Video Transcript
In this video, we will learn how to
use ammeters in electric circuits to measure the current through a component in the
circuit.
An ammeter is a device used to
measure the electric current in a circuit. It often looks like a box with a
dial on the front and two terminals into which we can connect wires in order to
connect our ammeter into the circuit. Occasionally, we may come across a
digital ammeter, which looks quite similar in many ways; it’s got the two terminals,
for example. But instead of a dial, it’s got a
digital screen where it shows the ammeter’s readout. In both cases, though, the capital
A on the device tells us that we are working with an ammeter rather than any other
device.
Now the way to use an ammeter is to
connect it into a circuit. For example, here we see a circuit
consisting of a battery and a bulb and the ammeter. And immediately, we can see that
the dial in pink has shifted to a new position. This style is now telling us that
the current in our circuit is five amperes. And so, if we were conducting an
experiment with this ammeter, then we would say that the current in the circuit, as
measured by our ammeter, is five amperes. Remember that the unit of electric
current is the ampere or amp.
Now, it’s very important to note
that our ammeter must be connected in series with the other components that we’re
trying to measure the current through. To understand what we mean by this,
let’s consider the current through the circuit that we’ve drawn here. Let’s think about the current
starting at the positive terminal of our battery. The direction in which our charge
flows is counterclockwise going through our bulb, thus lighting it up, and then
continuing to travel counterclockwise through our ammeter out the other side and
then back into the negative terminal of our battery.
In this circuit, our ammeter is
indeed connected in series with the battery and the bulb because all of the charge
that flows, say, through our bulb ends up also flowing through our ammeter. And hence, our ammeter is not, for
example, connected on a separate branch of the circuit. It’s on the same branch.
To see this even more clearly,
let’s consider circuit diagrams. Let’s start by recalling that this
is the circuit symbol for a battery. It looks like multiple cells
connected together with some dots in the middle. And then we can draw the wire that
connects our battery to the bulb, which is this piece of wire in our circuit, after
which we can recall that this is the circuit symbol for a bulb. It’s a circle with a cross through
it. And finally, we complete our
circuit diagram by adding in the ammeter, at which point we see that the circuit
symbol for an ammeter is a circle with a capital A inside it.
So once again, on this diagram, we
can follow the movement of charge throughout the circuit. In other words, we can consider the
current through the circuit. And starting at the positive
terminal of our battery, we once again go counterclockwise through the bulb, which
causes it to light up, of course, and continue to go counterclockwise through the
ammeter and then all the way back around to the negative terminal of our
battery. We can see that all of the charge
that’s traveling through our battery and our bulb does indeed end up going through
the ammeter, which is why we can say confidently that this ammeter is connected in
series.
However, if we were to connect our
ammeter into the circuit like this, for example, then we’d see that as soon as the
current got to this junction here, it would have to split. Some of it would go this way, and
some would go this way into the ammeter. In this scenario, we can say that
the ammeter is connected in parallel with the bulb rather than in series. And in this situation, our ammeter
would not work because it’s not connected properly. And so, for an ammeter to work
properly, it must be connected in series.
Another important thing to note is
that an ammeter can often look very similar to some other devices. The most common culprit is the
voltmeter. The voltmeter can have many
very-similar-looking features, for example, a dial and two terminals colored the
same way. And sometimes, the only real
obvious difference is the fact that a voltmeter has a V on the front, whereas an
ammeter has an A on the front. Now it’s important not to get
voltmeters and ammeters mixed up. They have very different functions
after all.
A voltmeter is used to measure the
voltage across a circuit component, whereas an ammeter is used to measure the
current through a circuit component. A voltmeter needs to be connected
in parallel, otherwise, it will not work properly, whereas an ammeter needs to be
connected in series as we’ve already seen. And so, if we’re going to use an
ammeter, it’s important that we look for the capital A on the front.
So now that we’ve learned a little
bit about ammeters, let’s take a look at a couple of example questions to help us
understand the topic a bit better.
The diagram shows an electric
circuit. How many ammeters are there in the
circuit?
Okay, so in this diagram, we’ve
been shown a circuit that has lots of components in it. In fact, it has one, two, three,
four, five, six, seven, and eight components in it. So, that’s quite a large
number. But we’ve been asked to find how
many ammeters are in this circuit. So to answer this question, let’s
first start by recalling that the circuit symbol of an ammeter looks like a circle
with a capital A inside it. So looking back to our circuit, we
can see one of these ammeters, and here is another one. All the other components are not
ammeters because here we’ve got a cell, here’s a bulb, here is an open switch, here
is another bulb, a third bulb here, and a closed switch. Therefore, the answer to our
question is that there are two ammeters in the circuit.
Let’s now take a look at another
example question.
Each of the following diagrams
shows a circuit containing a cell, a bulb, and an ammeter. Which one shows how the ammeter
must be connected to the circuit in order to measure the current in the circuit?
Okay, so in order to answer this
question, we should first recall that an ammeter is in fact used to measure the
current in a circuit. However, it has to be connected in
a very specific way into the circuit in order for it to work. We’ve been told that each one of
these circuit diagrams (A), (B), (C), and (D) shows us a cell, a bulb, and an
ammeter. Let’s recall that the circuit
symbol of a cell looks like this, a short line to represent the negative terminal of
the cell and a long line to represent the positive terminal. A bulb is drawn like this, a circle
with a cross through it. And an ammeter is drawn this way, a
circle with a capital A inside it. And we can see that each one of
these circuit diagrams does, in fact, have a cell, a bulb, and an ammeter in it.
Now, let’s also recall that for an
ammeter to work, it must be connected in series in our circuit. So let’s go through each circuit
and see if our ammeter is indeed connected in series. Let’s start with circuit (A). We can imagine charges flowing from
the positive terminal of our cell. In other words, we’re considering
the current in our circuit. And in this particular case, as
soon as we get to this junction, some of the charges must be going off in this
direction, whereas the others go off in this direction. And then we see the ammeter
connected on one of these branches. Therefore, we can say that our
ammeter is not connected in series in this circuit.
An additional problem is that the
circuit is not even complete. We can see that the end of this
wire here is not connected to anything; it’s just sort of left there. Therefore, we can definitively say
that circuit (A) is not the correct way in which to connect an ammeter in order to
measure the current in our circuit.
Moving on to circuit (B) then, we
can once again begin at the positive terminal of our cell and consider the charges
flowing in this direction. We can see that all of the current
does indeed pass through our ammeter and then through the bulb before continuing
clockwise and arriving at the negative terminal of the cell. At which point we’ve gone around
the entire circuit and all of the current has passed through our ammeter. It is connected in series. Therefore, option (B) is a good
candidate for a correct circuit.
Moving on to option (C) though, we
can once again begin at the positive terminal of the cell, going this way
clockwise. Then we can see that all of the
current does pass through the bulb. But as soon as we get to this
junction here, the current must split. Some goes in this direction and the
rest in this direction. And then we see that the ammeter is
again on one of these branches. Additionally, we’ve got the same
problem as in circuit (A). The circuit is actually
incomplete. This end of the wire is not
connected to anything. So because our ammeter is not
connected properly in circuit (C) and it’s also an incomplete circuit, we can say
that this is not a correct answer.
Finally, moving on to circuit (D)
again, beginning at the positive terminal, we go around clockwise until we get to
this junction here. Now, at this point, we may have
been able to say that our ammeter was at least connected in parallel if the circuit
had been completed, for example, if we had a wire that looked like this dotted
line. And in that case at least, the
ammeter would have been connected in parallel with our circuit, and the circuit
would have been a complete circuit. That still wouldn’t mean that the
ammeter was functioning because again it needs to be connected in series. And in fact, for the given circuit,
neither is the ammeter in series nor is the circuit complete. So, option (D) is out of the
question as well. At which point we can say that
option (B) shows us how to correctly connect an ammeter in order to measure the
current in a circuit.
Okay, so having looked at a couple
of example questions, let’s now summarize what we’ve talked about in this
lesson. In this video, we first saw that
ammeters are used to measure the electric current in a circuit. We also saw that in a circuit
diagram ammeters are represented by a circle with a capital A inside. Thirdly, we saw that ammeters must
be connected in series in order to properly measure the current in a circuit. And finally, we saw that, in real
life, ammeters can often look like other devices. One common example is a
voltmeter. However, ammeters can easily be
distinguished by the capital A on the front. This is a summary of ammeters.
