Video Transcript
Which of the following correctly
shows an alternating current on a graph of current against time? Is it option (A), (B), (C), or
(D)?
In this question, we must determine
which of the current–time graphs shows an alternating current. Recall that an alternating current
has a periodically changing direction. For each of the graphs, notice that
the horizontal axis shows values of time, while the vertical axis shows values of
current.
It is also important to notice that
the vertical axis of the graph extends above and below the horizontal axis. Values of current above the
horizontal axis are positive, and values below the horizontal axis are negative. The sign of a value of current
corresponds to the direction of the current. A positive-valued current and a
negative-valued current are in opposite directions.
Knowing this, let’s analyze the
current–time graphs that we have been given. Graphs (B) and (D) can be
immediately eliminated because their lines never cross the horizontal axis to become
negative. And thus, the lines cannot show an
alternating current.
Similarly, graph (C) can be
immediately eliminated as its line never crosses the horizontal axis to become
positive.
For graph (A), however, there are
alternating positive and negative values of current. This means that the current
periodically changes direction. So we know that graph (A) properly
shows alternating current. Therefore the correct answer is
(A).
