Lesson video

Hi, I'm Mrs. Hudson, and today we're going to be learning about changing voltage.

This is a key stage three science lesson in the physics topic, and it comes under the unit titled "Series Circuits." So let's get going.

The outcome of today's lesson is I can describe and apply the rules for voltage in a series circuit.

There will be some words that are frequently used in today's lesson.

These are the key words, and today they are voltage, volts with a V, battery and electrical cell.

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

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

A voltmeter measures the voltage or potential difference in units called volts, which we use the letter V to represent.

Two or more cells connected in series form a battery.

And finally, an electrical cell is a component that uses a chemical reaction to make electric charge flow around a circuit.

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

Today's lesson on changing voltage is going to be split up into three different parts.

In the first part of the lesson, we're going to look at voltages across components.

Then we're going to move on to look at connecting batteries.

And then in the final part of the lesson, we'll be looking at voltage, current and energy.

But let's get going with the first part of today's lesson, voltage across components.

So here we've got a service circuit that is made up of a cell and an individual lamp, and we can see that the lamp is emitting light.

And here if we added another circuit where we've added in an extra lamp.

And then finally a final circuit where we've got three lamps.

increasing the number of lamps within the circuit will reduce their individual brightness.

So the brightness of the lamps will decrease the more lamps you add into the circuit, as long as you keep the number of cells the same.

So each one of these circuits has got one cell, which is pushing electrons around the circuit.

And if I increase the number of lamps with just still that one cell, the brightness of the lamps will decrease.

Here we've got a circuit with one cell still, but this time there's three lamps.

And then in the next circuit I've got two cells which make up a battery and we've still got three lamps.

And in the final cell, there are three cells with three lamps.

And here, increasing the number of cells increases the lamp brightness.

So if I increase the number of cells but I keep the number of lamps the same, then the brightness of the lamps will increase.

Let's quickly check our understanding of that.

Which circuits will have the brightest lamps? So there's three circuits here.

A, which has one cell and one lamp.

B, which has three cells making up a battery and three lamps.

And C, which has three cells, again making up a battery and two lamps.

So you have to work out which circuit will have the brightest lamps.

Have a go and then come back when you're ready for me to give the answer.

Hopefully here, we recognize that the answer was C.

The reason it's C is because there are three cells and only two lamps, whereas if you compare that to B, there are three cells but three lamps and in A, there's one cell and one lamp.

So there are more cells in ratio to the number of lamps in C.

There's effectively only two lamps there, but there would be three cells that are pushing electrons around that circuit.

So well done if you managed to get that right.

The bulbs in lamps are designed to work best at specific voltages.

So if we have a look at this bulb in the picture in the middle, if we zoom in to look at the writing on that bulb in a little bit detail, this bulb works best at around 220 and 240 volts.

Now if you remember, the electric main supply in the UK is 230 volts.

So this bulb is designed to work best with the UK electrical mains voltage.

If the voltage is too high, the bulb could blow, which means that it would no longer work anymore.

And if the voltage is too low, the bulb will be too dim.

So it's really important that bulbs are designed to work with the correct voltage so that they either don't blow because the voltage is too high or they don't have too little voltage going through them, which would mean that the bulb is too dim.

So a 1.

5 volt cell will light a 1.

5 volt lamp correctly.

So we can see here that that 1.

5 volt cell within that series circuit is lighting that lamp and a lot of light is being transmitted.

A 1.

5 volt cell cannot light three 1.

5 volt lamps as brightly because there's still the same push of electrons from that 1.

5 volt cell.

But now there are three lamps and so the lamps are going to emit less light.

The lamps will only light dimly as they require more cells to supply the necessary voltage.

Let's check our understanding of that.

What will happen to a 1.

5 volt bulb in a lamp if it is connected to a 6 volt battery? A, the bulb will blow.

B, the bulb will be dim.

Or C, the bulb will be bright.

Hopefully here, we recognize that the answer is A.

If you connect a 1.

5 volt bulb to a 6 volt battery, there will be too much current flowing through that bulb and therefore the bulb will blow.

So well done if you managed to get that right.

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

And in this you need to build the following circuits, then record the voltages measured and describe the brightness of the lamps using the words very bright, bright or dim.

And you can see that there are six different circuits here.

Some of the circuits have got one cell and some of them have got two cells, some of them have one lamp and the others have two lamps.

And then in each circuit you have got a voltmeter, which is either across one bulb or it is across both of the bulbs.

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

Pause the video, write down your results, and then come back when you're ready for me to go through the answers.

Let's see how we did.

So first of all, your readings might be slightly different to the answers that we've got on here, but as long as your values follow the same trend as ours.

So for example, they roughly double or roughly half at the right point, then that's absolutely fine.

So if we start at the top left hand circuit, here you've got one cell and one lamp, and the reading around that lamp is going to be here 1.

4 volts, so roughly 1.

5 volts and the lamp is going to be very bright.

If we now compare that to the top middle hand circuit, the voltmeter here is across both of the bulbs, but the voltage is going to remain the same.

So it's going to remain at 1.

4 volts, but the brightness now is going to be dimmer.

So those bulbs in comparison to the first circuit will be dimmer.

Now going across to the top right hand circuit, this time we've added in another cell.

So we've got a battery made up of two cells and we've got one lamp in that circuit.

And this time because there are two cells, the voltage is going to double and so therefore you should have double what you've got in the previous two circuits.

So in this case on our answers it's 2.

8 volts.

And because the voltage is double, the brightness is very bright.

Now if we go down to the bottom circuits on the left hand side, we've got one cell here with two lamps and the voltmeter is across one of those individual lamps.

And because there's two lamps within that circuit, then the voltage across one of the lamps is going to be half of what it would be if it was across both of the lamps.

So 0.

7 volts in this case.

Now the brightness again is going to be dim.

Now looking at the bottom circuit in the middle, again this is very similar to the previous example, it's just that the voltmeter is around the other lamp within that circuit.

So the voltage again is going to be half 0.

7 volts and the lamp brightness will remain dim.

Now looking at the bottom right hand circuit, in this case, rather than there being one cell, there's going to be two cells forming a battery and the voltmeter is across both of those lamps.

So you should have the same voltage as what you did in the top right hand circuit at 2.

8 volts.

And the brightness of the bulbs though in this case is going to be not very bright.

It's going to just be bright.

So really great job if you manage to get that right.

Well done.

Great job so far.

We now understand about voltages across components.

So let's move on to look at voltage from batteries.

In this circuit, the battery has a voltage of 3 volts, so we can see that there's two individual cells there with a voltage of 1.

5 each making the total battery voltage 3 volts.

A voltmeter will measure 3 volts here.

So we can see that we've put the voltmeter across the battery and it will measure a reading of 3 volts.

It will also measure 3 volts if we connected the voltmeter across the circuit.

And it would also measure 3 volts if the voltmeter was connected across both of the lamps within that circuit.

And it's very important here that we recognize the voltmeter is across both of the lamps, not just one.

Each identical lamp will have a 1.

5 volt across it.

So this time, rather than measuring the voltage across both of the lamps, if you put a voltmeter across each individual lamp, what you'll notice is that the voltage across each lamp is half of 3, which is 1.

5 volts.

That would be assuming that the bulbs that you are using are identical.

So the total voltage pushing current through both lamps adds up to 3 volts.

Let's check our understanding of that.

So here we've got a circuit and the lamps in the circuit are shown, are identical.

It's asking, what is the voltage across lamp three? And if we just look at the circuit now there is a battery made up of two cells which has a voltage of 8 volts and there are four identical lamps within that circuit.

So you need to say what would be the voltage across lamp three? A, 8 volts.

B, 4 volts.

C, 2 volts, or D, 1 volt.

This is C, so well done if you've got 2 volts.

And that's because the voltage is going to be shared equally across the identical lamps.

So 8 divided by 4, which is the number of lamps, is going to be 2 volts across each lamp.

So well done if you got that right.

Let's have a go at this question now.

So this time we've got a circuit again which has got two cells making up a battery which has a 6 volt rating.

And then there are three lamps within that circuit and the voltmeter is over two of those lamps.

And the question is, the lamps in the circuit shown are identical.

What is the reading on the voltmeter? A, 2 volts, B, 3 volts, C, 4 volts or D, 6 volts.

Hopefully here we went for C, 4 volts.

And the reason it's 4 volts is because the 6 volt battery is going to supply 2 volts to each bulb and the voltmeter is across two, so it's gonna be 2 times by 2 volts, which is 4 volts.

So really big well done if you managed to get that right.

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

So here this would be representing one cell and if we added in more than one cell, we call that collection of more than one cell a battery made up of three cells.

Now this is quite confusing because in everyday life we may use one cell, but in everyday life we use the term battery.

But in science it's really important to make sure if we're just referring to one cell, that we use the word cell and only say battery if there is more than one cell.

Cells are used to form batteries in these devices.

So there's three different devices here.

If we look at this device, there are two triple A cells forming a battery in a wifi remote.

And let's say that each cell had a 1.

5 volt rating, there would be a total voltage there of the battery of 3 volts.

This here is three double AA cells forming a battery in a camera.

And again, if each AA cell had a 1 volt rating, if there were three cells within the battery, the total voltage would be 3 volts.

And then finally in this example, there are three triple A cells forming a battery in a torch.

A 12 volt car is made up of six, 2 volt cells connected in series.

And we can see very kind of loosely if we look at this picture that there are six 2 volt cells within that battery, which gives a total voltage of 12.

Electric car batteries have a voltage of 400 volts or 800 volts in some cases.

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

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

So we can see here that you've got the circuit symbol for a battery made up of two cells and you can see the positive terminal, which is the longer, thinner line and the negative terminal, which is the shorter, wider line.

And it's really important that the positive and the negative terminals are in the same direction.

And then the total voltage is the individual voltages of each cell added together.

So a way of representing that would be with two cells that make up a battery.

And if each cell had a 1.

5 volt rating, the total voltage of the battery would be 1.

5 plus 1.

5, which is 3 volts.

Let's check our understanding of that.

What voltage is this torch designed to work at? There are triple A cells there that each have a voltage of 1.

5 volts.

So is it A, 1.

5 volts, B, 3 volts, C, 4.

5 volts, or D, 6 volts? So hopefully here we recognize the answer is C, 4.

5 volts because there are three cells each with a voltage of 1.

5.

So 1.

5 plus 1.

5 plus 1.

5 is 4.

5 volts.

So really good job if you got that right.

The current flows through the cells, one after the other like in this kind of wiggly, green line on here.

And so you'll have 1.

5 volts, then 1.

5, and then 1.

5.

If cells are not aligned in the same direction, they push current in opposite directions, canceling each other out.

So if we look at this diagram here, we can see that there are two cells, but the positive and negative terminals are not in the same alignment and therefore, the they're gonna push the current in opposite directions and cancel each other out.

And that would look like this here if we are representing it with two cells that we might use to make up a battery.

So here you would have 1.

5 volts and then you would minus 1.

5 volts because the current's playing in the opposite direction.

And overall it would give a rating of naught volts.

Generally speaking though, cells don't have exactly 1.

5 volts.

It might be slightly lower, so therefore there might be some voltage that's given out, but it would be very, very small.

But the key learning from this slide is that cells should always be aligned correctly.

Let's check our understanding of that.

What is the voltage across this combination of 1.

5 volt cells? So if we look here, we need to look at how many different individual cells there are and how many of them are correctly aligned.

So is this A, 1.

5 volts, B, 3 volts, or C, 6 volts? Hopefully here we have gone for B, 3 volts.

So there are four individual cells in this battery, but two of the cells are not aligned in the same direction.

That's the first two cells.

so their voltage is canceled out.

And then you've got two cells that are aligned correctly afterwards and they both have a voltage of 1.

5, 1.

5 plus 1.

5 is 3, and that's why the answer's B.

So well done if you've got that right.

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

And your job here is to build the following circuits, write down the voltages measured and describe the brightness of the lamps using the words either bright or dim.

So there are six circuits to set up.

They each have one lamp within them and one volt meter, but there is a different examples of cells and the way that they are aligned here.

So a different number of cells that make up batteries and they're aligned in different ways.

Now, just so you are aware, I know that we have said previously that we should always make sure that cells are aligned in the correct direction.

The reason that we are doing this is to for us to understand what happens to voltage if cells are not aligned in the same direction, but we should always make sure cells are aligned in the same direction.

This is just for learning purposes.

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

Pause the video and then press play when you're ready for me to go through the answers.

Let's see how we did.

So again, you might have slightly different values, but as long as they follow the same trend, then that's absolutely fine.

So in the top left hand column, we can see that both the cells are aligned in the correct direction.

And if they were roughly both 1.

5 volts, then the voltage would be around 3 volts.

But in this case it read 2.

8 volts and the brightness of the bulb was very bright.

If we go across to the top middle circuit here, this time the cells that make up the battery, they are still in the correct alignment.

It's just that they are in the opposite direction.

So in this case, the voltage is still 2.

8, but it's just minus 2.

8 volts, but the bulb still remains very bright.

And then in the final top one, on the right hand side, there are four cells in this circuit, but actually all of the cells are aligned incorrectly.

So the first two are in the opposite direction, and then the last two also in the opposite direction.

So there is no voltage and the light is off there, the bulb is off.

So well done if you got that right.

Going down to the bottom row now.

In the bottom left hand one again, those two cells that make up the battery are not aligned properly.

So there's zero volts across the lamp and the lamp is off.

And then in the middle circuit at the bottom, there are three cells here, two of which are not aligned properly in the first two cells, and then the final one is aligned properly.

So there's a voltage of just one of those cells, 'cause the first two cancel out, so it's minus 1.

4 volts.

And then the brightness of that lamp is dim.

And then in the very final circuit here, you've got four cells, two of which are not aligned properly, but the first two cells are aligned properly.

So therefore there's a voltage of roughly 3.

So in this case it read 2.

8 volts and the brightness is bright.

So really great job if you managed to get those right.

Well done.

We're ready now to move on to the third part of today's lesson, which is voltage, current and energy.

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

Voltage is a measure of the strength of this push.

And when that cell pushes the current around that circuit, energy is then transferred from the chemical store, which is inside of the cell, and it's transferred to the light in the lamp, which causes the lamp to light up.

So if you increase the voltage across a circuit, it's going to push electric charge harder around that circuit.

It will also cause the energy transfer to be faster, so the lamp will be brighter and it will also cause a higher current within the circuit.

Let's check our understanding of that concept.

So which three of the following statements about batteries with larger voltages are correct? A, they push electric charges harder.

B, they transfer energy more quickly.

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

So which three of the following statements are correct? Hopefully here we went for A, high voltage does push electric charges harder.

B, that means that there's transfer of energy more quickly.

And D, this does cause a higher current.

So massive well done if you recognize those three statements as being correct.

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

And this is looking at the model that can be used to represent electric circuits.

The model here is being shown in this little video where there is somebody pulling on some tape and the tape then is moving around in a circle and it's been held in place by four still hands.

In this case the pulling hands, they're representing the cell or the battery.

The rope is representing the current and the flow of electrons around that circuit and the hands are representing components.

So your task is to use the rope, the tape loop model, to explain the following.

A, what makes a lamp light up? B, why after a long time does a battery stop working? And C, why does a larger voltage make a lamp brighter? I'm gonna give you a few things to help you with these.

So what makes a lamp light up? We have to think about the fact that energy transfer is taking place there.

And then for B, we have to think about the fact that in a battery there are chemicals which are then transferring energy around the circuit.

And then for C, we have to think about the fact that a higher voltage will be the hands pulling that tape harder.

And what effect will that have on the movement of the rope and what the rope in this instance is representing? I'm sure you're gonna do a really great job of this.

So pause the video, give it your best go, and then press play when you are ready for me to go through the answers.

Let's see how we did.

So looking first at A, what makes a lamp light up? When the tape is pulled, it is like a battery moving charges to cause a current.

The tape rubs against the hands making them warmer.

This is similar to charges transferring energy to the filament, causing it to heat up and light the lamp.

Then for B, why after a long time does the battery stop working? Well after a while, the person pulling the tape will get tired and stop.

This is like the battery running out of energy and ceasing to provide current.

And this happens when the batteries run out of any of the chemicals within it, that the chemical reactions within are transferring energy.

Then for C, why does a larger voltage make a lamp brighter? If the tape is moved with more force, it will move faster.

This is like a larger voltage that causes a larger current, transferring energy to the lamp more quickly and making the lamp brighter.

If you need to the video now to add any detail into those answers, then please do, but if not, well done for finishing it and we're going to move on now to summarize everything we've learned in today's lesson.

So today we've been looking at changing voltage and we started off by saying that increasing the number of cells in a battery will increase the voltage that it can supply.

We said cells are the individual components that make up a battery.

And we said there we have to be very careful because in everyday language people refer to one cell sometimes as a battery.

But in science, if something is just made up of one cell, we don't use the term battery.

Cells should be connected in the correct alignment.

And remember, if you put these cells in an incorrect alignment, the current will flow in opposite directions and there will be zero voltage.

Electrical devices are designed to work at specific voltages, so the correct voltage should always be used.

Remember as well there we said that if you use a too high a voltage for a specific bulb, for example, the bulb will blow.

And if you use a too low a voltage for a bulb, the bulb will be too dim.

And then finally we said the voltage of a battery is equal to the sum of the voltages across the components in a circuit.

So well done with today's lesson.

You've done a really great job.

I've really enjoyed teaching it.

I hope you have too and I look forward to seeing you next time.