Hello, I'm Dr.

DeMello and I'm going to be teaching you today's lesson.

Today's lesson is called Measuring Voltage, and it's part of the Series Circuits unit.

Let's begin.

The outcome of the lesson is I can describe how to measure voltages and the effect of different battery voltages on an electric circuit.

Let's look at today's keywords.

First of all, we have volt metre.

A volt metre is a device that measures the push of a battery through a circuit.

The next keyword is voltage.

The vol metre measures the voltage in a circuit and voltage is measured across components.

Potential difference is the next set of words, and these are the same as voltage.

It's a more scientific term for voltage.

Then we have vault.

That is the unit that voltage or potential difference is measured in.

And finally we have battery.

A battery pushes current through a circuit.

Here are the definitions for the keywords.

If you'd like, pause the video now so you can have a read of them.

Look out for these keywords in the remainder of the lesson.

This lesson has two parts.

The first part is using a volt metre, and in the second part we look at what is voltage.

So let's start with the first part using a volt metre.

This photo shows a volt metre.

It's a standard component that you find in electrical circuits.

It's used to measure voltage across components.

It shows the voltage in volts, and you can see there's a capital V on the connector for the volt metre.

Potential difference is another term used for voltage.

It's the more scientific term, and the symbol for a volt metre is a circle with a capital V in it.

two wires come out either side.

We can now do a check for your understanding.

Which of the following images shows a volt metre? Is it A, B, or C? Pause the video now, make your choice, and then come back to check your answer.

Welcome back.

If you chose answer A, that's correct.

The clue is in the capital V that you can see on the volt metre connector.

Option B is an ammeter and option C is a motor.

When we use volt metres, they should be placed across a component.

That means on either side of the component.

Here you can see a volt metre that's been placed either side of this lit bulb or lamp.

Volt metres can also be placed across the battery.

Here you can see the volt metre placed on the positive and the other negative side of this battery.

When placing volt metres in circuits, they should not be placed in series.

This volt metre in series with a bulb blocks the current and the lamp will not light.

Let's see what you can remember.

Which of the following three circuit diagrams shows the volt metre correctly measuring the voltage across the lamp? Is it A, B, or C? Pause the video now, make your choice, and come back to check your answer.

Welcome back.

If you chose answer C, that's correct.

In circuit C, the volt metre is placed across the lamp.

The ammeter is placed correctly in series.

If you chose answer A, that's not right because even though the ammeter is correct in its placement, the volt metre isn't.

it shouldn't be in series.

If you chose answer B, that's not correct because the volt metre and the ammeter are in each other's positions.

They should be swapped around.

Well done if you got that right.

Wires themselves have no effect on voltage.

Here we've got simulation with some bare leads.

Measuring the voltage along these leads or wires without any components along them does not affect the volt metre readings.

The reading stays at 1.

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

You can see the left hand probe moving up and down, and you can see the right hand probe moving up and down.

The volt metre reading stays at 1.

5 volts.

In the following circuits, the volt metre is measuring the same voltage in each of the following circuits.

So starting with the first one, the volt metre is placed above the battery and either side of it.

In the second circuit, the volt metre is now moved below the battery, but it's measuring the voltage either side of it.

These two volt metres are measuring the same voltages in the circuit.

In this third example, the volt metre is now above the bulb and is measuring the voltage across the bulb.

It's exactly the same as this circuit where the volt metre is below the bulb, but it's still measuring the voltage either side across the bulb.

And in this circuit, the volt metres below the bulb, but the wires are connected in the middle of the circuit.

It's the same sort of circuit, and the volt metre reading will be the same.

And finally, the vol metre is in the middle of the circuit and the connections are across either the bulb or the battery.

It's going to measure the same voltage.

All six of these circuits are exactly the same in terms of voltage.

They're all measuring the same value for voltage.

The thing to remember is that the wires, the leads, have no effect on the volt metre.

It's just the position around either the battery or the bulb.

And in this case, it is the same.

Let's do a check to test your understanding.

This time it's a true or false.

The circuit shown will correctly measure the voltage across the lamp.

Is this true or is it false? Pause the video, make your choice, and then come back and check your answer.

Welcome back.

If you chose true, that's correct.

Even though the volt metre seems to be connected to one side of the battery and then the other side of the bulb, it is actually measuring the voltage across the bulb or lamp.

Now's the time to justify your answer.

You have two choices, either A, the leads do not affect the voltage measured, or B, the volt metre should connect to either side of the lamp.

Pause the video now, choose your answer, and come back when you're ready.

Welcome back.

If you chose answer A, that's correct.

The leads do not affect the voltage measured.

If you chose answer B, that is correct, but it isn't the correct justification for the true answer.

Well done if you got that right.

Real circuits can be really confusing.

There's wires everywhere.

So in drawing circuit diagrams, we try to keep them as simple and as clear as possible so that they're easy to understand.

So here are a couple of pointers.

Draw the volt metre near the component it is testing, and then the connections will be quite close to the component being tested.

So it's obvious that the volt metre is testing the voltage of this bulb.

Let's check your understanding.

Which of these following three circuit diagrams best shows how to measure the voltage across the lamp? Remember, we're thinking of the best way of showing it.

Pause the video, make a choice, and then come back When you want to check.

Welcome back.

If you chose answer A, that's correct.

This circuit diagram is the one that best shows how to measure the voltage across the lamp.

Answers B and C also measure the voltage across the lamp, but they're not necessarily the best way of showing how to measure that voltage.

They could easily be construed as measuring the voltage across the battery, but again, that's the same as the voltage across the lamp in this case.

Now's your chance to try a practise task.

Aisha and Jun are making images of circuits they're studying.

The picture of the circuit is shown between them.

Aisha says, "This image shows a volt metre measuring the voltage across the resistor." Jun says, "No, it's measuring the voltage across the battery." Explain whether each pupil is correct and then justify your answer.

Pause the video now, write out your answer, and then come back to check it when you're ready.

Welcome back.

Let's check your answer.

Aisha is actually correct that the volt metre is measuring the voltage across the resistor.

The position of the volt metre leads does not affect the measurement.

As long as there are no other components between them and the resistor.

Remember, the leads themselves do not affect the voltage.

Jun is also correct.

The voltage metre is measuring the voltage across the battery.

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

Again, remember that the leads do not affect the voltage, so both Aisha and Jun are correct.

We're now ready to do the second part of this lesson.

What is voltage? The voltage of a battery is a measure of the strength with which it can move current round the circuit.

These students are doing a simulation of a battery moving current around the circuit.

Pulling the tape harder is like using a higher voltage.

The loop will go faster, a bit like a higher current.

On the other hand, gripping the tape tighter, makes the loop slow down.

So if the people at the four corners tighten their grip on the tape, it would make the loop slow down and it would be like a lower current.

Let's check your understanding.

Which of the two following statements are correct when the tape is being moved more slowly in the simulation? Is it A, a higher voltage, B, a lower voltage, C, a higher current, or D, a lower current? Pause the video, make two choices, and then come back to check your answer.

Welcome back.

So if you chose a lower voltage and a lower current, you're correct.

The slower movement of the tape would be like a lower voltage and lower current.

Let's look at some typical voltage that are around us.

So if you have an AA battery, the voltage on the AA battery would be 1.

5 volts.

Your mobile phone battery is inside the mobile phone and its voltage is about four volts.

A petrol car battery would be 12 volts.

UK mains electricity is at roughly 230 volts, and electric cars are usually up to 400 volts, though some of the newer ones go up to 800 volts.

So there's a range of different voltages for devices around us.

Different devices need different voltages to work properly.

Mains voltage about 230 volts is too high to charge a laptop.

We use an adapter to lower the voltage to about 20 volts, which is safe for the laptop.

In this case, the adapter is in the middle of the wire.

Sometimes the adapter itself plugs into the socket in the wall.

The laptop works safely with this lower voltage.

If the voltage was too high, it would cause too much current flow and would damage the laptop.

It's important to have the right voltage going through the different devices we use.

Small batteries can have a higher voltage than larger ones.

This lithium cell has a three volt rating, so it gives out three volts.

This bigger battery, an alkaline one, only gives out 1.

5 volts.

So the small cell will actually push current harder than the big battery.

The voltage depends on the chemical reactions in the battery.

If a higher voltage is needed, you can actually combine batteries to increase the voltage.

Let's check your understanding.

Which battery lights the bulb the brightest? The same type of bulb is used in each case.

So you have option A where there's a 1.

5-volt battery, you have option B where there's a six-volt battery, and then option C where there's a nine-volt battery.

Three batteries are of different physical sizes.

Pause the video, make your choice, and then come back to check your answer.

Welcome back.

If you chose answer C, that's correct.

Answer C is a nine-volt battery.

It pushes the current around the circuit the hardest.

The other batteries are lower voltages, and so the bulb will be less bright in these cases.

these batteries use the same chemical reaction.

They're both rated at 1.

5 volts.

They're both alkaline batteries.

The larger battery contains a larger store of chemicals, and so it'll work for a lot longer than the smaller one.

For the smaller one, all the energy will be transferred from the chemical store sooner than the bigger one, so it wouldn't last as long.

Let's do a check for your understanding.

We've got two batteries here, A and B, and they're placed in identical circuits.

They use the same chemicals.

Which statement about the batteries is correct.

So statement A is they will last the same time.

Statement B, A will last longer than B, and statement C, B will last longer than A.

For the video, make your choice, and come back to check your answer.

Welcome back.

If you chose answer B, that's correct.

A will last longer than B.

Because it is a larger battery, it will have more chemicals in it, and so it will last longer.

Both batteries provide the same voltage because they're using the same chemicals and in this case it's three volts, but they won't last the same time.

Here is a circuit with one battery and one bulb.

Adding a battery to this circuit causes a bigger push and more current to flow.

This will mean the bulb gets brighter.

If you add an extra bulb to that original circuit, it makes it harder for the current to flow.

As the battery cannot push harder, the bulb will get dimmer.

Here's a test for understanding.

Why does adding a second battery to the circuit make the current increase? The battery is rated at 1.

5 volts and the bulb is rated at 1.

5 volts.

The choices of the answers are A, two batteries move current with more force than one, B, it is easier for two batteries to move current through the bulb, C, there is more current in two batteries than in one.

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

Welcome back.

If you chose answer A, that's correct.

Two batteries are able to move twice as much current than one battery so they move more current.

Answer B is not really correct because it's just as hard for the two batteries to move current through the bulb.

It's the same bulb.

And answer C is not right because there isn't more current in the batteries.

The batteries actually have chemicals in them that react to push the current.

Well done if you've got that right Now we'll try a slightly different check.

Why does adding a second lamp this time to this circuit make the current decrease? Again, it's a 1.

5 volt battery and a 1.

5 volt bulb.

We're gonna add a second 1.

5 volt bulb to the circuit.

The choices are A, two lamps use up more current than one, B, the battery cannot push harder and two bulbs make it harder to move current, and C, the battery does not push as hard when there are two bulbs.

Pause the video, make your choice, and then come back when you're ready.

Welcome back.

If you chose answer B, that's correct.

Well done.

The battery cannot push harder and two bulbs will make it harder to move the current.

If you chose answer A, remember current does not get used up.

The lamps just make it harder for the current to flow.

And answer C, the battery does not push as hard.

Well, it's pushing just as hard as before.

It's just that the two bulbs make it more difficult.

Now you're ready to do a practise task.

Explain how the rope loop simulation shows what happens to the current when the voltage is increased, and then explain how the rope loop simulation shows what happens to the current when more components are added to a circuit.

Pause the video, write out your answers to these two questions, and then come back to check your answer.

Welcome back.

Question one.

Explain how the rope loop simulation shows what happens to the current when the voltage is increased.

So in the simulation, a higher voltage is shown by moving the rope with more force.

The rope is moved more quickly, which shows that the current is increased.

Question two, explain how the rope loop simulation shows what happens to the current when more components are added to the circuit.

So the answer is to show more components being added, the rope can be gripped by more hands, making it harder to move round.

A battery cannot change how hard it pushes, so the rope moves round less quickly, which shows the current is decreased.

In this case, we're talking about more hands, rather than hands gripping the rope tighter.

That would be a different example.

Well done if you got that right.

So let's summarise what we've learned.

A volt metre measures the voltage or potential difference, the more scientific term, across a component.

The units of voltage are volts, capital V.

Volt metres do not allow current to flow through them.

Electrical leads between components do not affect the voltage, and the voltage across a battery and a single component will be the same.

Well done on completing this lesson.

I hope to see you again soon.