How is an alternating current produced ?
An alternating current is produced by an electric generator. An electric generator
consists of a magnet
and a loop of wire which rotates in the magnetic field of the magnet. As the
wire rotates in the magnetic field, the changing strength of the magnetic field
through the wire produces a force which drives the electric charges around the
wire. The force initially generates an electric current in one direction along
the wire. Then as the loop rotates through 180 degrees the force reverses to
give an electric current in the opposite direction along the wire. Every time
the loop rotates through 180 degrees the direction of the force and therefore
the current changes. The changing direction of the force after every 180 degrees
of rotation gives the alternating current. As well as having the magnet and
wire an electric generator also has slip rings which make sure that the ends
of the wire are always connected to the same side of the electric circuit. This
makes sure that the direction of the current changes every half revolution of
the wire.
The diagrams below show how the force that produces the current in the wire
varies as the loop of wire rotates through the magetic field. The force is given
a special name, it is called the "electromotove force" or e.m.f. It is similar
to the potential difference that gives the flow of electrons in dc currents
and is also measured in Volts.
Lets now consider what happens to the current in the loop of wire as the wire rotates through the magnetic field. First take the magnetic field to point from the left to the right and the loop of the wire to be initially at right angles to the magnetic field. We will consider the current at the bottom end of the wire labelled B. The loop of wire is rotated in a counter clockwise direction so initialy the end B is moving parallel to the magnetic field. The direction of movement is given by the blue arrows. When the wire is moving parallel to the field there is no emf or electrical current.
As the loop rotates counter clockwise the bottom end moves up and the movement of the loop is now at an angle to the magnetic field. When the movement of the loop is at an angle to the magnetic field an e.m.f. is produced and an electrical current flows through the loop. At the point B the e.m.f produces a current that points out of the screen (at T the e.m.f produces a current pointing into the screen, since both ends of the loop are moving in opposite directions). As the loop moves counter clockwise it becomes aligned with the magnetic field and the movemement of the loop is now perpendicular to the field. At this point the emf becomes it largest and the current is its strongest. As the loop continues to rotate in the clockwise direction the movement becomes more aligned with the field and the emf and current gets smaller. When the loop is again in the vertical position with B at the top of the diagram the direction of rotation is aligned with the magnetic field and the emf and current drops to zero. The loop has rotated through 180 degrees.
As the loop rotates past 180 degees the loop is again moving at an angle to
the magentic field and a current is generated. However this time the force that
creates the current, points in the opposite direction since the end of the loop,
B, is moving down and not up. Since the e.m.f is in the opposite direction the
current at B now points into the screen. The current at B (and also T) has
alternated in direction and we now have our alternating current. The current is
again its strongest when the loop is parallel to the field and the rotation is
perpendicular. As the loop rotates back to and through the starting position the
current again drops to zero and reverses direction. This process continues while
the loop continues to rotates and only stops when the loop becomes stationary.
The faster the loop rotates the larger the force on the electric charges and the
larger the electric current.