Lesson 1/Learning Event 2
This can be illustrated if we compare AC to the swinging pendulum of a clock. Current, like the
pendulum, starts from one extreme point to the right or left and very slowly builds up to fastest
speed at about one-half of its travel. Then, its speed begins to fall off until it comes to a complete
stop for just an instant before swinging again in the opposite direction. AC is defined as a current
that is constantly building up or falling off and periodically changing direction.
When the electron flow returns to the same point it started from, just as the pendulum completes
its swing to and fro, this completes one cycle. The number of cycles that occur each second is
known as the frequency of the current. The current supplied to your home, in this country, has a
frequency of 60 cycles per second. Lately, the term "Hertz" is often used instead of cycles. So
remember that both "60-Hertz AC" and "60-cycle AC" mean the same thing.
Electric current that flows in one direction only is called DIRECT CURRENT. The term direct
current is commonly shortened to DC. Batteries cannot reverse the electrical charge, as must be
done to produce AC, so they always supply DC. Since automotive vehicles use batteries, most
electrical circuits that you will be working on use DC.
In automotive electricity, we are sometimes faced with the problem of changing AC to DC. For
instance, if the vehicle is equipped with an AC generator, that current cannot be used to charge the
vehicle battery. In this case, the AC must be changed to DC.
Parts called RECTIFIERS or diodes are used to change AC to DC. A rectifier works like a one-
way check valve in a water system. Water or current can flow through the check valve or rectifier
in one direction, but flow is blocked in the opposite direction. Rectifiers are made from materials
that are constructed in such a manner that the atoms allow electrons to move in one direction but
not the other.