Lesson video

Hi, I'm Mrs. Hudson and today I'm going to be teaching you a lesson called "Measuring electric current." This is a Key Stage 3 science lesson and it comes in the unit titled "Series circuits." So let's get going.

The outcome of today's lesson is I can measure current and describe the rule for current in a series circuit.

And as part of today's lesson, you'll get some opportunity to practice building series circuits.

There will be some keywords in today's lesson that are used frequently, and those are ammeter, amps with the symbol A, and series circuit.

So let's have a look a little bit closer at what they mean.

An ammeter is a device that measures the current flowing in a circuit.

An ammeter measures the current in units called amps, which has the symbol capital A, which is short for amperes.

A series circuit is an electric circuit with one complete loop from one end of a battery to the other end.

If you want to pause the video to make a note of those keywords, then please do and then press play ready for me to carry on with the rest of the lesson.

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

In the first part of the lesson, we're going to be looking at using an ammeter, and then in the second part we're going to look at current in circuits with an ammeter.

But let's get going with the first part of the lesson, using an ammeter.

This picture here is showing you an ammeter and it is used to measure the current that flows in a circuit.

It shows the current in amps, which has the symbol capital A, which is short for amperes, and you can see on that image the best way to identify an ammeter is by looking for the big A which represents the amps on the front.

The symbol for an ammeter is a circle with a capital A inside of it.

Let's check our understanding of that so far.

Which of the images below is of an ammeter, A, B, or C? Hopefully here you got B.

Now remember that an ammeter has the capital A on the front.

A is showing you a voltmeter which has a V on the front, and C is showing you a lamp which has got the lamp holder and the bulb inside of it.

Well done if you managed to get that right.

Ammeters are always placed in series with components and we can see here that we've got a series circuit that's made up of a cell and a lamp and they are in one continuous loop, so it is a series circuit.

So if we added an ammeter into the circuit, we would place it within the same continuous loop.

This ammeter is part of the circuit's single loop, so all the current passes through it.

And we can see that the purple line there is representing the current, the flow of electrons around the circuit, and you can see that it is flowing through each component in a single loop.

When an ammeter is placed in a circuit, the current remains the same.

So we've got the same series circuit here with a cell, a bulb, and an ammeter.

And ammeters measure the current without affecting its flow.

I could place that ammeter anywhere in the series circuit and it wouldn't affect the flow of the current, so the current reading would be exactly the same even if I move the ammeter so it was placed to the left of the lamp.

It doesn't matter where the ammeter is placed, the current is going to remain the same.

Let's check our understanding of that.

So we can see here that we've got two cells that make up a battery, a motor, and two lamps with a closed switch.

So if an ammeter was placed in the circuit, which one of these statements would be correct? A, if it is placed before the motor, the motor will slow down.

B, wherever it is placed, there will be no effect.

C, if it is placed between the two lamps, only the lamps will get dimmer.

Or D, wherever it is placed, the motor will slow down and the lamps will get dimmer.

Hopefully here you went for B, wherever the ammeter is placed, there will be no effect because remember, wherever you put an ammeter in a series circuit, it will have no effect on the current.

Well done if you managed to get that right.

An ammeter can be placed anywhere in a series circuit.

So again here we've got a series circuit made up of a cell, a bulb, and an ammeter.

And if I moved that ammeter to that position, the current would remain the same.

If I moved it there, the current would be the same.

If I moved it to the left of the cell, the current would remain the same.

And the same there.

If I moved it again, the current remains the same everywhere in a series circuit.

All the current stills flows through it, so the current it measures will be the same.

So the key learning point from this slide is that wherever an ammeter is placed in a series circuit, the current can still flow through it, and so the current measures exactly the same at any point in a series circuit.

Ammeters do not affect the current in a circuit.

So we can see here again we've got a series circuit with a lamp and an ammeter in it.

And if I added lots more ammeters within this circuit, the lamp is going to receive the same current and remain just as bright even if all of those ammeters are added.

So key learning from this slide is that ammeters do not affect the current in a circuit.

It doesn't matter how many ammeters you have in a series circuit, the current will remain the same and therefore the lamp will remain the same brightness.

Let's check our understanding with a true or false question.

So we've got a series circuit here with a cell, two ammeters, and a lamp.

True or false? The reading on ammeter 1 one is greater than on ammeter 2.

What do you think? Hopefully here you went for false.

Now justify your answer.

A, the current is the same throughout the circuit, or B, some of the current is used up by the lamp.

This is A, the current is the same throughout the circuit.

It doesn't matter how many ammeters you add, the current remains the same.

Great job if you managed to get that right.

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

So you need to build the following circuits, write down the currents measured, and describe the brightness of the lamps with using the words bright or dim.

And there are six different circuits for you to set up exactly like what the diagram is to record the current, either say if the lamp is bright or dim.

Now be careful to make sure that you use the correct components in the correct order here, but I'm sure you're going to do a really wonderful job.

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

Welcome back, let's see how we have done.

So on here there are some examples of currents that you might have had readings of, but just so you know, yours might vary slightly from this and that's absolutely fine as long as your values follow the same trend by either staying roughly the same or roughly halfing when I say so.

So if we start at the top row on the left-hand side, you have a circuit there in series made up of a cell, an ammeter, and a lamp.

And the current reading was 0.

15 amps, your value might be slightly different to that, and the brightness of that lamp was, it was very bright.

Compare that to the middle top circuit.

The only difference there is that the ammeter's moved position and therefore the current should be roughly the same and the brightness of the lamp is roughly the same, so the lamp remains bright, and you can see the current there's 0.

14 amps which is slightly lower but it is roughly the same.

Sometimes the ammeter might vary a little bit, but it would be roughly the same value.

Then if we look at the very right-hand top circuit, the difference here is that you've added in an extra ammeter and we said if you add in multiple ammeters, it has no effect on the current, so the current's going to remain the same at roughly 0.

15 amps, and the lamp will remain bright.

Moving on now to the bottom circuit, so if we look at the bottom left-hand circuit, what we can see there is that you've added in an extra lamp in comparison to the very top circuit, so there's now two lamps within that circuit, and what that does is it reduces the current within the circuit and it should be roughly half the amount you had on the top circuits.

So you can see it's gone from 0.

15 amps to 0.

07.

So as long as your value's roughly half, that seems right.

And therefore, because the current is less, the lamps will become dimmer in comparison to the top circuits.

Now moving on to the middle circuit, we can see there that the only difference is the ammeter's moved position, so that has no effect on the current, so therefore the current should remain roughly the same as whatever you got for the previous circuit, so in this case, 0.

07 amps, and the lamps are going to remain dimmer.

And then if we look at the final circuit, again, it's just the position of the ammeter that has changed, so the current should remain the same at 0.

07 amps, and the lamps will remain dim.

So the key learning points from this are that current is the same across any point of a series circuit.

It doesn't matter where the ammeter is placed within that series circuit, the current reading will be the same.

And it doesn't matter if you add in multiple ammeters, it will have no effect on the current.

However, if you add in a lamp or a resistor, then the current will decrease.

So a massive well done if you managed to get that right.

If you need to pause the video to add anything into your answers, please do, but we're going to carry on now with the rest of the lesson.

We're ready now to move on to the second part of our lesson, which is current in circuits with an ammeter.

In a series circuit, the current measured by an ammeter is the same regardless of where it is placed, which we've previously spoken about.

So in this series circuit here we can see we've got a cell, two ammeters and a lamp.

And the current is going to be the same on both of those ammeters and it doesn't matter where those ammeters are placed, the current will remain the same.

The current is also the same through the lamp, the cell and the wires.

So the current through the cell is 0.

15 amps and the current through the lamp is also 0.

15 amps.

The current is the same everywhere in a series circuit.

Adding another lamp decreases the current throughout the circuit.

So here we've got a circuit that is in series that consists of a cell, two ammeters, and one lamp.

And we can see that the current being read on both ammeters is 0.

15 amps.

If you added in another bulb, though, the current would decrease, which we can see now has gone down to 0.

07 amps in the ammeters.

Adding a resistor also decreases the current.

So if we add a resistor into that circuit, the current then decreases further and it goes down to 0.

04 amps.

So adding lamps or resistors into a series circuit will decrease the current.

Let's check our understanding of that.

Which of these circuits has the greatest current? The components are identical.

So we've got three different circuits here with a different number of components within them.

Which one has the greatest current, A, B, or C? So hopefully here we've recognized that it was going to be A.

The reason it's A is because A has got two lamps and one resistor, so three components within it, whereas B and C have got four components, so B has got three lamps and one resistor and C has got two resistors and two lamps.

So therefore A will have the greatest current because it's got the least number of components within it.

Well done if you got that right.

We can see here that we've got a series circuit containing a cell, two ammeters, and a lamp, and adding a component anywhere within a series circuit, so if we look at those arrows we could add in a component at any point in that series circuit, it will change the current throughout the entire circuit.

And that's represented here by showing you the green line around the entirety of that series circuit, which is representing the current.

And if you add in a component at any point in that series circuit, the current will be affected at any point in the entire series circuit.

Let's check our understanding of that.

What happens when a lamp is added to a series circuit? A, the current before the lamp decreases.

B, the current throughout the circuit increases.

C, the current throughout the circuit decreases.

Or D, the current after the lamp decreases.

Hopefully here we went for C and said the current throughout the circuit decreases.

So well done if you got C, that's brilliant.

Here you can see there's a series circuit that contains a cell, two ammeters, two lamps and a resistor, and changing the positions of the two lamps and the resistor does not affect the size of the current.

So you can see here the resistor's changing position and the current remains the same.

The lamp can change position and the current remains the same.

The resistor again and the lamp again and the current will always remain the same, it doesn't matter where the resistor and the lamp here is within that circuit.

Some pupils here have made a model of a series circuit and you can see that there's one person who is pulling a piece of rope and then there are two people using their hands to make a kind of rectangular shape.

And you can see that the person is pulling the rope around and it's causing the rope to move through the fingers of the people who are holding it.

Now there's two ways that we could change how the rope is moving here.

The person who's pulling on the rope could pull either harder or they could pull softer, and the people who are holding the rope could either loosen their grip or tighten their grip.

And what this model is trying to represent is a circuit similar to this where you've got the cell.

The cell is representing the hands, which is what is pushing those electrons around the circuit.

And then the people who are holding the rope are representing four different components, which we can see here are two resistors and two lamps.

So in this model, different parts are being represented by different things.

So the pulling hands move the tape around the circuit, and this is representing the cell in the circuit, so it's pushing the electrons around the circuit.

So the pulling hands represent the cell.

The tape represents the current in the wire, so the tape is flowing around the circuit, which is representing the current flowing around the circuit too.

And then finally, the four hands, they represent the components, which were in this case two lamps and two resistors.

Let's check our understanding of that so far.

Match the parts of the tape model to those in a circuit.

So you've got pulling hands, still hands, and tape.

And you need to match that to the thing that they're representing, which is either the current in the wire, the cell or the battery, or the lamps or resistors.

So if you need to pause the video to have a go at this, please do and then press play ready when to come back.

Welcome back.

So here you should have the pulling hands representing the cell or the battery.

The still hands are representing the lamps or the resistors, so the components within the circuit, and then the tape are representing the current in the wire.

So really fantastic job if you managed to get that right.

Well done.

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

And here, Alex and Izzy are predicting how a circuit they have built will work, and we can see that there's a series circuit there that contains two cells making a battery, there's three lamps and there's a closed switch.

Now, Alex has said, "Lamp 1 will be the brightest, followed by 2, then 3." And Izzy has said, "Lamps 1 and 3 will be bright, while lamp 2 will be dim." Your job is to explain why you think Alex and Izzy have made their predictions, then explain where their predictions differ from the actual outcome and what the correct answer is.

Pause the video, I'm sure you're gonna do a really great job, and then press play when you're ready for me to go through the answers.

Let's see how we did.

So Alex thinks that the current will decrease as it passes through each lamp, so the first lamp gets the most while the last gets the least.

Izzy thinks that the lamp closest to the battery gets the most current, so they will be brighter than the lamp in the middle, which is the farthest from the battery.

They are both mistaken, which means they're both wrong, because the current is the same throughout the circuit and all the lamps receive the same current, making them equally bright.

So your job was to try and identify where each person had gone wrong and then give the correct answer.

So if you need to pause the video to add anything into your answer to make it really great, then please do.

If not, we're gonna summarize everything we've learned this lesson.

Great job, we've reached the end of the lesson on "Measuring electric current," so we're gonna summarize everything that we've learned so far.

First of all, we said that an ammeter measures the current in a circuit and is placed in series, and we can see there there's an image of an ammeter, which has the capital A, how we recognize it is, and then the symbol that we use in a circuit diagram for an ammeter is a circle with an A inside of it.

Current is measured in units called amps, with a capital A, which is short for amperes.

Ammeters do not affect the current they measure, so you could place as many ammeters in the same series circuit and the current would always be the same.

In a series circuit, the current is the same throughout, and adding component anywhere in the circuit will affect the current everywhere in the circuit.

So we said if you had a series circuit and you added in an extra lamp, that this would decrease the current, but it didn't matter where you added the lamp in the circuit, that the current would remain the same everywhere.

And then finally, we spoke about how we can use models to help explain how circuits work, and the model we talked about was a piece of tape which was being pulled around with four people holding that tape, and the cell was being represented by the pulling hands, the tape was representing the current, and the hands were representing the resistors and the lamps within the circuit.

Really great job in today's lesson.

I've really enjoyed it, I hope you have too, and I'll see you next time.