Alternating Current:

        The electricity supplied to our homes, schools, businesses and
industries is generally alternating current or ac. Also, the radio and
television signals that travel from a transmitting station to a reciever
are produced by alternating currents in the trasmitting antenna. In this
section we will investigate the methods of generating alternating current
and learn some of the characteristics of this type of power as compared
to direct current power.

Electromagnetic Induction:
        Magnetism can be produced by passing an electric current through
a conductor, The reverse is also true: A conductor which moves across or
cutsa magnetic field develops a current. This is the principle of
Electromagnetic Induction, which is the process of developing a 
voltage in a wire that is either cutting or being cut by a magnetic field.
This idea is the basis for most electric Generatorsand similar power
sources.

        If you place two magnets near each other so that the two attract
(the N attracts the S and vice versa), and pass a length of wire between
the two, a voltage will be induced in the wire. There is a magnetic field
between the two magnets and passing the wire through the field or the field
across the wire produces an induced voltage in the wire.

        The strength of the induced voltage depends on several factors. Two
of these are the length of the conductor cutting the field and the rate at
which the magnetic lines are cut.

        The conductor cutting the field may have different shapes. It may
be a straight wire. More often, it is a coil or a loop or loops of wire.
No matter what the shape, the principle is the same. The longer the part
of the conductor cutting the magnetic field, the greater the voltage induced
in the conductor.

        The amount of induced voltage also depends on the rate of cutting of
the magnetic field. The rate of cutting depends on the speed of conductor
movement and the strength of the field. The greater the conductor speed, the
greater the induced voltage. Slowing down the speed of the conductor 
decreases the cutting rate, lowering the induced voltage.

        We can also change the induced voltage by changing the field strength
Changing the field strength actually changes the number of lines cut by
a moving conductor during a certain period of time. If we increase the 
field strength, there are more lines of force for a given area. In other
words, the lines are closer together. Packing more lines in the same space
means that the conductor will cut more lines for each inch of travel. The
more lines cut, the greater the induced voltage.

        Another factor which can affect the rate of cutting is the angle
at which the lines are cut. For the same rate of speed and distance moving
across the field at an angle produces less voltage.

        Moving the conductor back and forth across the field causes the
voltage being induced to be of a reversed polarity, if you cut the lines
of force in one direction, the polarity is one polarity and when you cut
the lines in the opposite direction, the polarity is of the opposite polarity.

        Considering these factors, these four factors control the amount
        of induced voltage:

        

        
The speed of the motion, the greater the speed the conductor
        cuts the magnetic lines, the greater the induced voltage.
        
The length of the wire that cuts the magnetic field. The longer
        the wire, the greater the induction. In the case of a coil, the
        more turns, the greater the induced voltage.
        
The strength of the magnetic lines, or the number of magnetic
        lines per unit area (the density). The stronger the flux, the 
        greater the voltage.
        
The angle of cutting the magnetic lines changes the induction.
        The more nearly the wire cuts squarely across the lines, the larger
        the induced voltage.
        

These factors can be summed up into one simple sentence which states:

        The Induced Voltage is Directly Proportional to the Rate
        of Cutting the Lines of Force. 

The term "rate" means the number of magnetic lines which are cut by the
conductor per second. The rate can be increased by increasing the speed 
of cutting, the lingth of the conductor cutting the field, the strength
of the flux, or the angle of cutting.

        An important point to remember is that the action induces a votage
in the coil or conductor, and if the conductor that cuts the magnetic flux
is a part of a complete electric circuit, then the induced voltage will
cause a current in the circuit. You may read a lot about induced current
but to be technically correct, it is a current caused by an induced voltage.

AC Generator:
        To put these basic ideas together, a simple explanation of an
ac generator is in order. An ac generator is often called an Alternator
The north pole of one magnet is placed near the south pole of another magnet.
A magnetic field exists between the two poles. The pole faces of the magnets
are formed so that conductors will easily turn between them. These magnets
are called field magnets and are referred to as the fields. This part of
the alternator is called the stator, the part that moves, or turns is called
the armature or rotor.
        The electrical circuit is connected to the armature and is made of 
many loops of wire which cut the fields of the magnet causing an induced
voltage called ac voltage. As one side of each loop passes through the
magnetic field an induced voltage gradually increases until it reaches
a maximum, then as it passes the peak it decreases until it reaches a
minimum at the same time as the other side of the loop of wire begins
to enter the magnetic field causing a voltage of the opposite polarity
to begin to increase. This rising and falling and changing of polarity
is demonstrated by the use of a graph called a sine wave. The number of
times this voltage rises and falls per second is called the cycle or
Hertz. The voltage entering the average home in the USA is called
120/240 volts 60 cycle ac. This is the method most used to generate
ac voltage and current, but there are other methods to create ac voltage
such as Inverters and Oscillators that use DC Direct Current to simulate 
ac current.
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