Lesson video

Hi, I'm Mrs. Hudson.

And today, we're going to be looking at a lesson called "Measuring voltage." This is a Key Stage 3 Physics lesson, and it comes under the unit titled "Series circuits." So let's get going.

The outcome of today's lesson is: I can describe how to measure voltage and the effect of different cell or battery voltages on an electric circuit.

There'll be some keywords used frequently in today's lesson, and they are voltmeter, voltage, potential difference, sometimes shortened to p.

d.

, volts with a V, and battery.

Let's have a look at what each of those words mean.

A voltmeter is a device that measures the voltage supplied by a cell or battery or across a component.

Voltage is a measure of the push from a cell or battery that moves charge around a circuit.

Potential difference, p.

d.

, is a more formal term for voltage; they can be used interchangeably.

A voltmeter measures the voltage or potential difference in units called volts, and we represent volts with a capital V.

And finally, two or more cells connected in series form a battery.

If you want to pause the video to make a note of those words, then please do, but we're going to carry on now with the rest of the lesson.

Today's lesson on measuring voltage is going to be split up into two parts.

The first part of the lesson, we're going to be looking at using a voltmeter, and then we're going to move on in the second part to look at what is voltage? But let's get going with the first part of today's lesson, using a voltmeter.

The component shown here is a voltmeter.

So we can see an image there of something that you might use on your building circuits to measure voltage or potential difference, and that is called a voltmeter.

It is used to measure the voltage across components.

So components within the circuit could be things like a lamp or a resistor, and you use a voltmeter to measure the voltage across those components.

Now, looking at that voltmeter, we can tell it's a voltmeter because there is a capital V on the front of it, and also above it, it says 20 volts.

And volts is the unit to measure voltage or potential difference.

It shows the voltage in volts, which we just said, and we use a capital V to represent volts.

Potential difference, p.

d.

, is a more formal term for voltage, and the two terms, potential difference and voltage, can be used interchangeably.

The symbol for a voltmeter is a circle with a capital V inside of it.

And remember, you draw the wire lines coming out of the circle with the V in.

Let's just quickly check if we've understood that.

Which of the following images is of a voltmeter? Is it A, B, or C? We should have gone here for A, and we know A is a voltmeter because we can see the capital V for volts on the front of it.

B has a capital A, so that is an ammeter, which measures current, and C is a motor.

So well done if you got A.

Voltmeters can be placed across a component in a circuit.

So here we can see that there's a series circuit that contains a cell and a lamp, and the voltmeter has been attached across the lamp.

So the wire of the voltmeter is on one side of the lamp, and the other wire of the voltmeter is on the other side of the lamp.

Voltmeters can also be placed across a cell.

So this is the same series circuit consisting of a cell and a lamp, but this time, the voltmeter has been placed across the cell rather than across the lamp.

Voltmeters should not be placed in series.

A voltmeter will block the current and prevent the lamp from lighting.

So if you look at this circuit here, there's a cell and a lamp, but the voltmeter has been placed in series with the cell and the lamp.

And therefore the voltmeter will block the current, so much so that it will actually stop the lamp from lighting up.

So you must always place a voltmeter across a component, or sometimes we say in parallel with that component.

Let's check our understanding of that concept.

So we can see here there are three images of circuits, and the circuits contain a cell, a voltmeter, a lamp, and an ammeter.

And the question is: which of the following circuit diagrams shows the voltmeter correctly measuring the voltage across the lamp? So we're looking for the correct diagram where the voltmeter is measuring the potential difference across the lamp.

Is it A, B, or C? Now, what we should have gone for here is C, so well done if you got that right.

Remember that the voltmeter needs to be placed across the component, which in this case was the lamp.

So we can see the wires of the voltmeter in C are either side of the lamp.

Now, A is wrong because the voltmeter is in series, so therefore it would block the current.

And B is wrong because the ammeter has been placed across the lamp and the voltmeter is in series, so the voltmeter and the ammeter are the wrong way round in B.

Well done if you recognized that C was right.

Measuring voltage along the wires without components does not affect the voltmeter reading.

So we can see there's a simulation here of a circuit that has a cell and a lamp within it, and there's a voltmeter where the probes are moving across the wires.

But in the simulation, the reading remains at 1.

5 volts even when the voltmeter probes are moved to different positions along the wires.

So as long as there is only one component within the circuit, moving the probes of the voltmeter will not change the voltmeter reading.

In each of the following circuits, the same voltage is being measured by the voltmeter.

So we can see that the circuit consists of a cell and one component, and therefore it doesn't matter where the voltmeter is connected to.

It is going to have the same reading on the voltmeter.

So if you look at the top circuit on the left-hand side, we can see the voltmeter is placed sort of in the middle.

It's not obvious if it's reading across the lamp or the cell, but the reading will still be the same.

Whereas in the green circuit outlined in the middle, the voltmeter is across the lamp, but the reading will still be the same.

On the blue circuit, because there's only one component, the voltmeter reading will be the same across the cell.

And it's the same for every single circuit on this slide.

The voltmeter reading will be the same in each of the circuits.

Let's check our understanding of that.

There's an image here of a circuit that consists of a cell and a lamp, and then there's also a voltmeter within that circuit.

And it has been put in parallel, so it's across the components.

True or false? The voltmeter in the circuit shown will correctly measure the voltage across the lamp.

Is that true or false? You answer that bit first.

Hopefully here we went for true, it does measure correctly the voltage across the lamp.

Now justify your answer.

A, the voltmeter leads do not affect the voltage measured, or B, the voltmeter leads should be connected to either side of the lamp.

This is A, so well done if you recognized that the voltmeter leads do not affect the voltage measured in this circuit because there's a cell and only one component.

B is actually a correct statement.

Voltmeter leads should be connected to either side of the lamp, but it's just not true for this particular circuit.

Real circuits can be complex, so circuit diagrams should be kept as simple and as clear as possible.

So we've got the same circuit we've been looking at here consisting of a cell and a lamp.

And you can see that the voltmeter has been placed across the lamp, and it's really clear that it is across the lamp.

So the voltmeter should be placed near the component that it is measuring, and it's really clear from this diagram that the voltmeter is measuring the potential difference across the lamp.

The connections should be close to and on either side of the component that is being measured.

Let's check our understanding of that concept.

Which of the following circuit diagrams best shows how to measure the voltage across a lamp? Is it A, B, or C? We should have gone here for A.

That diagram is very clearly showing you that the voltmeter is measuring the potential difference across the lamp.

For both B and C, it's not clear whether the voltmeter is measuring the potential difference across the cell or the lamp.

So well done if you got that right.

We're ready now to move on to the first task of the lesson, Task A.

And Aisha and Jun are making images of circuits they are studying.

You can see there there's a voltmeter, a cell, and a resistor, and the voltmeter is across the circuit.

Aisha has said, "This image shows a voltmeter measuring the voltage across the resistor." And Jun has said, "No, it's measuring the voltage across the cell." Explain whether each pupil is correct and justify your answer.

I'm sure you're gonna do a really great job of this.

Give it your best go, pause the video, and then press play when you're ready for me to go through the answers.

Let's see how we did with that.

So let's start with what Aisha said.

Aisha is correct that the voltmeter is measuring the voltage across the resistor.

The position of the voltmeter leads does not affect the measurement as long as there are no components between them and the resistor.

The leads themselves do not affect the voltage reading.

You might also have said here, though, that from the circuit diagram, it does look more like the voltmeter is reading the potential difference across the cell.

But from what we've also learned in this learning cycle, that because there's only one component in that circuit, the reading would be the same.

So Aisha technically is correct.

Now look at Jun.

Jun is also correct.

The voltmeter is measuring the voltage across the cell.

The voltage across the cell will be the same as the voltage across the resistor as the resistor is the only other component in the circuit.

Once again, the position of the voltmeter leads does not affect the voltage measurement.

So, fantastic job if you recognized that.

If you want to pause the video to add in any detail to your answer, then please do, and then press play when you're ready to continue.

Well done so far.

Really good job.

We know how to use a voltmeter, so let's look now at what is voltage? Voltage is a measure of the push from a cell or battery that moves charge around a circuit.

And we can see here that there's a model of a circuit.

And there's somebody who is pulling some tape around the circuit, and there's four still hands that are holding the tape in the corner.

Pulling the tape round harder causes the loop to move faster.

This could represent an increase in voltage, which leads to a higher current.

So in this model here, the pulling hands are representing the cell or the battery, the tape is representing the current, and the still hands in the corner are representing components.

And what we're saying here is that if you increase the voltage, what that looks like here is that the hands pulling the tape would pull harder.

What that would mean is, the tape would then move around faster, and that's representing the higher current, which is going to flow around the circuit.

Gripping the tape tighter causes the loop to slow down, and this could represent a decrease in the current.

So if the components are the hands, if the hands grip the tape harder because they maybe have a high resistance, then that's going to slow the tape and the current will decrease.

Let's check our understanding of that concept.

When the tape is moved more slowly in the simulation, this could represent which two of the following? A, a higher voltage, B, a lower voltage, C, a higher current, or D, a lower current.

And remember we're talking here about the tape being moved more slowly.

So which two are correct? So we should have gone for B here and D.

If the hands pull more slowly, then that means that there is a lower voltage.

Also, if the hands do pull more slowly, then the tape is going to move more slowly, and that's representing a lower current.

If you wanted to increase the voltage, then what you would need to do is pull the tape harder, which would be representing a harder push from the cell or the battery, and then in turn, that would cause the tape to speed up, which would be representing a higher current.

So well done if you got those right.

Great job.

Here are some typical voltages that are used in everyday life.

So a AA cell normally has a voltage of 1.

5.

A lithium-ion battery, which you might find in a phone, for example, is going to have 4 volts.

A car battery of a petrol car is going to have a 12 volt battery.

The mains electricity supply within the UK is 230 volts.

And a car battery in an electric car is around about 400 volts.

So that's quite a lot higher than the battery in a petrol car.

Different devices require different voltages to operate correctly.

So you can see here that there's a laptop plugged in to the mains voltage.

And remember, the mains voltage is around about 230 volts.

So mains voltage is actually too high to charge this laptop.

An adapter, which we can see labeled there, which has 20 volts on it, lowers the voltage down to 20 volts.

The laptop operates safely within this lower voltage.

If the voltage were to remain too high, it would cause too much current to flow, potentially damaging the laptop.

So the laptop there has an adapter which lowers the voltage from 230 to around 20, which allows the current to be reduced to a range which is safe for the laptop.

Small cells can have a higher voltage than large ones.

So you can see here that there is a lithium cell which is smaller than the alkaline cell underneath it.

So the little round silver cell is lithium, and then underneath that's an alkaline cell.

And we can see that the lithium cell has a 3 volt rating and the larger cell underneath has a 1.

5 volt rating.

The voltage depends on the chemical reactions within the cell.

If a higher voltage is needed, cells can be combined in series to form a battery of cells, increasing the voltage.

And remember, a battery is where you have more than one cell.

Let's check our understanding of that.

Which cell or battery makes the bulb light the brightest? The same type of bulb is used each time.

Is it A, with a 1.

5 volt rating, B, with a 6 volt rating, or C, with a 9 volt rating? We should have gone for C here.

It's going to be the.

It's going to be the battery that has the highest voltage rating, which in this case is C at 9 volts, even though it's a smaller looking battery than B.

So well done if you got that right.

So here we've got an image of two different cells, and these cells use the same chemical reaction and have a voltage of 1.

5 volts.

But we can see that one of them is much larger than the other, but they both have the same voltage.

The larger cell contains more chemicals, allowing it to last longer.

The smaller cell will transfer all of its energy more quickly due to it having fewer chemicals.

So if you have a larger cell and a smaller cell that have the same voltage, the larger cell is probably going to last for longer because it has more chemicals within it.

Let's check our understanding of that.

We've got two cells here.

One of them's larger than the other, so A is larger and has a 3 volt rating, and B is smaller with also a 3 volt rating.

The question is: Cells A and B are used in identical circuits and contain the same chemicals.

Which of the following statements is correct? A, they will last the same amount of time, B, A will last longer than B, or C, B will last longer than A.

We should have gone for B here.

Because A is larger and they've got the same voltage, then A's going to have more chemicals within it, which will mean it will last longer than B.

Really great job if you managed to get that right.

So here we've got a circuit which consists of a cell and a lamp.

Adding a second cell to the circuit to make a battery causes a bigger push and more current flows.

This means that the bulb gets brighter.

So we've got the same circuit here.

We've got one cell with one lamp.

However, this time, if you added a bulb to this circuit to make there two bulbs, it makes it harder for the current to flow.

As the cell cannot push harder, the bulb gets dimmer.

So if you add a cell, then the bulbs will get brighter, but if you keep the number of cells the same and increase the number of bulbs, the bulbs will get dimmer.

So let's have a look if we've understood that.

We've got a circuit here that consists of a cell and a lamp.

They both have a 1.

5 volt rating.

And the question is: Why does adding a second 1.

5 volt cell to this circuit make the current increase? A, a 3 volt battery will move current with more force than a 1.

5 volt cell, B, it is harder for two cells to move current through the lamp, or C, there is more current in two batteries than in one.

So we should here have gone for A.

A 3 volt battery, which is what it would be if you added two 1.

5 volt cells, will move current with more force than a 1.

5 volt cell.

Well done if you got that right.

Let's have a look at this question now.

We've got the same circuit.

Why does adding a second lamp to this circuit make the current decrease? A, the two lamps use up more current than one, B, the cell cannot push any harder and two lamps make it harder to move current, C, the cell does not push as hard when there are two lamps.

We should have gone for B here.

The cell cannot push any harder and two lamps make it harder for the current to move.

A is wrong because if it says two lamps use up more current, it's not that they use up the current, so that's not correct.

And then C is wrong.

The cell does not push as hard when there are two lamps, so the cell does push the same amount when there are two lamps.

It's just that the cells make it harder for that current to move.

So well done if you selected B.

Really good job.

We're ready now to move on to the last task of the lesson, Task B.

And so we're looking again at the model of the circuit here.

The two questions you need to answer are, number one, explain how the rope loop simulation shows what happens to the current when the voltage is increased.

For this question, we need to think about what would we do to show that the voltage was being increased? And then for number two, explain how the rope loop simulation shows what happens to the current when more components are added to a circuit.

Now, just to give you a little bit of a help here, to remind you, the pulling hands are representing the cell or the battery and how hard the current is being pushed around the circuit.

The rope and tape is representing the current, so how fast it is will dictate how high the current is.

And then the hands, the still hands, are representing the components within the circuit.

I'm sure you're going to do a really great job of this.

Give it your best go, and then come back to me when you're ready for me to go through the answers.

So let's see how we did.

For number one, in the simulation, increasing the voltage is represented by applying more force to move the rope.

The increased force causes the rope to move more quickly, which illustrates an increase in the current.

So really great job if you got that right.

And then for number two, when more components are added to the circuit in the simulation, the rope is gripped by more hands, making it harder to move.

Since the battery cannot increase the force with which it pushes, the rope moves more slowly, representing a decrease in the current.

Now, you might have written your answer slightly differently to that, so I'd recommend pausing the video and checking you've got all the details into yours and adding anything in that you might have missed.

But we're gonna move on now to summarize what we've learned in today's lesson.

So really good job today on our lesson called "Measuring voltage." Let's recap everything that we've learned.

So we've got an image there of a voltmeter and then the circuit symbol underneath.

And we said that a voltmeter measures the voltage, or sometimes called the potential difference, across a component.

The units of voltage are volts, which we represent with a capital V.

And we said that voltmeters do not allow current to flow through them.

They have a very high resistance.

We said electrical leads between components do not affect the voltage.

And finally, we said the voltage across a cell or battery and a single component in the same series circuit will be the same.

Really great job with today's lesson.

I've really enjoyed teaching it to you.

I hope you've enjoyed it too, and I will see you next time.