Question Video: Identifying a Graph of Alternating Current against Time | Nagwa Question Video: Identifying a Graph of Alternating Current against Time | Nagwa

# Question Video: Identifying a Graph of Alternating Current against Time Science • Third Year of Preparatory School

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Which of the following correctly shows an alternating current on a graph of current against time? [A] Option A [B] Option B [C] Option C [D] Option D

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### 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 properly shows an alternating current. Recall that an alternating current has a periodically changing direction. On each graph, the horizontal axis 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. The sign of a value of current indicates the direction of the current. A positive-valued current and a negative-valued current are in opposite directions.

Now let’s use this information to analyze the current–time graphs that we have been given.

We can immediately eliminate graph (A) as the line never crosses the horizontal axis. This means that the current never changes direction to become negative, so we know that the graph cannot represent an alternating current.

Similarly, we can eliminate graph (D) as the line on this graph also never crosses the horizontal axis. The current never changes direction to become positive, so it is not an alternating current.

For graph (B), we can see that there are alternating positive and negative values of current. Is graph B showing alternating current then? Well, if we look at the graph carefully, we can see that in the time interval between one second and two seconds, the value of the current is zero. Such intervals of zero current recur periodically. And in those intervals, the current takes on a constant value for an entire second. An alternating current should have a value that’s always changing and so is never constant. Thus, we know that graph (B) does not show an alternating current, so we can eliminate this option.

For graph (C), notice that the value shown by the line is always changing. Here, the line shows alternating values of both positive and negative current. In order to do this, the line must cross the horizontal axis. So notice that the value of the current is zero at zero seconds, one second, two seconds, and so on. But the value of current is zero only for the instant in which the direction of a current changes.

Therefore, graph (C) correctly shows alternating current, so this is the correct answer.

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