Video: Describing the Definition of Alternating Current

Which of the following statements correctly describes what is meant by the term “alternating current”? [A] An alternating current is an electric current that varies periodically but does not change direction. [B] An alternating current is any electric current that changes direction at least once. [C] An alternating current is an electric current that gradually decreases over time. [D] An alternating current is an electric current which periodically reverses direction. [E] An alternating current is an electric current that gradually increases over time.

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Video Transcript

Which of the following statements correctly describes what is meant by the term alternating current? A) An alternating current is an electric current that varies periodically but does not change direction. B) An alternating current is any electric current that changes direction at least once. C) An alternating current is an electric current that gradually decreases over time. D) An alternating current is an electric current which periodically reverses direction. E) An alternating current is an electric current that gradually increases over time.

Okay, looking at this question, we see that what we’re after is the correct definition of the term alternating current. One way to figure this out is to draw a miniature set of axes, say these will be our axes right here, where on the vertical axis we’re plotting current, and on the horizontal we’re plotting time.

Now, if we had a circuit with an AC, alternating current, power supply, and we were to plot the value of that current over time on these axes, then that curve might look something like this. It would start out at zero and then go up to a certain value and then come back down to zero then go into the negative values and then come back up to zero once more. And then, following this pattern, the current would once more go up and back down, then down, then back up, and so forth and so on. What we see is this wave pattern. In fact, it looks like a sine wave.

Looking at this graph, there are a couple of things we can notice. First, we see that the current sometimes has a positive value and it sometimes has a negative value. We know that physically what this corresponds to when current changes sign is that it’s moving in a different direction in a circuit. For example, if positive current represented current flow in a clockwise direction through a circuit, then when the current switches over to negative, that means it’s going the opposite way, counterclockwise.

Along with this, we can notice that this change from positive to negative current and back again happens over and over in a regular way. That is, once the current has started out from zero, gone to its maximum positive, back to zero, its maximum negative, and then back to zero again. That pattern is then repeated. Once more, it goes up, back to zero, down, and back to zero once more. And this happens over and over and over again. If our graph extended out farther to the right, we could see that. Each of these cycles that the current goes through shows us that the current in an AC circuit varies in a predictable way. It varies periodically.

Recalling all this about alternating current, let’s revisit our answer options and see which one matches what we’ve seen. Option A says that an alternating current is an electric current that varies periodically but does not change direction. We’ve seen from our graph that, indeed, alternating current does vary periodically. But it also changes direction. It goes from positive to negative and back again. So, this last part of the definition in option A is incorrect, which makes this option, overall, one we won’t choose.

Moving on to option B, this says, an alternating current is any electric current that changes direction at least once. Well, it’s true that alternating current does change direction, and it does that at least once. But from our graph, we see that there’s more to it. For alternating current, the change in current direction happens periodically in an even, regular way. Option B doesn’t mention the periodic behavior of alternating current, so we won’t choose that as our answer either.

Then, option C, an alternating current is an electric current that gradually decreases over time. Well, looking once more at our curve, we see that indeed there are periods where this curve is decreasing, where it’s getting smaller in value. But that’s not the only way it behaves. There are also sections of this curve where the current is increasing. And in fact, there’s an even mix between increasing current values and decreasing. Option C leaves out those increasing values, and so we won’t choose that as our answer either.

Option D says that an alternating current is an electric current which periodically reverses direction. Considering this definition, we see that it’s a match for the curve that we’ve drawn. It talks about periodic behavior, and it also says that the current reverses direction periodically. That’s exactly what we see in our curve, with the current going from positive to negative to positive and back again. Since this option describes current that reverses direction and does so periodically, it looks like option D may be our answer.

Just to make sure, though, let’s consider our last choice, option E. This says that an alternating current is an electric current that gradually increases over time. Just like we saw with answer option C, there is some truth to this description. There are times over which an alternating current is increasing in value, but there are also times where it’s decreasing. Option E doesn’t account for those decreasing values and so it’s not a complete description of alternating current. We come back then to option D, which is indeed our final choice. An alternating current is an electric current which periodically reverses direction.

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