Hello, I'm Dr.

De Mello and I'm going to be teaching you today's lesson on electricity.

Today's lesson is about measuring current and it's from the series circuits unit.

By the end of this lesson, you'll be able to measure current and describe the role of current in a series circuit by carrying out tests with real circuits.

So let's begin.

We have three keywords today.

An ammeter is a device that's used to measure the size of the current that's passing through a circuit.

The ammeter measures current in amps or amperes.

We use the symbol capital A for the unit for short.

The final keywords are series circuit.

An ammeter is always placed in a series circuit, always in line with the main loop of current.

These are the keywords defined.

If you need to, pause the video and read through them.

Remember to look out for them as you pass through the slides.

Our lesson on measuring electric current today has two parts.

The first part is about using an ammeter correctly.

The second part is looking at the currents in circuits with an ammeter, and you'll be doing some of these measurements.

So let's start with using an ammeter.

This is an ammeter.

It's used to measure the current that flows in a circuit.

It shows the current in amps, and we use the symbol capital A for the unit, which is short for amperes.

The symbol for an ammeter is a circle with an A in it, and that's placed in a series circuit.

Let's see what you can remember.

Which one of these images shows an ammeter? Pause the video, make a choice, and then you can come back and check your answer.

Welcome back.

If you chose answer B, you're correct.

Well done.

That's an ammeter.

The clue is in the connector on the front you can see a capital A for the unit amps.

On the left we have a volt metre, and on the right we have a lamp, a bulb in a lamp holder.

Well done if you got that right.

This is a series circuit.

Ammeters are always placed in this type of circuit.

So here is an ammeter placed in this series circuit in line with a single loop.

You can see that all the current passes through the ammeter.

This purple loop shows you how it does this.

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

Ammeters measure the current without affecting how it flows.

Let's check what you've understood so far.

So here's a question.

An ammeter is placed in series in this circuit shown.

Which statement is true? We'll take a moment to look at the circuit first.

There are two cells and a battery, there's a motor, there's two lamps and a switch, and they're all part of a series circuit.

So if the ammeter is placed in it, which of the following is true? Is it 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 bulbs, only the bulbs will get dimmer.

And finally D, wherever it is placed, the motor will slow down and the bulbs will get dimmer.

Pause the video, make your choice, and then come back and check how you did.

Welcome back.

If you chose answer B, well done.

That's correct.

An ammeter has no effect on a series circuit.

Wherever it is placed, it won't affect the other components.

This ammeter is measuring the current through this single series circuit.

It can be placed anywhere in the circuit, so it can be moved along to the right, or it can be near to the battery, or it can be after the battery or further along the wires.

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

Ammeters do not affect the current in the circuit.

You can add all these ammeters and the bulb still gets the same current.

It's going to be just as bright if all the ammeters are added.

Let's test your understanding with a true or false question.

The question refers to this circuit which has two ammeters, ammeter number one and ammeter number two, and a lamp that's lit with a battery.

So the statement is: the reading on ammeter 1 is bigger than on ammeter 2.

Is that true or false? Pause the video, have a look at the circuit, make a decision and come back to check your answer.

Welcome back.

If you chose false, that's correct.

The reading on ammeter 1 is not bigger than on ammeter 2.

Now is the time to justify your answer.

The reason would be either A, the current is the same all around the circuit, or B, some of the current is used up by the bulb.

Again, pause the video, think about the question and then come back when you've made your choice.

Okay, welcome back.

If you chose answer A, that's correct.

The current is the same all around the circuit.

The ammeters don't affect it at all on either side.

The current will stay the same.

We've come to the end of this section.

So this is task A for you to practise what you've learned.

So you're going to build the following six circuits one at a time, and you're going to write down the currents measured and describe the brightness of the lamps.

For example, are they bright or dim? In the top row of the circuits, you'll see that there's one lamp with a single battery.

The number of ammeters may change and the positions may change, but it's one lamp and one battery.

In the bottom row there are two lamps for the single battery and the positions of the ammeter change.

Pause the video now.

Go ahead and test out these circuits, and when you're done, come back and we'll check your answers.

Welcome back.

If you managed to build those circuits and test them, that was excellent.

Let's look at the answers that you might have got.

These are an example set of answers.

Compare yours with them and see what you got.

In the top row, the single lamp would've been bright for all the three circuits.

We have one battery and one lamp.

All the lamps should have been equally bright.

On the first one we have 0.

15 amps.

On the second one we have 0.

14 amps, which isn't that different.

And on the third one, both ammeters are measuring 0.

15 amps.

So what we found is in that type of circuit, the current is the same wherever it is placed.

And if you've got two ammeters, they don't affect the current.

Moving to the bottom row.

In this case there's two lamps for the single battery and this time, the lamps would've been dim or a bit dimmer.

The current this time is roughly half what it was before, 0.

07 amps.

And if you look at the first one, the current again is 0.

07 amps.

The second one in the middle at the bottom, it's 0.

07 amps.

And in the last one where the ammeter is in between the two lamps, it's 0.

07 amps.

So what that tells us again is, the position of the ammeter does not affect the current.

And secondly, the current is the same all around the circuit.

Good job with doing that practical.

We'll now start the second part of this lesson, current in circuits with an ammeter.

In a series circuit, the current is measured by an ammeter and it's the same wherever the ammeter is.

So we have here a battery and a lamp with two ammeters.

The current in these two ammeters will be the same, 0.

15 amps in this case.

The current is also the same in the lamp, the battery, and in the wires, 0.

15 amps.

Adding another lamp would reduce the current all around the circuit.

So as another lamp is added, the current drops to 0.

07 amps.

And again, both ammeters will measure 0.

07 amps.

That's what you found when you did the practical.

If you added a resistor as well, that would further reduce the current.

And the current has now gone down to 0.

04 amps.

Again, both ammeters will be measuring 0.

04 amps.

So to summarise, the current is the same all around the circuit.

Ammeters will measure that same current and they won't affect it.

But adding other components like lamps and resistors will reduce that current all around the circuit as measured by the ammeters.

Here is a test to see how you've understood that work so far.

There are three circuits.

Which of these circuits has the highest current? And the components in all these circuits are identical for their type.

The lamps are all the same and the resistors are all the same.

Pause the video, make a choice, and come back and check your answers.

Welcome back.

If you chose answer A, that's correct.

If you look at the circuits, answer A has three other components in it, two lamps and one resistor.

Circuit B has three lamps and one resistor, so it would prevent the current flowing, the current would be lower.

And circuit C has two lamps, but it has two resistors.

The extra resistor would reduce the current.

So B and C would have a lower current.

The highest current would be in circuit A.

Well done if you got that right.

So what we've found so far is adding a component anywhere in a series circuit, so anywhere as shown by these arrows, so the top part or the sides or the bottom, between the components, in front or behind them, would change the current everywhere in the circuit.

So looking at this loop going around the circuit, the current would change everywhere along that loop and it would be the same.

If you can remember those rules, you'll be able to solve circuit problems quite easily.

Let's check how well you've understood the work so far.

So what happens when a lamp is added to a series circuit? A, only the current before the lamp is reduced.

B, the current increases.

C, the current everywhere in the circuit is reduced.

And D, only the current after the lamp is reduced.

Pause the video, make your choice, and then come back and we'll check your answers.

Welcome back.

If you chose answer C, that's correct.

The current everywhere in the circuit is reduced.

Well done if you got that right.

The next thing we'll look at is whether changing the positions of components in a series circuit affects the current.

We've got a circuit here with one battery.

We then have an ammeter on the left followed by two lamps, a resistor, and then an ammeter on the right, and the circuit goes back to the battery.

So it's a series circuit with three components and two ammeters.

Changing the positions of the two lamps and the resistor does not change the size of the current.

So if the resistor was moved up to beside the battery, the current would still stay the same.

If one of the lamps was moved up to the other side of the battery, the current would still be the same.

Moving a resistor down to the bottom part of the circuit would still cause the same current, and even moving the bulb back down to the bottom would result in no change.

Positions of components in a series circuit does not affect the circuit.

These pupils are using a loop of tape to simulate a circuit.

It's a model of a series circuit.

One pupil at the top is pulling the loop round.

Two other pupils are holding the loop loosely, so it's roughly a rectangular shape.

The pupils have some choices.

The pupil pulling the rope can pull with more or less force.

Those gripping the rope can either grip loosely or more tightly, and you could also have other pupils joining in and holding the rope.

The sort of circuit this represents is a circuit where there's a battery at the top and then there are four components.

Going clockwise, after the battery we have a resistor.

Then at the bottom we have a bulb, another resistor.

And finally at the top, another lamp that leads to the battery.

So looking at the model, the pulling hands move the tape around.

The tape is like the current in the wire leads.

And the four still hands are like components.

Let's check what you can say about this tape model of a circuit.

On the left we've got pulling hands, still hands, and tape.

And on the right we have current in the wire, battery, lamps and resistors.

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

Pause the video, make your matches, and then come back and check your answers.

Okay, welcome back.

So if you said pulling hands with a battery, that's correct.

If you said the still hands were the lamps and resistors, that's correct.

And that means the tape would be like the current in the wire.

We've come to the end of the section.

This is task B, which will allow you to practise what you have learned.

Alex and Izzy are predicting how a circuit they have built will work.

And if we look at the circuit, there are two batteries at the top and then there's three lamps at the bottom and there's a switch to turn on and turn off the circuit.

Alex says, "Lamp 1 will be the brightest, followed by 2, then by 3." Izzy says, "Lamp 1 and 3 will be bright, while lamp 2 will be dim." Explain why you think Alex and Izzy made their predictions and then explain why they're both wrong and what the correct answer is.

Pause the video now, have a look again at the circuit, and then write out your answers and come back to check them when you're ready.

You may have written something like this.

Alex thinks that the current will reduce as it passes through each lamp.

So the first lamp gets the most while the last gets the least.

Izzy thinks that the lamps closest to the battery get the most current, so they'll be brighter than the lamp in the middle, which is the furthest from the battery.

And then finally, you should have written that they're both wrong because the current everywhere in the circuit is the same.

All the lamps get the same current and so all the lamps are equally bright.

If you managed to get that right, that was brilliant.

That's quite a hard task.

Well done.

You've completed the lesson on measuring electric current.

Great work.

Here's a summary of what you've done so far.

An ammeter measures the current in a circuit and is placed in a series circuit.

The unit of current is the amp, capital A, and this is short for the word ampere.

Ammeters do not affect the current they measure.

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

And finally, we can use models to help explain how circuits work.

Well done on completing this lesson and I hope to see you soon.