Lesson Video: Ammeters Science

In this video, we will learn how to use ammeters in electric circuits to measure the current through a component in the circuit.

09:53

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.

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