Question Video: Identifying Direct Current on a Graph of Current Against Time | Nagwa Question Video: Identifying Direct Current on a Graph of Current Against Time | Nagwa

Question Video: Identifying Direct Current on a Graph of Current Against Time Science • Third Year of Preparatory School

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This graph of current against time shows five different sources of current. Which of the lines represent direct current?

03:09

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).

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