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• 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• 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=sy05B32XTYY

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• 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.

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• 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.