Direct current is like water supplied to a house. The water enters the
house through the supply pipe, is distributed to the kitchen and
bathrooms, then the waste water goes down the drain and all collects in
the one sewer pipe. The water flows in one direction only; in the
supply pipe and out the sewer pipe. This is like direct current (DC).
Electricity is delivered in much the same way. If you look at a modern
wall socket (in the U.S.) you will see three contact holes: a long one,
a short one and a round one. Essentially, the electricity comes out of
the short contact and exits through the long contact; the short contact
is like the faucet and the long contact is like the drain.
This is not the whole story because the electricity delivered on the
power grid is alternating current (AC). This means that the direction
of current flow repeatedly changes. For half of the time electricity
flows from the short contact to the
long contact. For the other half of the time it flows from the long
contact to the short contact. Each time the current goes back and forth
is called a cycle. When the current flows one way that is called a half
cycle. When it reverses and flows the other way that is the other half
cycle.
Current
flow for one half-cycle
The
other half-cycle
Alternating
current reverses
direction many times every second. Half the time the current flows out
of the short contact and into the
long contact. The other half of the time it flows out of the long
contact into the short contact. This is alternating current.
In the above diagram the line representing current is broken on the
left side to emphasize that the long contact (connected to the
“neutral” wire) remains at zero volts (referenced to ground). The short
contact (connected to
the “hot” wire) alternates between positive and negative voltage
(review Polarity
in DC circuits). From this frame of reference the short
contact appears to alternately push out current then suck it back in.
The long contact appears to give no impetus to the flow of the current.
Recall the breathing analogy from Polarity
in DC circuits. The short contact
can be imagined as breathing electricity in and out.
If you imagine the analogy of water delivered to a house, half the time
the water comes out the faucet and goes down the drain. The other half
of the time the faucet sucks water back out of the drain.
Rotating Battery Model
One way to visualize
alternating current is with a rotating battery. The straight arrow
shows the direction of conventional current flow as the battery rotates.
Another way of looking at alternating current is as alternating
voltage. Think of it as a battery that rotates. Half of the time the
battery is oriented one way and half of the time it is oriented the
other way. Consequently, the current that flows through the circuit
will alternate directions.
We usually think of alternating current as the kind of electricity that
comes form the power receptacle. However, we find alternating current
in many circuits. When sound pressure is converted to electrical
impulses by a microphone, the result is alternating current. The output
from a microphone can change as slowly as 20 times per second and as
fast as 20,000 times per second. This is called audio frequency
AC or simply AF. Radio circuits also work with alternating current. The
main difference is that the current may change direction hundreds of
thousands of times per second or as often as several billion times per
second. Circuits that work at these frequencies are called radio
frequency or RF circuits. AF and RF circuits will be discussed later.
In the following pages we will be examining AC from the power grid.
However, the rules for alternating current apply to all frequencies.
