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### Transcript of 21.3 Electric Energy Generation and Transmission .

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21.3 Electric Energy Generation and Transmission

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Traffic lights change colors to control the flow of cars.

Without electricity, many activities would have to stop after the sun goes down.

The question is: Where does all the electrical energy come from?

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Generating Electric CurrentAll the electrical energy comes from the two

aspects of the electromagnetic force.

In the last section of this chapter, we learned that electric current produces a magnetic field.

Also, the reverse can happen: A magnetic field can create an electric current.

This is called electromagnetic induction.

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Generating Electric CurrentElectromagnetic induction

was discovered in 1831 by Michael Faraday (an English scientist).

According to Faraday’s law, a voltage is created in a conductor by a changing magnetic field.

How can you change the magnetic field?

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Generating Electric CurrentYou can change the magnetic field by moving a magnet.

So moving a magnet through a coiled wire that is in a circuit will produce an electric current.

Once you stop moving the magnet, the current will stop.

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Generating Electric CurrentYou can also move the coil and keep the magnet

still to produce a current.The way to produce a current is to have the

magnet and coil move relative to one another.

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GeneratorsMost of the electrical energy used in homes is

produced with generators at large power plants.

A generator is a device that converts mechanical energy into electrical energy.

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GeneratorsGenerators contain a turbine - a device with

fanlike blades that turn when pushed.

To produce electrical energy, the turbine rotates the coils of a generator, or it rotates magnets around the coils of wire.

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AC GeneratorsThere are two types of generators: AC and DC.The AC generator produces an alternating current.How does it work?A wire coil in the generator is attached to metal

bands called slip rings.

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AC GeneratorsThe slip rings are in contact with metal brushes that

are in turn attached to a circuit. As the loop of wire is rotated, the magnetic field

induces a current in the wire.This current is in one direction and then when the

loop turns halfway around, the current reverses direction.

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DC GeneratorsA DC generator produces a direct current.

Its design is like the design of an AC generator except that a commutator replaces the slip rings.

The commutator only allows current to leave in one direction (thus DC).

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TransformersMost power plants produce voltages that are too high to

be handled safely in homes.

So, in order for it to be safe, the voltage must first be changed, or transformed.

This is where a “transformer” comes in.

A transformer is a device that increases or decreases the voltage and current of two linked AC circuits. QuickTime™ and a

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Changing Voltage and CurrentThere are two types of transformers, a “step-up” and a

“step-down” transformer.

Each transformer has two sets of coils wrapped around a ring-shaped iron core.

When there is an alternating current in the primary coil, the current creates a changing magnetic field in the iron ring.

This induces a current in the other (secondary) coil.

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Changing Voltage and CurrentThe step-down transformer decreases voltage.It does this by having the primary coil have more loops

than the secondary coil.By having less loops in the secondary coil, the

secondary coil is not able to collect as much of the magnetic field to help push electrons through it.

A step-up transformer increases voltage.

The primary coil has less loops than the secondary coil.

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Electrical Energy for Your HomeSo in order to power our homes, the power plant

generates electrical energy with generators.From the power plant, the electricity travels through a

series of transformers, which reduce the voltage before it gets to our homes.

After this voltage is reduced, then it is safer to use.

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