ELECTRICITY FUNDAMENTALS

Electricity is an apparent force in nature that exists whenever there is a net electrical charge between any two objects.

Basics of Electrostatics:

Properties of Electricity:

OHM's LAW

Is there a relation between I, V, and R ? Let's do an experiment:




In the above circuit there is a battery (V), some resistors (R), and a light bulb that can only be activated if the right number of amps reach it. We can control this buy putting the right resistance in the circuit for a given battery Voltage.

Experimental results then lead to Ohms law:

V = R * I

This is a linear relation. If you double the voltage (V) then for the same value of R you get twice the current. If you want to keep the current the same value after doubling V, you would have to double the resistance (R).

Example:

Your electricity bill essentially measures the amount of current that you use but you use this current as Power .

Power = V * I

So the toaster has a power of 120x8 = 960 Watts.

Energy = Power * Time (and its energy --> kilowatt hours that you pay for - a 100 watt light bulb left on 10 hours = 1 kilowatt hour. )

If you leave your toaster on for one hour, than that would also be approximately 1 KWH (960 watt-hours if you want to nit pick).

1000 Watt-hours = 1 Kilowatt hour (KWH); A KWH will be our basic unit of energy in this class. You purchase KWHs from the electric utility whenever you use power in your home.

The Discovery of Electricy and Magnetism and the Generation of Electricity.

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

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 .

Diagram of a simple electric generator:

In this position there is no current flow but their is a large potential difference (a large voltage)

In this position the Voltage is now zero and the current flow 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:

Why do transmission lines carry such high voltages?

Consider the following:

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 if we increase 120 Volts to 1200 Volts we have only 69.4 watts of energy loss and a 99% energy efficient delivery system This is why high voltage (typically 760 thousand Volts or 760 kiloVolts) transmission lines are required to delivery electricity from central generating sources (e.g. a hydroelectric dam) to consumers/grids hundreds of miles away.

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

Vout x Iout = Vin x Iin

Since Vin is very high, Iin is low and (to prevent transmission loss); when Vin is stepped down to produce Vout (what you get at your house), Iout increases so you can run your stuff.

And that's the way the world works.