Video Transcript
Which of the following correctly
shows a direct current on a graph of current against time? Is it graph (A), (B), (C), or
(D)?
This question asks about direct
current. We must determine which of the
current–time graphs shows a direct current. We recall that a direct current has
a constant value and direction. The horizontal axis of the
current–time graph shows values of time, while the vertical axis shows values of
current. It is 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
values.
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. A direct current can be either only
negative valued or only positive valued. If the line on a graph is
horizontal throughout the graph, the graph shows a direct current. So let’s analyze the current–time
graphs that we have been given. We can immediately eliminate graph
(C) as the line on graph (C) is not horizontal at any point. The line also crosses the
horizontal axis and so shows a current that changes direction. We can also eliminate graph (A) as
the line on graph (A) is not horizontal throughout the graph. At a time of 3.5 seconds, the value
of the current starts to change, approaching zero.
Graph (B) has a set of broken
lines. Each line is horizontal. Each line is positive valued. Is this then a direct current? Looking closely at the graph, it
can be seen that there’re in fact no lines connecting two neighboring horizontal
lines. For example, at a time of one
second, a current of 1.6 amperes changes to a current of 0.8 amperes. This means that the current changes
from 1.6 amps directly to zero and then changes from zero directly to 0.8 amps. These changes occur in a very short
time interval, too short to be visible on the graph. We see then that the value of the
current changes at a time of one second. The current therefore does not have
a constant value and so is not a direct current. We can eliminate graph (B).
Graph (D) shows a line that is
horizontal throughout the graph. This represents a direct current,
and as such graph (D) is the correct choice.
