Transcript Generating Electricity

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
Electromagnetism
• It was already known that the opposite was
true, that a metal placed inside a current loop
could become magnetized.
Generators
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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.
Generator
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
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
Wimshurst Machine
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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
Van de graf generator
• an electrostatic
machine which uses a
moving belt to
accumulate very high
electrostatically stable
voltages on a hollow
metal globe.
Van de graaff generator
• Video:
http://www.youtube.com/watch?v=sy05B32X
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Faraday’s 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
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.
Pixii's dynamo
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
Modern electrical power plants
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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
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In a nuclear power plant, the fission
chain reaction of splitting nuclei
provides the source of heat.
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.
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.
Getting the electricity from the plant
to the light switch
Power transmission
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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.
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
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
Transformers
• No not these guys…..
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.
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.
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.
Power Transmission
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.