Hello, my name's Dr.

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

Welcome to today's lesson.

It's from the unit series circuits.

It's called Changing Voltage.

Today's outcome is I can describe and apply the rules for voltage in a series circuits.

Let's begin.

These are today's keywords.

The first keyword is voltage.

Voltage is a measure of the push that current is given when it flows around a circuit.

The next keyword is potential difference.

Potential difference is a more scientific term for voltage, it's the same thing really.

Our third key word is volt.

This is the unit that voltage or potential difference is measured in and we often use a capital V as the short form for it.

Next we have battery batteries.

The common use for a device that sends electricity through a circuit.

A battery is actually a combination of a series of components that will give the final voltage, and those components are electrical cells.

The individual units that make up a battery are cells.

We don't often use the word cell in everyday language, but in scientific language, cell is the correct word to use.

These are the key words with their definitions.

If you'd like, pause the video now, read through them and then remember to look out for them in the lesson.

Our changing voltage lesson has three parts.

The first part is about the voltages we see across different components.

The second part is where we connect batteries, and the third part is where we look at voltage, current and energy all together.

Let's start with the first part.

Voltages across components.

Here is a simple circuit.

It has a battery and it has a lamp.

The lamp is lit increasing the number of lamps in this circuit so that we have two lamps would cause the bulbs to get dimmer.

If we add another lamp, so there's three lamps.

The bulbs are even dimmer still, the brightness is reduced.

What about if we change the number of batteries? So in this circuit we have one battery to start with and we have the three dim lamps.

If we add another battery, the lamps will get a little bit brighter.

If you add a third battery, the lamps get even brighter still.

So increasing the number of batteries in a series circuit increases the lamp brightness.

Let's check your understanding.

So we have three circuits over here.

Which circuit will have the brightest lamps? Is it A, B, or C? Pause the video.

We'll look at the circuits carefully counting each of the components.

Make a decision and then come back to check your answer.

Welcome back.

If you chose circuit C, that's correct, those lamps will be the brightest.

Let's analyse this a bit further.

If you look at circuit A, there's one battery for one lamp.

If you look at circuit B, there are three batteries for three lamps.

So you could say each battery supplies one lamp.

If you look at circuit C, there are three batteries, but there's only two lamps, so each lamp gets a bit more than one battery's worth.

Those lamps will be the brightest.

Well done if you got that right.

Bulbs are designed to work best at certain voltages.

This is a normal bulb that you could buy from a supermarket and it's designed to light a room.

If you look carefully at the lettering, you can see that this bulb works best at about 220 to 240 volts.

Our mains supply in the UK is about 230 volts, so this bulb is set to work for UK Maine.

If the voltage is too high, the bulb could get damaged or sometimes we say blow.

If the voltage was too low, the bulb would be too dim, it wouldn't work.

It would allow electricity through, but it wouldn't light as it's supposed to.

Bulbs and batteries are often rated, so a 1.

5 volt battery can light a 1.

5 volt bulb correctly.

If you added more bulbs that were rated 1.

5 volts to this sort of circuit, the 1.

5 volt battery cannot push enough current through all three 1.

5 volt bulbs.

The bulbs would only like dimly.

They need more batteries to push more current through them.

Now is a chance to check your understanding.

What would happen if a 1.

5 volt bulb is connected to a 6.

0 volt battery? Would A, the bulb blow, B, the bulb would be dim or C, the bulb would light? PAuse the video, read the answers, make a choice, and then come back to check.

Welcome back.

If you chose answer A that's correct, the bulb would blow.

It's only designed to take 1.

5 volts.

6.

0 volts would be too high for it.

The high voltage would push too much current through it and damage it.

It would probably blow.

If you chose answer B, that's unlikely to have happened because the bulb would've had more than enough voltage pushing through it.

If you chose answer C, that's incorrect.

The bulb might have lit, but then it would've blown because of the high voltage pushing too much current through it.

If you got that right, really well done.

Let's move on.

Now's a chance to put into practise what you've learned.

Task A requires you to build some circuits and measure the voltages with a volt metre and describe the brightness of the bulbs.

The circuits are a combination of one or two bulbs and one or two batteries in series.

A volt metre is put across the bulbs to measure the different voltages.

Pause the video now.

Carry out the tests, noting down your answers, and then come back to check some sample answers.

Here are some sample answers using 1.

5 volt batteries and bulbs rated for 1.

5 volts.

So starting on the left at the top we've got 1.

4 volts and a bright bulb.

Often batteries don't give out the full voltage if they've been used for a while, so 1.

5 volts is reasonable.

The bulb is bright, so it seems to be working fine.

If we go to the circuit below it on the left, we've got two volts and they're both dim.

They're being lit by one 1.

5 volt battery.

This time the voltage across one of the bulbs is naught 0.

7 volts.

This is about half the 1.

5 volt rating from the battery, so that seems sensible, and of course the bulbs are dim because they're sharing the voltage from the battery.

Moving onto the top middle circuit, we have one battery and two bulbs again.

This time the volt metre is placed across both bulbs and reads 1.

4 volts, which is as expected.

Also, the two bulbs are both dim because they're sharing out the voltage.

Going down to the circuit at the bottom, again, we have one battery and two bulbs.

The bulbs are again dim.

This time we're measuring the voltage on the right hand bulb and it comes out as 9.

7 volts, which is the same for the voltage on the bulb on the left as expected.

Moving over to the last two circuits.

At the top on the right we have two batteries and we have one bulb.

This time the bulb is very bright.

Hopefully it won't blow.

The voltage on this bulb is 2.

8 voltages, so it's twice the voltage from one battery.

Moving down, we've got two bulbs with two batteries and they're both right.

The voltage across both bulbs is 2.

8 volts.

If you've done this practical, you may have had different voltages, but you should have had similar patterns where the voltage doubles or halves in the different circuits.

Well done if you manage to complete that.

We've now finished the first section and are ready to move on to the second section.

Voltage from batteries.

In this circuit, the battery is rated at three volts.

The battery is lighting up two lamps.

If a volt metre is placed across the battery as shown here, it'll measure three volts.

If the connections are placed on either side of the circuit, it'll still measure three volts in this position and if it's placed across the two bulbs, it will also measure three volts.

It measures three volts in all these positions.

Breaking down the circuit further, the lamps will each have 1.

5 volts.

The voltage pushing current through each bulb adds up to the voltage, pushing current through both bulbs or three volts, so 1.

5 volts plus 1.

5 volts gives you three volts, the voltage of the battery.

The identical bulbs share the three volts between them so they each get 1.

5 volts.

Here is a chance to check your understanding.

What is the voltage across lamp three.

In this circuit, the lamps are identical, so we have an eight volt battery and we've got four identical lamps in series.

What is the voltage across lamp three? Is it a eight volts, B four volts, C two volts, or D one volt? Pause the video, make a choice and come back to check your answer.

Welcome back.

If you chose answer C, that's correct.

Each bulb gets two volts.

If you add up four lots of two volts, you end up with eight volts.

Remember, the bulbs are identical so they each get the same voltage.

Let's try another check of understanding.

What's the reading on the volt metre in this circuit of identical lamps.

This time we have six volts from the battery and we've got a volt metre connected across two lamps is the reading two volts, three volts, four volts or six volts.

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

Welcome back.

If you chose answer C four volts.

That's correct.

We have six volts shared between three bulbs.

Each bulb is identical, so each bulb gets two volts.

Two volts times three gives you six volts.

So the voltage across two bulbs is two volts plus two volts, which gives you four volts, if you got that right really well done.

The scientific word battery actually means a collection of electrical cells.

One of these on their own is called a cell.

When you add a few of them together, it's a battery, and in this case this is a battery of three cells.

This is a little bit confusing 'cause if you go to a shop and ask for a battery, just a single battery, you'll be given one cell.

In science it's important to get the terminology correct, so one of these on their own is a cell.

A collection of them together is a battery.

Here are three devices that use batteries of cells, so starting at the top we've got a wifi clicker that uses two AAA cells to make a battery of two cells.

If each of the cells is rated 1.

5 volts, then the battery has a total voltage of 1.

5 plus 1.

5 or three volts.

Next we have a camera that uses a battery of three AA cells.

Again, if these are 1.

5 volts, the total voltage of the battery is 1.

5 plus 1.

5 plus 1.

5 or 4.

5 volts.

Finally, we have a head torch that has a battery of three AAA cells.

Again, combining the batteries gives a higher voltage if needed.

Metro cars use 12 volt car batteries.

This battery shown is made up of six two volt electrical cells connected together in series.

You can just about see them through the plastic.

Electric car batteries are rated at 400 volts and some at 800 volts.

They use a large number of individual cells connected together to produce this voltage.

When batteries are aligned in the same direction in series, they push current in the same direction and their voltages add together.

So we have a symbol over here of a battery of two cells.

The two cells would look like this lined up in the same direction.

If each cell is 1.

5 volts, when you add them together, you'd get a total of 3.

0 volts.

Let's check your understanding.

What voltage is this torch designed to work at? There are three AAA cells and each of them has a voltage of 1.

5 volts.

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

Welcome back.

If you chose 4.

5 volts, that's correct.

If you look at the cells connected together, the current flows through one cell after the other and if you add up 1.

53 times, you get 4.

5 volts.

Well done if you got that correct.

If batteries are not aligned in the same direction, they push current in opposite directions and they cancel each other out.

So if you place two batteries with the same polarity end together, they will cancel out and give you zero volts.

1.

5 minus 1.

5 volts in the opposite direction will give you zero volts.

In reality, batteries don't give you exactly the right amount of voltage.

They vary slightly.

Maybe one of them will be 1.

4 volts, so you will get a low voltage rather than exactly zero volts.

Batteries should always be aligned correctly for this reason.

Let's check your understanding so far, what's the voltage across this combination of 1.

5 volt cells? Is it a 1.

5 volts, B, 3.

0 volts, or C, 6.

0 volts? Pause the video, carefully look at how the cells are connected together and then make your choice of answer.

Come back when you're ready.

Welcome back.

If you chose three volts, that's correct.

Let's look at these cells carefully.

On the left, there are two cells that are pointing in opposite directions.

They will cancel each other out.

On the right, there are two cells that are pointing in the same direction, so in total we've got two cells pointing in the same direction two times 1.

5 is three, and so this combination should give 3.

0 volts.

Remember, connecting cells like this should not be done.

They should always be connected in the same direction.

We've come to the end of the section and now's a chance to practise what you've learnt.

Build these circuits as shown and write down the voltages for each of the voltmeter measurements and the lamp brightness in each case.

Remember, batteries should not be connected in the wrong polarity normally.

We are just testing to see how they would work in these combinations.

Pause the video, go ahead and make these observations and then come back to check what you found.

Welcome back.

We've got some results using 1.

5 volt rated cells and 1.

5 volt rated bulbs.

So in the first circuit on the left at the top we've got two cells pointing in the same direction and they give a voltage of 2.

8 volts, which is nearly three volts.

The bulb is bright.

Underneath that the two cells are pointing in opposite directions.

This time the voltage is zero volts and the bulb doesn't light, it's off.

Going to the middle top circuit, we've got the cells pointing to the right.

This time the voltage is minus 2.

8 volts, so if you like, it's going backwards and the bulb still lights brightly.

The direction doesn't matter for the bulb.

Looking at the bottom row in the middle, we've got three cells.

The two cells on the left cancel each other out, leaving one cell pointing to the right.

This time the voltage is minus 1.

4 volts and the bulb is dim.

Going to the top right, we've got four cells.

The first two cells on the left point in opposite directions, so they cancel out the two cells on the right point in opposite directions as well, so they cancel out giving a voltage of zero volts.

In this case, the bulb is off.

And finally, bottom right, we have two cells on the left pointing to the left in the same direction, and we've got two cells on the right pointing against each other, so that gives us 2.

8 volts and a bright bulb.

If you had similar sorts of results, that's really well done.

These are quite complex, but remember, batteries should not be connected in the opposite polarity.

We've only done this to check how they work.

We've now reached the last section where we relate voltage, current and energy.

The cell in this circuit provides the push on electrical charge causing a current to flow and light the bulb.

Voltage is a measure of the strength of this push.

Energy is transferred from the chemical store in the cell causing the lamp to light up.

Let's check how much you've learnt.

Which three of the following statements are correct about batteries with larger voltages? Is it A, they push electric chargers harder? B, they transfer energy more quickly.

C, they cause a lower resistance, or D, they cause a higher current.

Pause the video, choose three of the statements that you think are correct and then come back and we'll check your answers.

Welcome back.

So if you chose A, they push electric charges harder.

That's correct.

Larger voltages, push electric charges harder.

If you chose B, they transfer energy more quickly.

That's also correct.

If the charges are being pushed harder, more energy is transferred.

Finally, if you chose D, they cause a higher current.

That's also correct.

A harder push will cause a higher current.

Well done if you've got those right.

Here is the final task.

We're going to use our rope or tape loop model to explain the following.

First of all, A, what makes a lamp light up? Use the rope loop model to explain that.

Then B, use the rope loop model to explain why after a long time a battery stops working.

And finally use it to explain why a larger voltage makes a lamp brighter.

Pause the video now, write out some answers for A, B, and C and then come back to check how you've got along.

Welcome back.

So starting with making a lamp light up, we can say when the tape is pulled, it is like a battery moving charger to cause a current.

The tape rubs the hands making them warmer.

This is like the charges transferring energy to the filament and making it hotter.

To explain why a battery stops working, you could have written after some time the person pulling the tape will get tired and stop.

This is like the battery running out of energy and stopping.

To explain why a bigger voltage makes a lamp brighter.

You could have said if the tape has moved with more force, it'll move the tape faster.

This is like a higher current that transfers energy to the lamp more quickly, making it brighter.

Well done if you've got those answers right.

We've reached the end of the lesson, so let's summarise what you've learned.

Increasing the number of cells in a battery will increase the voltage it can supply.

Cells are the individual components in a battery.

Cells should be connected in the correct alignment.

Electrical devices are designed to work at certain voltages and the correct voltage should always be used.

The voltage of a battery is the same as the voltages across the components in a circuit added together.

You've done really well in completing this lesson.

I hope to see you again soon.