Electromagnetism by ewghwehws


• 1820, Oersted finds that a current carrying
  wire will cause a compass needle to deflect
• Implication: the current carrying wire itself
  behaves like a magnet (since it can exert a
  force on another magnet… like a permanent
  magnet would)
• Let’s use field theory to understand what’s
Magnetic Field for a coil of wire
      called a solenoid
Magnetic Field for Solenoid with an
             Iron Core
Another interesting application…
         electric motors
    Electromagnetism (cont’d)
• 1820, Oersted finds that a current carrying
  wire will cause a compass needle to
  deflect… so there was a discovery waiting
  to be made…
• 1832, Michael Faraday found that a
  changing magnetic field could produce an
  electric current… electromagnetic induction
• Principle underlying electric generators
               Four Stations
1. Build an electromagnet - design & execute an
   experiment to test how the number of coils
   affects the strength of the electromagnet
2. Build your own telegraph
3. Build an electric motor - try to determine the
   number of rpms of it
4. Measure the voltage and current produced by a
   hand-crank generator over a 10 second time
NOTE: You must create your own data tables for
   stations 1 & 4
 Contrasting Electric motors and
• Electric motors – convert electrical energy
  into mechanical energy… the motion of
  electrons into the motion of something else
• Electric generators – convert mechanical
  energy into electrical energy… the motion
  of something into the motion of electrons
Electrical Transmission
        Electrical Transmission
• Transformers only work with alternating current.
• Using direct current will create a magnetic field in
  the core but it will not be a changing magnetic
  field and so no voltage will be induced in the
  secondary coil.
• Using a step up transformer to increase the voltage
  does not give you something for nothing. As the
  voltage goes up, the current goes down by the
  same proportion. The power equation shows that
  the overall power remains the same.
• P=V x I Power = Voltage x Current
 Electrical Transmission (cont’d)
• Power output is always less than the power input because
  the changing magnetic field in the core creates currents
  (called eddy currents) which heat the core. This heat is
  then lost to the environment, it is wasted energy.
• Electricity is first produced at the power plants and then
  sent to step-up transformers where low-voltage electricity
  is changed to high voltage to facilitate the transfer of
  power from the power plant to the customer. Voltage must
  be increased so that the electric current has the "push" it
  needs to efficiently travel long distances.
• Ohm’s Law V = I R
 Electrical Transmission (cont’d)
• From the step-up transformer, transmission lines carry the
  high voltage electric current long distances through thick
  wires mounted on tall towers that keep the transmission
  lines high above the ground. Insulators made of porcelain
  or polymers are used to prevent the electricity from leaving
  the transmission lines.
• High-voltage transmission lines carry the electric current to
  substations where the voltage is lowered so it that can be
  distributed locally on smaller power lines known as
  distribution lines.
• Distribution line voltage levels are typically 4 kV or 12
  kV. These voltages are reduced one last time at smaller
  pole-top transformers to utilization voltages, typically 120
  and 240 volts, to make the power safe to use in our homes.
               Equipment needed
•   Large nails
•   Strong cylindrical magnets
•   Small nails
•   FOSS equipment
    • Circuit boards, coils of wire, wire, batteries, switches,
      iron pieces, long wires, battery holders
•   Hand-crank generators
•   Voltmeters
•   Ammeters
•   Foil
•   Scissors
•   Galvanometer, coils, wires, switch

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