### Video Transcript

In this video, we will learn what
the main types of electric current are and what sources produce them. We’re going to see that there are
two main types of electric current, and each is produced in different ways.

To get started, imagine that we
have a cell as well as a light bulb. And the cell and the bulb are
connected by wires to make a completed circuit. When this happens, electric charge,
electrons, will flow through the wire. As these charges flow through the
bulb, they make it light up and then continue on through the rest of the
circuit. As long as this cell is not
depleted, charge will continue to flow in the circuit. It will always travel in the same
clockwise direction, and the size or the magnitude of the current will also stay the
same. The current provided by this cell
is one type of electric current.

In order to understand it better,
say that we attached an ammeter, a device for measuring current, to our circuit. We can do this by opening up a gap
in the circuit and then closing that gap by placing the ammeter there. Now, all the current in the circuit
must pass through our current measurement device. If we watch our ammeter and wait,
we’ll see that this needle does not move. That tells us two things about this
electric current. First, it consists of charge that
always flows in the same direction in the circuit. If the current changed direction,
then sometimes the needle would point on this side of zero and other times it would
point on this side. So the fact that the needle is
always on the same side of zero means that charge is always traveling in the same
direction in this circuit, in this case in a clockwise direction.

The second thing we can learn from
this needle staying still is that, as we said, the current always has the same size
or magnitude. Not only does charge flow clockwise
in the circuit, but the same amount of charge passes any given point in the circuit
any given second. We could use our ammeter
measurements to make a plot of current against time. We’ll say that when charge flows
clockwise — that is, the needle on our ammeter is to the right of zero — that that
is positive current. At the initial instant in time, our
current then has some positive value. Let’s say that value is here on the
vertical axis. Current is measured in units of
amperes, so this will be some number of amps of current.

We’re more interested here, though,
in the shape that this graph will take. Since the needle on the ammeter
doesn’t move over time, this means at any other point in time the current in the
circuit will have the same magnitude. This means we can extend this point
so that it becomes a horizontal line. Current that always points the same
direction and always has the same magnitude is called direct current.

Direct current is one of two main
types of electric current. Direct current is generated by
cells as well as batteries. Now, let’s say we break apart our
circuit by removing the cell. The reading on our ammeter returns
to zero and there’s no more charge flow. Say then that we get a bit of
equipment called a dynamo. This is a type of electrical
motor. Connecting our wires to the dynamo,
once more we have a closed circuit. Charge will flow in the circuit,
but as we’ll see not like it did before.

Keeping an eye on our ammeter
needle, we see that at the instant the wires are connected to the dynamo, the
reading stays at zero. But then, due to work done by the
dynamo, charge flows and the needle swings to the right. However, it doesn’t stay there. A short time later, the needle
moves back to zero. Then, it moves past zero, swinging
to the left. When the needle points to the left
of zero on our ammeter device, that means electric charge in our circuit is now
flowing in a counterclockwise direction. An instant in time later, the
needle returns to zero and then moves past zero to the right so that once more
current is traveling in a clockwise direction.

Every time the ammeter needle is to
the right of zero, to what we’ve called positive current, charge flows through the
circuit in a clockwise direction. When the needle points on the other
side of zero though, to what we can call negative current, charge now flows through
the circuit in the counterclockwise direction. When a dynamo supplies current to a
circuit, that current changes direction over and over and over again.

To plot the current measured by the
ammeter now, we’ll want to extend our vertical axis below zero. This is because, as we said,
whenever our ammeter needle points to the left of zero, we’re calling that negative
current. With this dynamo then, our current
goes back and forth from positive to negative to positive and so on. If we begin our plot when the
current measured is zero right here and then track the current from that instant up
until the needle is as far right as it gets on our ammeter, the plot of current
against time for that interval would look like this. At this point, though, the needle
begins to swing back to zero. When it gets back to zero, our
graph would look this way. Then our needle swings to the left
past zero; this is where current is negative. So our section of graph will look
this way.

We remember that for our circuit,
negative current means current in the counterclockwise direction. Positive current is in the
clockwise direction. Once the ammeter needle has gone as
far left of zero as it ever goes, then it swings back to zero. Our graph now looks like this. And now, this cycle begins to
repeat. The ammeter needle swings to the
right again, then back to zero, then to the left, and once again back to zero. This type of current we’re seeing
on our graph is called alternating current. We can see how it gets this
name. It’s constantly changing or
alternating direction. At every instant when this current
has a positive value, we know it’s traveling in the clockwise direction. On the other hand, when the current
value is negative, it points counterclockwise.

There are a couple of things to
notice about this alternating current graph. First, the maximum value of
positive current here has the same magnitude as the maximum negative current. At each of these instants in time,
the magnitude of this current is the same. The second thing to notice here is
that the amount of time it takes for current to change direction is always the
same. That is, this amount of time right
here, where the current is always positive pointing clockwise, is equal to this
amount of time right here. And that’s equal to this amount of
time here and this amount of time here and so on. Because these intervals of time are
the same, we say that alternating current changes direction periodically. That is, it does so at a steady
constant rate. Knowing all this about alternating
and direct current, let’s now look at a few examples.

Which of the following sentences
correctly describes direct current? (A) Direct current has a variable
magnitude and always has the same direction. (B) Direct current has a variable
magnitude and its direction can reverse. (C) Direct current has a constant
magnitude and its direction can reverse. (D) Direct current has a constant
magnitude and always has the same direction.

Clearing some space at the top of
our screen, say that we have a graph of current, that’s 𝐼, plotted versus time. We’ve set up our graph so that
current can have positive as well as negative values. This just means that the current
can change direction. In other words, charge would flow
in different directions in a circuit. If we were to plot a current
described by answer option (A) on our graph, that current might look like this. Say it starts out positive and then
changes in value, that is, changes in magnitude. But since the current is always the
same sign, positive, it always points in the same direction.

Next, let’s plot the current
described in option (B). Current with a variable magnitude
whose direction can reverse might look like this. Sometimes this current is positive,
and sometimes it’s negative. That shows that it changes
direction. We also see that the magnitude or
size of this current changes over time; it’s variable.

If we were to make a curve
described by answer option (C), that could look like this. The magnitude or size of this
current is always the same, it’s constant, but its direction does reverse. Finally, let’s make a curve for
answer option (D). A current with constant magnitude
always pointing in the same direction could look like this. It appears as a horizontal line,
and this is a sign that this current is direct current.

The two conditions for a current to
be direct current are that its magnitude must be constant and it always has the same
direction. We choose answer option (D).

Let’s look now at another
example.

Which of the following sentences
correctly describes alternating current? (A) Alternating current is a
current that never reverses direction. (B) Alternating current is a
current that periodically reverses direction. (C) Alternating current is a
current that can sometimes reverse direction. (D) Alternating current has a
constant magnitude and always has the same direction.

The first thing we can say about
alternating current is that the name comes from the fact that it does indeed change
direction. Considering a closed loop of wire,
if alternating current existed in this wire, sometimes the charge would flow in this
direction, and sometimes it would flow the opposite way. That’s what it means for current to
change direction.

Reviewing our four answer options,
option (A) says that alternating current never reverses direction and option (D)
says that alternating current always has the same direction, in other words that it
doesn’t reverse direction. Neither of these answer options is
correct. Alternating current does change
direction; that’s how it gets its name.

Both of our remaining choices
describe current that either does change direction or current that can. Not only does alternating current
change direction, but it does so at even intervals of time. This means that if alternating
current, say, spends five seconds traveling in a clockwise direction, then it will
spend the next five seconds traveling in a counterclockwise direction and then five
more seconds in a clockwise direction then five more in a counterclockwise. It goes back and forth like this,
and it does so periodically. This is another way of saying the
current changes direction at even intervals of time. For our answer, we choose option
(B). Alternating current is a current
that periodically reverses direction.

Let’s look now at one last
example.

The graph below shows the electric
current over time for two different circuits. Which one of the following
sentences is correct? (A) Line one represents an
alternating current and line two represents a direct current. (B) Line one represents a direct
current and line two represents an alternating current. (C) There is not enough information
to tell which line represents which type of current.

Looking at the graph given, we see
line one is the line in red and line two is that in blue. Focusing first on line one, we see
that this line has both positive values of current as well as negative values. This tells us that the current
represented by line one changes direction. Along with this, if we start from a
time of zero seconds, we can see that the amount of time for which the current is in
the positive direction — that’s this time interval here — is equal to the amount of
time the current then switches to point in the negative direction. This pattern then repeats for the
rest of the changes in direction of the current represented by line one. We can say then that this current
reverses direction periodically. That tells us that line one
represents alternating current.

Next, let’s consider line two, the
blue line on our graph. If we were to trace over this line,
we would see that it always has the same positive value of current. This means the current always
points in the same direction, and it always has the same magnitude. That tells us the current
represented by line two is direct current. Answer option (A) then is the one
we’ll choose. Line one on this graph represents
an alternating current and line two represents a direct current.

Let’s now finish our lesson by
reviewing a few key points. In this video, we saw that the two
main types of current are direct current and alternating current. Direct current has a constant
direction and magnitude and is supplied by cells and batteries. Alternating current is current that
periodically reverses direction and is created by a device called a dynamo. This is a summary of types of
electric current.