### Video Transcript

The graph below of current against
time shows four different sources of current. Which one of the four lines does
not represent an alternating current?

Here, we are given a graph with
four lines that represent four different sources of current. We are asked to look at them and
pick out the one that does not represent an alternating current.

First, let’s discuss what an
alternating current is. An alternating current is a current
that has a varying magnitude. The direction of an alternating
current repeatedly reverses, with equal time intervals between reversals of
direction. Taking a look through the lines,
all of them have a current value that changes with time.

Now, what about the other property
of an alternating current, repeatedly reversing direction? On a current–time graph, a change
of direction of current is represented by the lines showing the current changing
from positive values to negative values or from negative values to positive
values. As the horizontal axis of the graph
is drawn at the zero value of current, crossing the horizontal axis must represent
the current changing direction.

Now that we know how to identify a
change in current direction from a line on the graph, let’s look through the lines
and see whether any of them cross the horizontal axis. Starting with line one, we can see
that it crosses the horizontal axis at intervals of 0.5 seconds. So this line does represent an
alternating current. Taking a look at line two, we see
that it does not cross the horizontal axis. It just repeatedly touches the
horizontal axis. The value of current is always
either positive or zero, never changing from positive to negative. So line two does not represent an
alternating current.

Just to be safe, let’s take a look
at the other two lines. Line three and line four both cross
the horizontal axis at regular intervals of one second. So both of them represent
alternating currents. We see then that it’s line two that
does not represent an alternating current.