Video Transcript
In this video, we will learn how to
use voltmeters in electric circuits to measure the potential difference across a
component in the circuit.
A voltmeter is a device used to
measure the potential difference across the components in the circuit. A lot of the time, it looks like a
box with a dial on the front and two terminals to which we can connect wires in
order to connect our voltmeter into the circuit. Occasionally, we may come across
digital voltmeters, which are similar. They have a box and two
terminals. But instead of a dial to display
its reading, it has a screen. In both cases, a capital V tells us
that it is a voltmeter rather than any other device.
Now, the way to use a voltmeter is
to connect it into a circuit. Here we have an example of a
circuit that consists of a battery, a light bulb, and a voltmeter. And we can see that the dial has
shifted to a new position. The dial is telling us that the
potential difference across the battery in our circuit is equal to five volts. So if we were doing an experiment
with our voltmeter, we would say that the potential difference across the battery,
as measured by the voltmeter, is equal to five volts. Remember that the unit for
potential difference is the volt.
Now, it’s very important to make
sure that our voltmeter is connected in parallel with the components that we are
trying to measure the potential difference across. To understand what we mean by this,
let’s consider the route that current takes through the circuit. Starting at the positive terminal
of the battery, charge can flow clockwise through the circuit, flowing through the
light bulb, causing it to light up, and back out the other side, before flowing
around to the negative terminal of the battery. However, there’s another route that
the current could take. Starting again from the positive
terminal of the battery, charge can flow counterclockwise until it reaches the
voltmeter, flows through it and out the other side, and back round to the negative
terminal of the battery.
In this circuit, our voltmeter is
connected in parallel with the component that we are trying to measure the potential
difference across, the battery. We know this because the current
can either go clockwise or counterclockwise through the circuit, meaning the
voltmeter is on a separate branch of the circuit. 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 the positive terminal of the battery to the light bulb. And connected to this is our light
bulb. Recall that the circuit symbol for
a light bulb is a circle with a cross through it. Then, we have our wire that
connects the light bulb to the negative terminal of the battery.
Next, we will look at the other
route that charge can take as it flows through the circuit. First, we draw the wire that
connects the positive terminal of the battery to the voltmeter. Then, we draw the voltmeter itself,
which we see has a circuit symbol which is a circle with a capital V inside it. Then, we draw the wire that goes
from the voltmeter to the negative terminal of the battery. Once again, we can consider the
route that current takes through the circuit. Starting from the positive terminal
of the battery, we see that charge can flow clockwise through the bulb, causing it
to light up, then back round to the negative terminal of the battery. Alternatively, charge can also flow
counterclockwise to the voltmeter and then back round to the negative terminal of
the battery. This shows us that the voltmeter is
on a different branch of the circuit and the components it is measuring the
potential difference across, the battery. This is because not all of the
charge that flows through the battery also flows through the voltmeter, because some
of it flows clockwise through the bulb instead. So we can confirm that our
voltmeter is connected in parallel with the battery.
However, if we were to connect our
voltmeter into the circuit like this, we would see that the current only has one
route through the circuit, meaning all of the charge that flows through the battery
also flows through the voltmeter. This means that the voltmeter is
connected in series with the battery and in this case would not work because it is
not connected properly. So, for a voltmeter to work
properly, it must be connected in parallel.
Another important thing to note is
that our voltmeter can often look very similar to some other devices. The most common of these is the
ammeter. An ammeter can look almost
identical to a voltmeter. It generally takes the shape of a
box, has a dial, and it also has two terminals to connect it into the circuit. Sometimes the only way to tell the
difference between an ammeter and a voltmeter is the capital A on the ammeter,
whereas our voltmeter had a capital V.
It’s very important not to get
ammeters and voltmeters mixed up. This is because they have very
different functions. A voltmeter measures the potential
difference across a component in a circuit, whereas an ammeter measures the current
through a component in the circuit. As we know, a voltmeter must be
connected in parallel with the component it is measuring the potential difference
across, whereas an ammeter must be connected in series with the component it is
measuring the current through. So if we’re going to use a
voltmeter, it’s very important that we look for the capital V on the front.
Now that we’ve learnt a bit about
voltmeters, let’s look at a couple of example questions to help us understand the
topic better.
The diagram shows an electric
circuit. How many voltmeters are there in
the circuit?
So we’ve been given a circuit
diagram that has many components in it. It actually has one, two, three,
four, five, six, seven, eight components in it, and that’s quite a large number. But we’ve been asked to find how
many voltmeters there are in the circuit. So, to answer this question, let’s
start by recalling the circuit symbol for a voltmeter, which is a circle with a
capital V inside of it. So, looking back to our circuit, we
can see one, two voltmeters. The other components are a cell, an
ammeter, three bulbs, and an open switch. The circuit symbol for the ammeter
looks quite similar to the one for the voltmeter, except it has a capital A inside
of it. So the answer to our question is
two voltmeters. There are two voltmeters in the
circuit.
Let’s now look at another example
question.
Each of the following diagrams
shows a circuit containing a cell, a bulb, a buzzer, and a voltmeter. Which one shows how the voltmeter
must be connected to the circuit to measure the potential difference across the bulb
only?
To answer this question, let’s
start by looking at the symbols for each of these circuit components. We have a cell, which has a circuit
symbol that looks like this, which has a long line that represents the positive
terminal and a short line that represents the negative terminal. We also have a bulb, which has a
circuit symbol that looks like this, which is a circle with a cross through it. And we have a buzzer, which has a
circuit symbol that is a semicircle with two lines coming out of it. And we have a voltmeter, whose
circuit symbol is a circle with a capital V inside of it.
The question asks us how the
voltmeter must be connected to measure the potential difference across the bulb
only. We can recall that for a voltmeter
to measure the potential difference across a component, it must be connected in
parallel with that component. So, in this case, the voltmeter
must be connected in parallel with the bulb only. So we must identify which of the
circuits from (A), (B), (C), (D), and (E) have the voltmeter connected in parallel
with the bulb only.
To work this out, we can follow the
current on the route it takes through each circuit. Let’s start with circuit (A). The charge flows out of the
positive terminal of the cell round to the bulb and through it. And then it reaches this terminal
here, at which point it splits, with some of the current going down to the voltmeter
and some of it going left to the buzzer. The charge flows through the
voltmeter and back up, where it rejoins the charge flowing through the buzzer at
this junction. The charge then flows back to the
negative terminal of the battery. From following the current on its
route through the circuit, we see that the voltmeter is in parallel with one of the
other components, because the flow of charge split and some of it went to the
voltmeter and some of it went to the buzzer. This means that the voltmeter is in
parallel with a component, but unfortunately this component is the buzzer, not the
bulb. So we can say that circuit diagram
(A) is not the correct way to connect the voltmeter in parallel with the bulb
only.
Next, let’s look at circuit diagram
(B). Charge flows from the positive
terminal of the cell round to this junction here, where it splits. Some of the charge flows down to
the voltmeter, through it, and out the other side until it reaches this second
junction here. The rest of the charge flows left,
through the bulb, and then through the buzzer, before rejoining the charge that
flows through the voltmeter at this junction. It then continues to flow round
back to the negative terminal of the cell. Here we see that the voltmeter is
in parallel with the bulb, because the flow of charge splits, with some of it going
to the voltmeter and some of it going to the bulb. However, the charge that flows
through the bulb also flows through the buzzer, meaning that the voltmeter is in
parallel with both the buzzer and the bulb. We are aiming for the voltmeter to
be in parallel with the bulb only. So we can say that this is not the
correct way to connect the voltmeter to the circuit.
Let’s look at option (C). Following the charge flow again, we
see that it splits at this same junction, with some going to the voltmeter, and
round, and some going to the bulb, before it rejoins at this junction here. And then the charge flows to the
rest of the circuit. Now, here we see that the voltmeter
is in parallel with the bulb. Some of the charge flows down to
the voltmeter and through it, and some of the charge flows through the bulb. The flows rejoin at this junction
here, meaning that the voltmeter is only in parallel with the bulb. This means circuit diagram (C) is a
good candidate for how the voltmeter must be connected to the circuit.
Moving on to option (D), following
the route that the current takes through the circuit, we immediately see that it
splits at this junction here. Some of the charge will flow up to
the voltmeter, through it, and back to this junction over here, while the remainder
of the charge will flow down to the light bulb and through the buzzer, before
rejoining the other charge. From this, it is clear that the
voltmeter is in parallel with both the bulb and the buzzer, which is actually the
same as option (B). So we can rule out option (D).
Finally, we can look at circuit
diagram (E). As before, we can follow the
current on the route it takes through the circuit. It splits at this junction here,
with some of the charge flowing through the voltmeter and over to this junction on
the left, with the rest of the charge flowing down to the bulb and then through the
buzzer, before reaching this junction. We can see that while some of the
charge is flowing through the voltmeter, the rest of the charge is flowing through
both the bulb and the buzzer. So the voltmeter is in parallel
with both the bulb and the buzzer. This is identical to the circuit
diagram (D) and (B). So we know that this is the
incorrect way to connect the voltmeter to the circuit. We can rule this out.
So we’ve identified option (C) as a
good candidate for how the voltmeter must be connected to the circuit. And we’ve ruled out all other
options. So option (C) shows us how the
voltmeter must be connected to the circuit to measure the potential difference
across the bulb only.
Okay, now that we’ve looked at a
couple of example questions, let’s summarize what we’ve talked about in this
lesson. In this video, we first saw that
voltmeters are used to measure the potential difference across a component in a
circuit. We also saw that in a circuit
diagram, voltmeters are represented by a circle with a capital V inside of it. We’ve also seen that in order to
work properly, voltmeters must be connected in parallel with the component they are
measuring the potential difference across. Finally, we’ve seen that voltmeters
can often look like other devices, such as an ammeter. But they can easily be
distinguished by the capital V on the front. This is a summary of
voltmeters.
