
  that lightning was just a large electric spark.
• The common belief that Franklin easily survived a
  lightning strike is not just wrong, it is
  dangerous it may convince kids that it's OK to
  duplicate the kite experiment as long as they
  "protect" themselves by holding a silk ribbon.
  Make no mistake, Franklin's experiment was
  extremely dangerous, and if lightning had
  actually hit his kite, he certainly would have
  been killed.
• Taken from "ELECTRICITY" MISCONCEPTIONS IN
  TEXTBOOKS - - - William J. Beaty
• at http//www.eskimo.com/billb/miscon/elect.html

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Coulombs Law

• The inverse square relationship
• Just like gravity
• The Law
• k 9.0 x 109 Nm2/C2
• More fundamental
• ?o 8.85 x 10-12 C2/N m2

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Coulombs Law Examples

• What is the Coulomb force between two 1C charges
  one meter apart? From here to Lincoln?
• What is the Coulomb force between two protons
  separated by r 1 x 10-15 m? Compare with
  gravitational force.
• Arrange three charged objects in an equilateral
  triangle with one free to move. Let them be 15
  cm apart and have masses of 250 g, with 2.0 ?C
  of charge. Describe the motion of the free
  particle. What if one of the fixed charges had
  -2.0 ?C of charge?
• This illustrates the principle of superposition

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The Electric Field

• To circumvent the idea of instantaneously
  propagating forces we posit the existence of a
  force field
• We call this the electric field, E
• E is a vector.
• Walkers notation
• A field is a quantity that has a value at every
  point in space.
• For example, a temperature map is a scalar field
  and a windspeed map is a vector field
• And guess what Fields are real!

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The Electric Field

• Definition
• Direction is defined by how a positive test
  charge qo would move
• For a point charge,
• Note the difference between q and qo

q
What is here?
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Electric Vector Fields

• Vectors can be found for any point near the
  charge configuration
• This defines the electric field

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The Electric Field

• Examples
• What is the electric field vector 12 cm away from
  a point charge of -42 ?C?
• An isolated electron is acted on by an electric
  force of 3.2 x 10-14 N. What is the magnitude of
  the electric field at the electrons location?
• A 3.2 ng dust particle contains an excess of a
  billion electrons. What is the direction and
  magnitude of an E-field that would keep the
  particle from falling?
• Two charges of 4.0 ?C and 9.0 ?C are 30 cm
  apart. Where on the line joining the charges is
  the electric field zero?

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Electric Field Lines

• Rules, p. 643
• Note that more lines/area means a stronger field
• Can field lines cross?
• Examples
• Single point Charge
• Like and unlike point charge pairs
• Dipoles
• See next page

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Electric Field Lines
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The Parallel-Plate Capacitor

• A Capacitor stores charge
• Between parallel plates the E-field is quite
  uniform.

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Charges on a Conductor

• The electric field is zero everywhere inside a
  charged conductor
• If it were not, the charges would re-arrange
  themselves until it is
• Any excess charge on an isolated conductor
  resides entirely on the surface of the conductor
• Since charges repel each other and are free to
  move in a conductor
• The electric field at the outer surface of a
  charged conductor is perpendicular to the surface
• Why?
• The excess charge on a conductor in electrostatic
  equilibrium is more concentrated at regions of
  greater curvature.
• Again, tangential components would move charges
• Therefore the E-field is concentrated at tips of
  conductors
• The lightning rod

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The Faraday Cage

• Movie Movie 2
• What is the safest place to hide during a
  lightning storm?

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The Faraday Cage

• Strangers Converse in a Berkeley Elevator
• (from Brad DeLong)
• Person 1 I'm getting into an elevator. I'm
  about to lose the connection...
• Person 2 An elevator is an admirably
  effective Faraday cage.
• Person 3 Somebody should make a cell phone
  that works inside a Faraday cage.
• Person 1 But surely the laws of physics...
• Person 4 It could work via gravitational
  radiation
• Person 2 Two charged, mutually orbiting
  micro black holes within the cell phone casing...
• Person 5 Surely the Hawking radiation would
  be too fierce?
• Person 2 I dunno. How long is the lifespan
  of a 10 kg black hole, anyway?
• Person 1 You'd carry around a 50 lb cell
  phone just so you could talk in elevators?