Generating Electricity

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Generating Electricity

• 1831 Michael Faraday discovers that by moving a
  magnetic bar near a loop of wire, an electric
  current can be induced in the wire.
• The magnetic field produced by the magnet applies
  a force on the electrons in the wire, causing
  them to move.
• When the north end of the magnet enters the coil,
  a current is induced that travels around the coil
  in a counterclockwise direction producing a
  positive current when the magnet is then pulled
  out of the coil, the direction reverses to
  clockwise producing a negative current.
• Known as electromagnetic induction
• This allowed the generation and transmission of
  electricity possible, along with electric motors
  and modern communications and computer systems
• Electromagnetic induction animation

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Electromagnetism

• It was already known that the opposite was true,
  that a metal placed inside a current loop could
  become magnetized.

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Generators

• Coil of copper wire mounted on a rotating
  armature
• Coils are rotated through a magnetic field
• This induces a current in the coils.
• But, the induced current resists the rotation of
  the coils, so we need an external energy source
  to rotate the coils.
• The current exits the rotating coil via slip
  rings that are in contact with carbon brushes.
• The direction of current flow changes as the coil
  rotates in the magnetic field. This produces an
  alternating current.

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Generator
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Alternating vs direct current

• Direct current flow of current in one direction.
  Produced by batteries, solar cells, dynamos
• Alternating current when the flow of current
  periodically changes direction(50-60 times per
  second). This is what is delivered to homes and
  businesses

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Before Faraday

• Electricity was generated via electrostatic means
• used moving electrically charged belts, plates
  and disks to carry charge to a high potential
  electrode.
• Charge was generated using either of two
  mechanisms
• Electrostatic induction or
• The triboelectric effect, where the contact
  between two insulators leaves them charged.
• Generated high voltage but low current, not good
  for commercial use

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Wimshurst Machine

• two large contra-rotating discs mounted in a
  vertical plane, two cross bars with metallic
  brushes, and a spark gap formed by two metal
  spheres.
• two insulated disks and their metal sectors
  rotate in opposite directions passing the crossed
  metal neutralizer bars and their brushes.
• imbalance of charges is induced, amplified, and
  collected by two pairs of metal combs with points
  placed near the surfaces of each disk.
• The positive feedback increases the accumulating
  charges exponentially until a spark jumps across
  the gap.
• The accumulated spark energy can be increased by
  adding a pair of Leyden jars, an early type of
  capacitor suitable for high voltages

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Van de graf generator

• an electrostatic machine which uses a moving belt
  to accumulate very high electrostatically stable
  voltages on a hollow metal globe.

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Van de graaff generator

• Video http//www.youtube.com/watch?vsy05B32XTYY

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Faradays Disk

• A copper disc rotating between
• the poles of a horseshoe magnet.
• produced a small DC voltage,
• and large amounts of current.
• First electromagnetic generator

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Dynamos

• First generator able to produce electricity for
  industrial purposes
• First dynamo was built by Hippolyte Pixii in
  1832.
• a stationary structure, which provides a constant
  magnetic field, and a set of rotating windings
  which turn within that field.
• Magnetic field may be provided by one or more
  permanent magnets or by one or more
  electromagnets, which are usually called field
  coils.

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Pixii's dynamo
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Dynamos

• Produce a direct current
• Basis for later devices such as the electric
  motor, the alternating-current alternator, and
  the rotary converter.
• Developed as a replacement for batteries

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Modern electrical power plants

• Boiler Unit Almost all of power plants operate
  by heating water in a boiler unit into super
  heated steam at very high pressures. The source
  of heat from combustion reactions may vary in
  fossil fuel plants from the source of fuels such
  as coal, oil, or natural gas. Biomass, waste
  plant parts, solid waste incinerators are also
  used as a source of heat. All of these sources of
  fuels result in varying amounts of air pollution,
  as well as carbon
• In a nuclear power plant, the fission chain
  reaction of splitting nuclei provides the source
  of heat.

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Modern electrical power plants

• The super heated steam is used to spin the blades
  of a turbine, which turns a coil of wires within
  a circular arrangements of magnets.

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Modern Electric power plants

• Cooling Water After the steam travels through
  the turbine, it must be cooled and condensed back
  into liquid water to start the cycle over again.
  Cooling water can be obtained from a nearby river
  or lake. An alternate method is to use a very
  tall cooling tower, where the evaporation of
  water falling through the tower provides the
  cooling effect.

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Getting the electricity from the plant to the
light switch
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Power transmission

• Power plants are not located near population
  centers
• Need to get the power from the plant to the users
• Edison created the first power system in New York
  City in 1882.
• Used direct current. Could only deliver
  electricity to customers closer than 1.5 miles
  away from the power station.
• Westinghouse proposed using AC current, which
  could be more easily and cheaply transmitted.
  Resulted in the War of Currents
• Edison waged a PR campaign, claiming AC current
  was far more dangerous as at frequencies near
  60HZ, it had a greater potential to cause cardiac
  fibrillations.
• He and his workers publically electrocuted
  animals to make their point.
• Edison opposed capital punishment, but in an
  effort to make his point about AC current, he
  secretly funded the development of the first
  electric chair.

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Power transmission

• Energy is lost in transmission lines
• Materials that allow electrons to flow through
  them (current) are called conductors.
• Every conductor has some resistance to the flow
  of current.
• Energy is lost as the current flows through the
  transmission lines
• Relationship between voltage, current and the
  resistance to current flow is given by V IR

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Power transmission

• The losses in the line are proportional to the
  resistance and the current squared or RI2 and the
  power in the line is proportional to VI (voltage
  times current)
• The solution to these losses is to transmit the
  power at much higher voltages than the users
  need, and step the voltage down along the way.
  That way the current in the line is low, so the
  power losses are low.
• So the voltage is increased before the
  electricity leaves the power station and then
  decreased as needed.
• This is accomplished with a device called a
  transformer

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Transformers

• No not these guys..

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Transformers

• A device that transfers energy from one
  electrical circuit to another using the concept
  of induction
• A changing current in the first circuit (the
  primary) creates a changing magnetic field. This
  changing magnetic field induces a changing
  voltage in the second circuit (the secondary).
  This effect is called mutual induction.

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Transformers

• The number of coils in the windings determine if
  the voltage is increased (stepped up) or
  decreased (stepped down)
• If the number of coils in the secondary is larger
  than the primary, voltage is stepped up, if it is
  less it is stepped down.

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Power transmission

• At the power station, the generator produces
  13-25kV.
• A step up transformer boosts this to 115 to 765
  kV.
• Substations reduce the voltages for local
  distribution.
• Transformers on power poles reduce it further to
  the 240 V generally fed into our homes.

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Power Transmission
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Health Risks from Power Lines

• Power lines are live, if you touch them (and are
  in contact with the ground) you provide the
  current a path to ground. AC currents can induce
  heart fibrillations and cause death.
• NO strong link to overhead power lines and
  increased cancer due to the lines themselves.