ELECTRICITY GENERATION

Remember Ohms law from last time. Here are two circuits below. If you know Ohms law you should be able to make the light come on in one only one try as the amperage, I, is now specified.

In the early 19th century the following similarity between two charged particles and two magnets was observed:

• both created "forces" that could operate in a vacuum
• charge had a postive and negative component; magnets had a north and south pole force could then be either attractive or repulsive.
  

• both the magnetic force and the electrostatic force strength decreased as 1/R²

In 1820 Oersted did this experiment:

and discovered that an electric current creates a magnetic field

Similarly, a coil of wire with a current passing through it generates a magnetic field. This is known as an electromagnet or solenoid .

So now we know that a current can create a magnetic field. If a magnetic field can create a current then we have a means of generating electricity. Experiments showed that a magnetic just sitting next to a wire produced no current flow through that wire. However, if the magnet is moving a current is induced in the wire. The faster the magnet moves, the greater the induced current.

This is the principal behind simple electric generators in which a wire loop is rotated between to stationary magnetics. This produces a continuously varying voltage which in turn produces an alternating current .

These ideas were first suggested by Michael Faraday and Farady's law states that the voltage is proportional to the rate of change of the magnetic flux. In the diagrams below, this magnetic flux is represented by blue vectors going from North to South.

Diagram of a simple electric generator:

	In this position the magnetic flux is essentially zero but its rate of change is large. Hence, the voltage is at a maximum here and current flow is also at a maximum.
	In this position the Voltage is now zero and their is no current flow. The magnetic flux is at a maximum

To generate electricty then, all we really want to do is have some (mechanical) mechanism turn a crank that rotates a loop of wire between stationary magnets. The faster we can get this crank turned, the more current we can generate.

Popular Methods of Turning the Crank:

• Let water fall on it (Hydro Power)
• Direct a nozzle of steam at it (Coal or Nuclear Fired Steam Plant)
• Let the wind turn it (windmill)

Why do transmission lines carry such high voltages?

Consider the following:

• Electricity is generated at the generating plant at 120 Volts and then delivered to the households over conductors.
• There are 10 households and each needs 1000 Watts (for their toasters)
• The electric company must therefore supply 10x1000 = 10,000 Watts.

• Power = I x V I = P/V
• so I = 10,000/120 = 83.3 amps
• But, electrical power is dissipated as heat according to P = I²R (this is how your stove works)
• Lets assume R= 1: We now have heat dissipation = (83.3)*(83.3)(1) = 6944 watts.
• Heat dissipation is energy lost by the system. This loss is unavoidable!

• To deliver the 10,000 watts that the consumer needs requires that we generate 16,944 watts and hence have an overall efficiency of 10,000/16,944 = 59% which the consumer would pay for

How to solve the loss problem:

Current = Power/Voltage; If we increase V by a factor of 10, then I lowers by a factor of 10 (at constant power) and the power dissipated as heat lowers by a factor of 10².

Hence at 1200 Volts we have only 69.4 watts of energy loss and a 99% enery efficient delivery system.

How to change the voltage: Use a Transformer

	A transformer uses alternating current in one coil to induce alternating current in another. The induced voltage is given by: Vout = Vin x N2/N1 where N1 = Number of coils in the Primary and N2= Number of coils in the secondary. When N2 is less than N1, we reduce Vout. This is why there are transformers on power lines to step the voltage down to 120 Volts by the time it reaches your house.

Energy conservation tells us that Power In = Power Out

so

V_out x I_out = V_in x I_in

Since V_in is very high, I_in is low and (to prevent transmission loss); when V_in is stepped down to produce V_out (what you get at your house), I_out increases so you can run your stuff.

And that's the way the world works.

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How Electricity Works

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