### 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.