Lesson Video: Alternating Current Physics • 9th Grade

In this lesson, we will learn how to identify alternating currents and calculate the properties of alternating currents.


Video Transcript

In this video, we’re talking about alternating current. And as we’ll see, this is current in an electric circuit that changes direction. Today, we find this type of current in most residential and commercial applications. The kind of electric current we can expect to get out of the wall is alternating current.

Now, before getting into current that changes direction, let’s consider a simpler type. Say that we have this electrical circuit here. The circuit consists of a cell as well as a resistor. This cell will start to power the flow of current through this loop. And if we’re thinking of what’s called conventional current, that is the flow of positive charges through a circuit. Then, that current will start out at the positive terminal of our cell, and it will move clockwise through the rest of the circuit.

Looking over at this two-dimensional plot we’ve created, on the vertical axis we have current, and on the horizontal axis we have time elapsed. Now, let’s say that at time equals zero, and our initial time, that’s when this cell is powered on and current starts to flow in the circuit. From the very beginning, from the outset, the current in this circuit will be at some particular value, and it will stay the same as time passes. Not only does the magnitude, or the overall value, of our current always stay the same, but we know that current will always flow in the same direction in the circuit. It will always move clockwise.

Electrical current that always flows the same way through a circuit is known as direct current. Now, here’s a question. What if we went over to the circuit and we flipped our cell around? That is, we reversed it, so the longer side was on the left and the shorter side was on the right. If we did that, then current would no longer be flowing clockwise in this circuit. But rather it would be going the opposite way. Now, based on the direction of our cell, current would be flowing counterclockwise, or anticlockwise.

Here’s how we could show this on our graph of current versus time. What we could do is extend our vertical axis downward so that now current can have negative values. After all, if we decided that current moving in a clockwise direction is positive current, then that must mean current flowing the other way is negative. With our cell flipped around, and say that occurred at this particular time value, then from then on, as the current flowed in its new direction, it would look like this on our graph. The magnitude, or overall value, that the current has is the same as it was before but now it’s just moving in the opposite direction. And so, it has the opposite sign.

Now, if we look at the overall picture of current in this graph, we can no longer call it direct current because, after all, it changes direction. We see that happens right here at this time value where the current switches from positive moving one way to negative moving the other way. But then, if it’s no longer direct current, what name can we give it?

To figure that out, let’s think about this. What we’re seeing in our graph is that after this amount of time has elapsed, the current in the circuit changes direction. And we can give this time amount a name. Let’s just call it 𝑡. Now, what if the current in our circuit switched directions every time 𝑡 seconds had passed. In other words, if we went another 𝑡 seconds out on our time axis, then we would see the current in the circuit do this, stay down here. But then, at this particular time value, it would reverse direction again so that now the current would look like this in the circuit.

And let’s say that this pattern continued. Every 𝑡 seconds of time that pass, the current switches direction, from positive to negative or negative to positive, back and forth. In that case, what we would be looking at in our graph is current that periodically changes direction. And that word, periodically, is important. It means that the changes in current direction happen over the same time intervals each time. Once we know the period of these changes, we could predict which way the current is flowing at a given time. And with the current reversing direction periodically, that means it spends just as much time moving in one direction in the circuit, say clockwise, as the opposite direction, counterclockwise.

The name for current that behaves this way is alternating current. Now, it’s important to point out that the way we’ve shown alternating current in this graph is a bit different from the way we would often find it. To see what that looks like, let’s shift our circuit a bit out of the way. And let’s also replace our cell with this power supply. This symbol that we see here, which looks like a little wave, indicates the kind of current that this power supply will give to the circuit. This is known as an alternating current, or AC, source.

As we consider how this source creates current in the circuit, let’s again call current that travels in the clockwise direction positive. And current that goes the opposite way, we’ll call negative. Now, the first interesting thing that we see about an alternating current source, when we start to track the current it creates over time, is that it’s not an on or off device, like our cell was. With our direct-current-supplying cell, the current was either on or off. And the same thing was true even if we flipped the cell’s polarity so that it was sending current the opposite way. Once again, the current was either on or off.

But moving to an AC, an alternating current, supply, we’ll see current that exists over a range of values. At the initial time value, we’ll call it time equals zero, the current in the circuit is also zero. And then, as time passes, the current starts to ramp up; its values become greater and greater.

Here’s one way to think of what’s happening then in our circuit. At time 𝑡 equals zero, current turns on and it starts flowing in the clockwise direction. As time passes, the current stays moving in that same direction, but its magnitude increases. And we could show that by increasing the number of arrows in our circuit. So, the direction of the current is not changing, but its strength is. And we see that happening on our graph as the current reaches greater and greater positive values.

But then, here’s what happens next. Our alternating current source continues to supply current in the same direction clockwise. But the strength of that current, its magnitude, begins to decrease. We could indicate this on our sketch by erasing some of the arrowheads, indicating an overall weaker current magnitude. The current is still moving in the same direction, but it’s just not as strong. And this decreasing in strength continues until the current reaches a value of zero.

At this single moment, we have no arrow heads on the current in our circuit because, technically, at that instant, the current is zero. But it doesn’t stay that way for long. Our source continues to supply current then, but this time in the opposite direction. Now, the current is moving counterclockwise, what we’ve called the negative direction. And then, this current too increases in magnitude, that is, gets more and more negative up to a certain point.

Then, when it reaches this point, it stays in that same direction, counterclockwise, but begins to become less negative over time. The current supplied by our source is becoming more and more positive until it reaches, once more, a value of zero. And then, we can see what will happen next. Our source will continue to supply current. But now that current will be in the positive, or clockwise, direction. And so, the cycle continues.

Now, this pink curve that we’ve drawn in on our axes is characteristic of alternating current in a circuit. The current rises in one direction and then descends in that direction back to zero. Then, it switches direction, we could call that reversing polarity, gets to a maximum value in that direction, and then comes back to zero once more. And this continues on in a wave shape that looks a lot like a sine curve. And the fancy name for a curve that looks like a sine is sinusoidal.

Looking at this sign curve, there are a couple of things we can notice. First off, there is a peak value in current that this curve reaches. And the peak value of positive current in our circuit is the same in magnitude as the peak value of negative current. So, for example, if our peak positive current was at a value of two amperes, then that would mean that the lowest point on our curve is at a current value of negative two amperes.

Another thing to notice about this graph is that every time it crosses the horizontal axis, the current is changing direction from positive to negative or negative to positive. Looking near this first crossover point, we see that up till this point, the current is positive. But then, here it becomes negative. And then, at the next crossover point, it’s negative, and it becomes positive.

Knowing all this about alternating current, and seeing how much more complicated it is than direct current, we may wonder why AC, alternating current, is the standard type of current we find in our buildings. The reason for that has to do with the challenges related to transferring electrical power from the power plant where it was generated to the place where it’s applied. Alternating current, it turns out, lets us make that transition in a way that’s much more effective and efficient than direct current. To get some practice with these ideas we’ve seen so far, let’s look at an example exercise.

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.

Let’s take a moment now to summarize what we’ve learned about alternating current in this lesson. Starting out, we saw that direct current, sometimes called DC, is current that flows in one direction only. Building on this, we learned that alternating current, sometimes called AC, is current that periodically changes direction. Lastly, we saw that alternating current looks like a sine wave when it’s plotted, remember that word sinusoidal, where changes in the direction of the current are indicated by changes in sign, s i g n. That is positive or negative signs.

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