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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. – PowerPoint PPT presentation

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Title: Generating Electricity


1
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

2
Electromagnetism
  • It was already known that the opposite was true,
    that a metal placed inside a current loop could
    become magnetized.

3
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.

4
Generator
5
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

6
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

7
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

8
Van de graf generator
  • an electrostatic machine which uses a moving belt
    to accumulate very high electrostatically stable
    voltages on a hollow metal globe.

9
Van de graaff generator
  • Video http//www.youtube.com/watch?vsy05B32XTYY

10
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

11
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.

12
Pixii's dynamo
13
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

14
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.

15
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.

16
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.

17
Getting the electricity from the plant to the
light switch
18
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.

19
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

20
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

21
Transformers
  • No not these guys..

22
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.

23
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.

24
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.

25
Power Transmission
26
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.
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