Video Transcript
This graph of current against time
shows five different sources of current. Which of the lines represent direct
current? Is it (A) line three and line four;
(B) line one and line two; (C) line five only; (D) line three, line four, and line
five; or (E) line one and line five?
In this question, we can see a
current–time graph on which there are five lines representing currents in the same
time interval. We are asked which of these lines
represent direct currents. 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 current. A positive-valued current and a
negative-valued current are in opposite directions.
We can eliminate any line that
shows both positive and negative values of current as these would correspond to
currents that change direction. Any line that crosses the
horizontal axis can then be eliminated, so we can eliminate line three and line
four. Line five has only negative values
of current and does not cross the horizontal axis. A direct current can be either only
negative valued or only positive valued. We cannot eliminate line five just
for being negative valued.
Consider though that a direct
current does not change in value. We see that the value of the
current represented by line five starts to become closer to zero at a time of
approximately 3.8 seconds. At this time, line five is not
horizontal. Because line five is not horizontal
throughout the whole graph, line five is not a direct current, and we can eliminate
this line.
Both line one and line two are
horizontal throughout the whole graph. We cannot eliminate line one or
line two. So the only lines on the graph that
represent direct currents are line one and line two. The correct answer is option
(B).
