Question Video: Recognizing Alternating Current on Graphs of Current against Time | Nagwa Question Video: Recognizing Alternating Current on Graphs of Current against Time | Nagwa

# Question Video: Recognizing Alternating Current on Graphs of Current against Time Science • Third Year of Preparatory School

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The graph of current against time shows four different sources of current. Which one of the four lines does not represent an alternating current?

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### Video Transcript

The graph below of current against time shows four different sources of current. Which one of the four lines does not represent an alternating current?

Here, we are given a graph with four lines that represent four different sources of current. We are asked to look at them and pick out the one that does not represent an alternating current.

First, let’s discuss what an alternating current is. An alternating current is a current that has a varying magnitude. The direction of an alternating current repeatedly reverses, with equal time intervals between reversals of direction. Taking a look through the lines, all of them have a current value that changes with time.

Now, what about the other property of an alternating current, repeatedly reversing direction? On a current–time graph, a change of direction of current is represented by the lines showing the current changing from positive values to negative values or from negative values to positive values. As the horizontal axis of the graph is drawn at the zero value of current, crossing the horizontal axis must represent the current changing direction.

Now that we know how to identify a change in current direction from a line on the graph, let’s look through the lines and see whether any of them cross the horizontal axis. Starting with line one, we can see that it crosses the horizontal axis at intervals of 0.5 seconds. So this line does represent an alternating current. Taking a look at line two, we see that it does not cross the horizontal axis. It just repeatedly touches the horizontal axis. The value of current is always either positive or zero, never changing from positive to negative. So line two does not represent an alternating current.

Just to be safe, let’s take a look at the other two lines. Line three and line four both cross the horizontal axis at regular intervals of one second. So both of them represent alternating currents. We see then that it’s line two that does not represent an alternating current.

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